Foundation + basement costs.

[puntloos]

Trying to make sense of a foundation quote I've received. These basement guys are effectively quoting me 40,000 for a 25m2 "box" (incl ceiling)

But my builder isn't willing to do foundation if someone else is doing basement. Probably fair enough to leave it to one team. 

The basement guys are quoting another 40,000 for effectively a 130sqm foundation

 

Does that sound fair? 40,000 for a barebones basement sounds reasonable - at least compared to the 80k quote I received for a 10sqm, but perhaps they're trying to squeeze me on the 40k cost they are asking for the foundation (which I'd normally argue has to be done anyway).

 

It does not include any groundworks, they are expecting a big hole in the ground I guess (ok, subbasement?)

 

 

 

Edited July 11, 2020 by puntloos

[dpmiller]

what are your ground conditions?

[Conor]

So, we got final tenders in last week. The winning tender has £39,500 for a 100m² basement. That includes:

 

200mm ground bearing slab and tanking (ground works done and compacted stones laid already)

All steel reinforcement

ICF walls up to ground level

Tanking for ICF

Perimeter drain, backfilling with clean stone

 

Fyi our ground works: site clearance, excavation, main drainage works, muckaway, under slab stoned with 150mm type 3, access road and working areas stoned, is coming in at about £14k. Still waiting for the final bill. That was basically a man with a digger @ £40/hr + materials.

 

 

What exactly is included in your 40k quote?

 

Edited July 11, 2020 by Conor

[Thorfun]

3 minutes ago, Conor said:

So, we got final tenders in last week. The winning tender has £39,500 for a 100m² basement. That includes:

 

200mm ground bearing slab and tanking (ground works done and compacted stones laid already)

All steel reinforcement

ICF walls up to ground level

Tanking for ICF

Perimeter drain, backfilling with clean stone

 

Fyi our ground works: site clearance, excavation, main drainage works, muckaway, foot printed stones with 150mm type 3, access road and working areas stones, is coming in at about £14k. Still waiting for the final bill.

 

Just make sure what is included in your quote, but sounds pretty good.

 

 

my quote for our foundations for the basement came in at £105k without a lid! £30k of that was just dealing with the digging of the hole, £30k for dealing with the muck away and the rest for an insulated slab. 3.1m deep, 130m2, clay on top of sandstone. seems expensive to me. 

 

the actual basement walls were reasonable though at about £30k, although no mention of waterproofing was made in the quote so I assume I'll be adding extra on for that.

 

will continue to shop around.

[Conor]

3 minutes ago, Thorfun said:

 

my quote for our foundations for the basement came in at £105k without a lid! £30k of that was just dealing with the digging of the hole, £30k for dealing with the muck away and the rest for an insulated slab. 3.1m deep, 130m2, clay on top of sandstone. seems expensive to me. 

 

the actual basement walls were reasonable though at about £30k, although no mention of waterproofing was made in the quote so I assume I'll be adding extra on for that.

 

will continue to shop around.

 

You could do a lot better with the ground works. Best looking for a local small firm or self employed digger driver. We got a place about 10miles away to take clay away at £150 per load. Think we ended up to 20 loads. We're on a sloping site with a fair amount of space... So we're able to store about half the excavated material on site. May end up having to get that carted off eventually!

 

Wee also a sloping site which makes things much easier.... I'd guess out £14k cost would double if it were a full basement.

 

Tanking is expensive.. makes up £15k of our £40k quote.

 

 

 

[puntloos]

2 hours ago, dpmiller said:

what are your ground conditions?

 

Haven't done soil analysis yet, but nearby boreholes list clay on chalk
0-40ft - Gravelly Soil/clay
40-200ft - Chalk
Water at 39ft (in winter) (from yet another different borehole, slightly further away)

 

More in general, the plot is quite flat, but there is a previous house on it. No idea if a removal job is hard.

[Tony C]

Hi, my groundwork is starting next week with 25m2 basement (2m depth) and additional 130m2 ground floor slab. I think the cost for the basement will be significantly different with the method and restriction, i.e. you have lots of space or you are working in confined area.

The quotation from the groundwork contractor was certainly varied greatly. Its a lot of unknowns when you are digging basement I guess.

 

In my case

-  Building in London, backland site (more £ on muck away cost!)

-  Basement is on the boundary. The groundwork contractor will dig basement partially, then insert metal sheeting support, dig a bit more, lower the metal sheeting.

This is slower process compