Basement contractor recommendations

[Thorfun]

Hi all,

our planned self-build has a basement that will be a habitable area consisting of a music room and a games room/entertaining area. the ceiling will be 3m high as I have an idea to put a golf net and simulator down there and need the height to swing a golf club!

I am at the point where I am looking to get quotes for the work and was wondering if those that have had basements done are able to recommend their basement contractor?

Also, I was as the Build It Live show in Kent the other weekend and spoke to a couple of chaps from Surrey Basements who seems to know their stuff and looking at their website seem to have done a large number of good looking basements in the London area. does anyone have any experience with them?

any opinions are, as always, very welcome.

many thanks in advance.

[Carrerahill]

I'd just get in a concrete contractor in and get it poured - almost every commercial building built in cites have basements, they don't use a specific "basement" contractor, the guys doing the pour just form a basement in a big hole - nothing special - that will open up your choices. 

 

 

[Thorfun]

19 minutes ago, Carrerahill said:

I'd just get in a concrete contractor in and get it poured - almost every commercial building built in cites have basements, they don't use a specific "basement" contractor, the guys doing the pour just form a basement in a big hole - nothing special - that will open up your choices. 

 

 

sounds good, but scares the cr@p out of me as it sounds like a lot of responsibility on my shoulders to get everything ready for the pour. or am I completely misreading this?

 

I'm alright at DIY but building a basement is a bit beyond my confidence levels.

[Moonshine]

33 minutes ago, Carrerahill said:

they don't use a specific "basement" contractor, the guys doing the pour just form a basement in a big hole - nothing special

 

A wouldn't be that blase about it, there are the small matters of water drainage and tanking that have to be carefully considered. 

 

@Red Kite is living the basement dream at the moment and has lots of insightful information here

 

 

 

[Thorfun]

2 minutes ago, Moonshine said:

 

A wouldn't be that blase about it, there are the small matters of water drainage and tanking that have to be carefully considered. 

 

 

this is what scares me and why I'd want a specialist or at least someone who will guarantee their work, not just the concrete pour. if it wasn't for all that I could probably just ICF it myself.

 

[Brickie]

I have a mate who has a basement company but I don’t know if sharing his details would breach forum rules. 
If the mods ok it,I’ll pm you his details. 
He’s in South London,btw. 

[Thorfun]

5 minutes ago, Brickie said:

I have a mate who has a basement company but I don’t know if sharing his details would breach forum rules. 
If the mods ok it,I’ll pm you his details. 
He’s in South London,btw. 

 

would be very useful to PM it if that's allowed. cheers.

 

we're in West Sussex south of Gatwick so South London is not that far.

[nod]

11 minutes ago, Brickie said:

I have a mate who has a basement company but I don’t know if sharing his details would breach forum rules. 
If the mods ok it,I’ll pm you his details. 
He’s in South London,btw. 

Personal recommendation is always the best option with any building service 

Book well in advance as good company’s are always busy and will be happy to share previous jobs with you 

Happy Custer’s = Low stress 

[Thorfun]

2 minutes ago, nod said:

Personal recommendation is always the best option with any building service 

Book well in advance as good company’s are always busy and will be happy to share previous jobs with you 

Happy Custer’s = Low stress 

agreed. I've read a lot about @Bitpipe's basement and will, if it's ok with him, send him a PM to get the name of the company he used as he seemed very happy with their work.

[Pocster]

1 hour ago, Thorfun said:

 

this is what scares me and why I'd want a specialist or at least someone who will guarantee their work, not just the concrete pour. if it wasn't for all that I could probably just ICF it myself.

 

Tbh it’s not that hard but is a lot of work . As mentioned waterproofing ( or managing water ingress ) is your biggest concern . I went with an internal membrane going into an internal perimeter drain then out to a pump . As ‘extras’ I put a land drain around the basement also thick polythene sheet externally - though admittedly these have or will fail .

Edited February 21, 2020 by pocster

[Thorfun]

Just now, pocster said:

Tbh it’s not that hard but is a lot of work . As mention waterproofing ( or managing water ingress ) is your biggest concern . I went with an internal membrane going into an internal perimeter drain then out to a pump . As ‘extras’ I put a land drain around the basement also thick polythene sheet externally - though admittedly these have or will fail .

the company I spoke to at Build It Live use an internal membrane to manage water ingress just like you've described. I'm sure I could fit it and do it all but what about future saleability (not that we plan to move but you never know!) or getting a mortgage? wouldn't they need some form of sign-off that it's all been done properly and given a guarantee?

[Pocster]

1 minute ago, Thorfun said:

the company I spoke to at Build It Live use an internal membrane to manage water ingress just like you've described. I'm sure I could fit it and do it all but what about future saleability (not that we plan to move but you never know!) or getting a mortgage? wouldn't they need some form of sign-off that it's all been done properly and given a guarantee?

Not sure with re sale . My architect ‘checked’ - not that he’s an expert either .

If you are requiring a self build mortgage then I guess they would want some kind of ‘sign off ‘ for that . But then as it’s a ‘self build’ they might want sign off for practically everything e.g foundations , roof etc etc ...

[Thorfun]

Just now, pocster said:

Not sure with re sale . My architect ‘checked’ - not that he’s an expert either .

If you are requiring a self build mortgage then I guess they would want some kind of ‘sign off ‘ for that . But then as it’s a ‘self build’ they might want sign off for practically everything e.g foundations , roof etc etc ...

yep. I need a self-build mortgage at the financial institution I've been speaking to do indeed need their surveyor involved from the outset to sign everything off during the build.

[Pocster]

1 minute ago, Thorfun said:

yep. I need a self-build mortgage at the financial institution I've been speaking to do indeed need their surveyor involved from the outset to sign everything off during the build.

Ok . Best ask them exactly . I took photos at every stage so I could always prove I did it . Wether I did it well or not is another question . I guess someone on here with a self build mortgage can answer better ...

[Carrerahill]

2 hours ago, Moonshine said:

 

I wouldn't be that blase about it, there are the small matters of water drainage and tanking that have to be carefully considered. 

Yep, we often have to spec duct penetrations which will be water tight to commercial (usually city centre) buildings with basements.

 

They still don't call the "basement" contractor - any proper concrete contractor worth his salt will know how to deal with this and will be well versed in these matters. If they are not, don't hire them.

 

I wouldn't be blase about any aspect of construction from the foundations to the ridge tile, each does it's own job and play an important part, what I am saying to the OP don't be getting hung up on a "basement" contractor - as soon as you give someone a name like that they double their fee!

 

 

 

 

[Carrerahill]

3 hours ago, Thorfun said:

Sounds good, but scares the cr@p out of me as it sounds like a lot of responsibility on my shoulders to get everything ready for the pour. or am I completely misreading this?

 

No, get someone in to do the whole contract of this stage, big contractors will use a wood faced metal form-work system like the Gilpi system which will be their own kit - they will do the lot. You would probably just need to supply a prepped site. 

 

I highlight the "wood faced" aspect as it reduces the water content against the form and improves surface strength of the concrete.

 

Unless you feel really confident. But concrete form work (temporary works) is a whole speciality in it's own right and will be holding back a huge weight of concrete desperate to escape, so really... I wouldn't!

 

 

Edited February 21, 2020 by Carrerahill

[Thorfun]

6 minutes ago, Carrerahill said:

what I am saying to the OP don't be getting hung up on a "basement" contractor - as soon as you give someone a name like that they double their fee!

I understand what you're saying and will take it on board. thanks.

 

2 minutes ago, Carrerahill said:

No, get someone in to do the whole contract of this stage, big contractors will use a wood faced metal form-work system like the Gilpi system which will be their own kit - they will do the lot. You would probably just need to supply a prepped site. 

 

I highlight the "wood faced" aspect as it reduces the water content against the form and improves surface strength of the concrete.

 

Unless you feel really confident. But concrete form work (temporary works) is a whole speciality in it's own right and will be holding back a huge weight of concrete desperate to escape, so really... I wouldn't!

 

thanks. I really don't fancy tackling form work!

any recommendations on who I could contact initially? or should I just Google it for now and then do more research?

[Jeremy Harris]

3 hours ago, Brickie said:

I have a mate who has a basement company but I don’t know if sharing his details would breach forum rules. 
If the mods ok it,I’ll pm you his details. 
He’s in South London,btw. 

 

 

No problem with passing on a personal recommendation, IMHO.

[Bitpipe]

I did a lot of research before building ours. I'm no expert but here is my experience.

 

Unless you're excavating under an existing building,  if you hire someone with 'Basement' in their company title then you're probably paying a premium for what is essentially a concrete box that any decent ground-worker firm can build - i.e. the type of firm that would build an underground carpark etc.

 

Here are the steps that I followed:

 

1) get our planning permission for the basement element of your build. In our case, we re-submitted the planning app once the above ground element was approved - our planners were mostly interested in the lightwells to the front of the house (which are only visible as grills in the front paving and not so much the emergency egress to the rear the footprint itself. We represented the basement as a big empty box with 'basement' on it - not not indicate any room division or use at that stage.

 

2) get your SE to commission a ground investigation spec that will give them the relevant data (soil bearing, composition, water levels etc) and will also satisfy any conditions imposed by your LA (we had archaeological and soil contamination conditions). You can shop the spec around yourself but don.t be tempted to come up with your own. Then choose your GI contractor and get them to do their stuff. In our case, they were probing and coring around the existing house (that we later demolished) but they got enough data points to satisfy the SE.

 

NOTE - steps 1 and 2 are committed costs, depending on what comes out of the report, you may decide that it's not cost feasible to build your basement, especially if it requires piling etc.

 

3) get your SE to design the basement, paying attention to supporting the loads imposed by the house above. In our case, we already had a MBC structural design with calcs so the SE was able to take these into consideration. BC will also need to be consulted to ensure that the basement meets regs regarding emergency egress in fire (either a separate exit to ground level or sprinkler system). You will also know your groundwater situation and this will inform your waterproofing strategy.

 

• What shape is it? Ours was a slightly off square box (11.5m x 10.5m) with an additional rectangle bit for the plant room. We used stud-work to create the internal rooms and a web of steel above to hold the suspended timber floor. Wet ground floor UFH is on spreader plates.
• Complicated shapes, corners etc will cost more to build as there is a lot of labour in the formwork and steel placement.
• How will light get into the basement? We sourced german GRP lightwells (MEA Bausysteme) and specc'd holes to fit. East then to get a window made (UpVC) to fit the hole. Cheaper than specifying made on site concrete lightwells. 
• Will you have wet services in the basement (shower, toilet etc). Wet services will need a means to connect to the ground level fouls (pump). We decided not to but do have a plant room in there for the gas boiler, MVHR, UVC etc. Just have to get rid of boiler/mvhr condensate and we have a external drain for the UVC tundish where the external exit stairs are.
• Will it have a solid lid or be open and rely on a suspended floor (we did this). Will have implications on how you heat the GF if you're thinking of UFH.
• How will it be insulated? We put ours on a layer of 300mm of EPS200 (the bearing strength, calculated by the SE) and applied 200mm EPS70 to the walls (with LE foam) which then met the insulation detail from MBC. This was not hard to do but that grade of insulation is not cheap.
• We did not heat our basement, it's always 20^oc year round due to being in the ground and having decent insulation. Also, there is lots of ambient heat released by the plant room and electrical appliances, people etc.

 

4) you will need a waterproofing strategy - really depends if water table is far below you or you're effectively building underwater. We had gw at 6m and as only digging 3.5m down type B alone was ok. 

• A is an external membrane - can be effective but only as good as the application - a problem will not be apparent until you have a leak
• B is waterproof concrete - uses an admix to the concrete,  water bar on vertical and horizontal pour joins and penetrations (like fouls) and special plugs to fill the formwork bracing holes.. We used a warrantied SIKA system that was inspected and signed off
• C is an internal membrane, sump & pump. In this situation you are expecting the walls to leak so collect water internally and have it run down to a precast sump which is continually emptied by a pump. If the pump fails, the basement floods so it's good practice to have two and some kind of warning system and plan for power failure.

If you're above ground water then you need a land-drain at the perimeter of your basement to a soak-away. Covering in shingle and using clean stone as backfill for the gap between excavation and wall will act as a giant french drain to deal with rain water.  

 

5) you need a build strategy. How will your basement be built? Concrete poured in situ using movable formwork (most common method), ICF? Precast insulated panels? Bored holes filled with RC? Blockwork?

How big is your site, how much plant can you fit on it during the build? How close are you to neighbours? Does the party wall rule come into effect (the 3/6m separation bit)

 

NOTE - you may choose to build in ICF  but be aware that for a waterproofing strategy that depends on the walls being a barrier, ICF has the challenge of not being able to see the quality of the pour (marshmallow finish due to etc) as both sides covered with insulation. Does not mean it will be a problem, just means you won't know if there are any issues and will have to trust your contractor to get the mix perfect and vibrate well.

 

Will you need to de-water during construction - we had friends who live by a river and they were in this situation, they used an (expensive) system that pumps ground water out of the excavation continuously during the build.

 

6) when you have all of this info (most from SE) you can get groundworking firms to tender. The SE quote should detail the concrete spec, bar schedule, waterproofing strategy, backfill spec etc. If not, you're leaving some discretion to your contractor so pay attention to what they propose. Most GW will sub out the concrete work to specialists so they essentially dig the hole, get rid of the spoil and then backfill when the concrete is done. You'll probably use them to do any pre-site work (demolition, clearance etc) and put in the services for the new build (fouls, water, ducts for gas & telecom, power).

 

You should make it clear that you expect a turnkey package based on the SE spec and make sure that all elements have been included - contractors will look to cut costs by using spoil to backfill (vs stone) etc which will lead to more settlement after.

 

At this stage you will know what your basement will cost  

 

You may want to get the SE to spec traditional foundations to understand the incremental cost / m2. in our case we had the passive slab quote from MBC as a comparison.

 

A basement company may do all of the above for you but some costs may be opaque. Our friends in the river used Glatthar and were very pleased, however for our basement their waterproof system was overkill and was about 40% more expensive than the route we took. They also don't do any groundworks. 

 

Do not be bounced into using the system a contractor is used to  if it's not necessary for your site - i.e. an internal draining membrane if you're miles above groundwater (we had one who was very pushy on this)

 

Make sure the critical elements are warrantied with insurance backing as you will depend on this to get your overall house warranty (if you need one).

 

Good luck, it's a cost effective and amazingly useful space if you get it right.

 

 

[Pocster]

Agree with a lot of the above .

Though I do my own soul samples and sent them to a lab rather than a contractor - saved a few K just on that .

[Bitpipe]

3 minutes ago, pocster said:

Agree with a lot of the above .

Though I do my own soul samples and sent them to a lab rather than a contractor - saved a few K just on that .

 

I'd hate to think what they made of your soul

 

But yes, for the contamination that is a good option and one I should have used. Part of the GW quote will depend on the classification of the spoil as their landfill will need to know.

 

Our site was also within 100m of historical workings and was clay on gravel on chalk so risk of 'solution features' (fancy word for holes) so we needed two sets of GI as the first was 'inconclusive' basically the supervising engineer made a bit of a hash of it so I had to pay (a discounted rate) for a more comprehensive study. By the end of that the site was like a teabag but we had good data. All went smooth after that.

 

You do not want a surprise when you start digging.

[Pocster]

2 minutes ago, Bitpipe said:

 

I'd hate to think what they made of your soul

 

But yes, for the contamination that is a good option and one I should have used. Part of the GW quote will depend on the classification of the spoil as their landfill will need to know.

 

Our site was also within 100m of historical workings and was clay on gravel on chalk so risk of 'solution features' (fancy word for holes) so we needed two sets of GI as the first was 'inconclusive' basically the supervising engineer made a bit of a hash of it so I had to pay (a discounted rate) for a more comprehensive study. By the end of that the site was like a teabag but we had good data. All went smooth after that.

 

You do not want a surprise when you start digging.

Lol

Bloody spell check - really annoying me today . My soul is dammed but my soil samples were ok ?

Theres also a website ( name escapes me ) where you enter your postcode and it tells you the expected ground structure . Was spot on for us —solid virgin rock - hard as f

[Bitpipe]

On 21/02/2020 at 12:54, Thorfun said:

the company I spoke to at Build It Live use an internal membrane to manage water ingress just like you've described. I'm sure I could fit it and do it all but what about future saleability (not that we plan to move but you never know!) or getting a mortgage? wouldn't they need some form of sign-off that it's all been done properly and given a guarantee?

 

How can they recommend a system until they know the specific site conditions? This is the type of conversation that would ring alarm bells for me.

 

As @Carrerahill says, there are crews that do this work to high quality day in and out, usually contracting to big developers to do basement car parks, service rooms etc. Give them a proper set of drawings and they will price accordingly. Simpler to have a groundworker to contract the whole thing as a package as they will usually have a few crews that they work with to do the steel & concrete works - they usually stick to digging the holes, removing spoil, backfill and supply of the materials.

 

Like any contractor, always inspect previous clients and ask to see them on site currently. Scrutinise their quote and when they're on site, watch them like a hawk to make sure they follow it. The only downside with subterranean works is when it's finished, all the evidence is covered so hard to see if they've followed the spec.

 

Internally, the fit out is like any house, we tacked electrics onto the bare concrete and used dot & dab to fix the plasterboard. Laytex to floor to even out the bumps and then karndean on top as tile would have been colder underfoot.

[Thorfun]

wow! thanks so much @Bitpipe for taking the time to give such detailed information. there were a lot of questions there and I'll try and go through them and answer as well as I can. but I think what I can take from your post is that I'm not even close to being able to get a decent quote from someone for the basement. I think I need to be more patient, it's just I've got 8 weeks of waiting for planning to respond so thought I'd use it wisely to get rough quotes in so once permission is granted (ever the optimist) we can make a decision and crack on! so naive I know but you've got to let me live in ignorance for a while yet.

 

 

1 hour ago, Bitpipe said:

• What shape is it? Ours was a slightly off square box (11.5m x 10.5m) with an additional rectangle bit for the plant room. We used stud-work to create the internal rooms and a web of steel above to hold the suspended timber floor. Wet ground floor UFH is on spreader plates.

like yours it's just off square under the house (it's a part basement) at approx. 9m x 8.5m but it has a sunken external courtyard as an outside space, to let light in to the basement and also to act as a fire exit (which should answer some of the other questions you asked about light and BC fire exits). here's a basic plan.

 

1 hour ago, Bitpipe said:

• Will you have wet services in the basement (shower, toilet etc). Wet services will need a means to connect to the ground level fouls (pump). We decided not to but do have a plant room in there for the gas boiler, MVHR, UVC etc. Just have to get rid of boiler/mvhr condensate and we have a external drain for the UVC tundish where the external exit stairs are.
• Will it have a solid lid or be open and rely on a suspended floor (we did this). Will have implications on how you heat the GF if you're thinking of UFH.

 

no wet services as we didn't want to bother with that. any guests who need the bathroom can go up the stairs and there's a toilet up there.

I haven't decided on the roof of it yet. we do want UFH on the ground floor and also want good sound insulation as the music room will be used for my band to practise and record in so don't want the sound to travel upwards. so I'm leaning towards a solid floor even though this goes against the sustainability requirements I have. but a sustainable build is all about compromise and I will think long and hard about this aspect.

 

1 hour ago, Bitpipe said:

• How will it be insulated? We put ours on a layer of 300mm of EPS200 (the bearing strength, calculated by the SE) and applied 200mm EPS70 to the walls (with LE foam) which then met the insulation detail from MBC. This was not hard to do but that grade of insulation is not cheap.

 

again, not sure on this yet. but from initial reading, and I believe that the water table is very high where we live (our neighbours have a cellar/basement built recently-ish that they use for food storage so not really habitable and they said that the water table is above their basement (which is surprising as we're on top of a hill). anyway, when I get the geotechnical survey done that will tell us all) so I will probably have to go with Type C waterproofing with Type A tanking as a belt and braces option. so I would assume we would insulate the slab and also the internal walls in-front of the Type C waterproofing layer. but I have lots more reading to do on this subject.

1 hour ago, Bitpipe said:

5) you need a build strategy. How will your basement be built? Concrete poured in situ using movable formwork (most common method), ICF? Precast insulated panels? Bored holes filled with RC? Blockwork?

How big is your site, how much plant can you fit on it during the build? How close are you to neighbours? Does the party wall rule come into effect (the 3/6m separation bit)

 

NOTE - you may choose to build in ICF  but be aware that for a waterproofing strategy that depends on the walls being a barrier, ICF has the challenge of not being able to see the quality of the pour (marshmallow finish due to etc) as both sides covered with insulation. Does not mean it will be a problem, just means you won't know if there are any issues and will have to trust your contractor to get the mix perfect and vibrate well.

 

Will you need to de-water during construction - we had friends who live by a river and they were in this situation, they used an (expensive) system that pumps ground water out of the excavation continuously during the build.

again, I would probably defer to the 'experts' on this but I'm leaning towards poured in-situ using movable formwork. 

We have just under an acre on the site but the existing bungalow takes up a bit of that space but we do have plenty of space to put/fit plant on site during the build. The sunken courtyard is closest to the existing bungalow and is about 6m away which should be enough that sheet piling shouldn't be required to support the hole.

nearest neighbours are between 50 - 70m away I think so no concerns about party walls!

we will most likely need to de-water but I'm hoping that the planets will align and we'll break ground before the end of the summer and so the rain might have stopped by then.

 

thanks for all the other information in your post. it is noted and I will revisit it many times in my journey.

 

1 hour ago, Bitpipe said:

How can they recommend a system until they know the specific site conditions? This is the type of conversation that would ring alarm bells for me.

 

this is a very valid point! it did seem that they were pushing Type C waterproofing but I'm not so naive to be led down that path without further investigation and double checking everything first. I will request a quote from them anyway to get a comparison but as has been previously mentioned, personally recommendations count for a lot and so will bear that in mind when making a final decision.

 

1 hour ago, Bitpipe said:

As @Carrerahill says, there are crews that do this work to high quality day in and out, usually contracting to big developers to do basement car parks, service rooms etc. Give them a proper set of drawings and they will price accordingly. Simpler to have a groundworker to contract the whole thing as a package as they will usually have a few crews that they work with to do the steel & concrete works - they usually stick to digging the holes, removing spoil, backfill and supply of the materials.

 

I have a good friend who's father works as a QS for a big groundworks company and he has already said he'd get him to get me a quote. so this is definitely a route that I will be pursuing.

 

thanks again for all the excellent advice. I know I have just started on this journey and still have a long long way to go and I'm trying to fill the time waiting for planning approval in a constructive and educational way. ?

[Internet Know How]

On 22/02/2020 at 12:59, Bitpipe said:

I did a lot of research before building ours. I'm no expert but here is my experience.

 

Unless you're excavating under an existing building,  if you hire someone with 'Basement' in their company title then you're probably paying a premium for what is essentially a concrete box that any decent ground-worker firm can build - i.e. the type of firm that would build an underground carpark etc.

 

Here are the steps that I followed:

 

1) get our planning permission for the basement element of your build. In our case, we re-submitted the planning app once the above ground element was approved - our planners were mostly interested in the lightwells to the front of the house (which are only visible as grills in the front paving and not so much the emergency egress to the rear the footprint itself. We represented the basement as a big empty box with 'basement' on it - not not indicate any room division or use at that stage.

 

2) get your SE to commission a ground investigation spec that will give them the relevant data (soil bearing, composition, water levels etc) and will also satisfy any conditions imposed by your LA (we had archaeological and soil contamination conditions). You can shop the spec around yourself but don.t be tempted to come up with your own. Then choose your GI contractor and get them to do their stuff. In our case, they were probing and coring around the existing house (that we later demolished) but they got enough data points to satisfy the SE.

 

NOTE - steps 1 and 2 are committed costs, depending on what comes out of the report, you may decide that it's not cost feasible to build your basement, especially if it requires piling etc.

 

3) get your SE to design the basement, paying attention to supporting the loads imposed by the house above. In our case, we already had a MBC structural design with calcs so the SE was able to take these into consideration. BC will also need to be consulted to ensure that the basement meets regs regarding emergency egress in fire (either a separate exit to ground level or sprinkler system). You will also know your groundwater situation and this will inform your waterproofing strategy.

 

• What shape is it? Ours was a slightly off square box (11.5m x 10.5m) with an additional rectangle bit for the plant room. We used stud-work to create the internal rooms and a web of steel above to hold the suspended timber floor. Wet ground floor UFH is on spreader plates.
• Complicated shapes, corners etc will cost more to build as there is a lot of labour in the formwork and steel placement.
• How will light get into the basement? We sourced german GRP lightwells (MEA Bausysteme) and specc'd holes to fit. East then to get a window made (UpVC) to fit the hole. Cheaper than specifying made on site concrete lightwells. 
• Will you have wet services in the basement (shower, toilet etc). Wet services will need a means to connect to the ground level fouls (pump). We decided not to but do have a plant room in there for the gas boiler, MVHR, UVC etc. Just have to get rid of boiler/mvhr condensate and we have a external drain for the UVC tundish where the external exit stairs are.
• Will it have a solid lid or be open and rely on a suspended floor (we did this). Will have implications on how you heat the GF if you're thinking of UFH.
• How will it be insulated? We put ours on a layer of 300mm of EPS200 (the bearing strength, calculated by the SE) and applied 200mm EPS70 to the walls (with LE foam) which then met the insulation detail from MBC. This was not hard to do but that grade of insulation is not cheap.
• We did not heat our basement, it's always 20^oc year round due to being in the ground and having decent insulation. Also, there is lots of ambient heat released by the plant room and electrical appliances, people etc.

 

4) you will need a waterproofing strategy - really depends if water table is far below you or you're effectively building underwater. We had gw at 6m and as only digging 3.5m down type B alone was ok. 

• A is an external membrane - can be effective but only as good as the application - a problem will not be apparent until you have a leak
• B is waterproof concrete - uses an admix to the concrete,  water bar on vertical and horizontal pour joins and penetrations (like fouls) and special plugs to fill the formwork bracing holes.. We used a warrantied SIKA system that was inspected and signed off
• C is an internal membrane, sump & pump. In this situation you are expecting the walls to leak so collect water internally and have it run down to a precast sump which is continually emptied by a pump. If the pump fails, the basement floods so it's good practice to have two and some kind of warning system and plan for power failure.

If you're above ground water then you need a land-drain at the perimeter of your basement to a soak-away. Covering in shingle and using clean stone as backfill for the gap between excavation and wall will act as a giant french drain to deal with rain water.  

 

5) you need a build strategy. How will your basement be built? Concrete poured in situ using movable formwork (most common method), ICF? Precast insulated panels? Bored holes filled with RC? Blockwork?

How big is your site, how much plant can you fit on it during the build? How close are you to neighbours? Does the party wall rule come into effect (the 3/6m separation bit)

 

NOTE - you may choose to build in ICF  but be aware that for a waterproofing strategy that depends on the walls being a barrier, ICF has the challenge of not being able to see the quality of the pour (marshmallow finish due to etc) as both sides covered with insulation. Does not mean it will be a problem, just means you won't know if there are any issues and will have to trust your contractor to get the mix perfect and vibrate well.

 

Will you need to de-water during construction - we had friends who live by a river and they were in this situation, they used an (expensive) system that pumps ground water out of the excavation continuously during the build.

 

6) when you have all of this info (most from SE) you can get groundworking firms to tender. The SE quote should detail the concrete spec, bar schedule, waterproofing strategy, backfill spec etc. If not, you're leaving some discretion to your contractor so pay attention to what they propose. Most GW will sub out the concrete work to specialists so they essentially dig the hole, get rid of the spoil and then backfill when the concrete is done. You'll probably use them to do any pre-site work (demolition, clearance etc) and put in the services for the new build (fouls, water, ducts for gas & telecom, power).

 

You should make it clear that you expect a turnkey package based on the SE spec and make sure that all elements have been included - contractors will look to cut costs by using spoil to backfill (vs stone) etc which will lead to more settlement after.

 

At this stage you will know what your basement will cost  

 

You may want to get the SE to spec traditional foundations to understand the incremental cost / m2. in our case we had the passive slab quote from MBC as a comparison.

 

A basement company may do all of the above for you but some costs may be opaque. Our friends in the river used Glatthar and were very pleased, however for our basement their waterproof system was overkill and was about 40% more expensive than the route we took. They also don't do any groundworks. 

 

Do not be bounced into using the system a contractor is used to  if it's not necessary for your site - i.e. an internal draining membrane if you're miles above groundwater (we had one who was very pushy on this)

 

Make sure the critical elements are warrantied with insurance backing as you will depend on this to get your overall house warranty (if you need one).

 

Good luck, it's a cost effective and amazingly useful space if you get it right.

 

 

Great explanation, helped me get my head around my own basement plans. Appreciated!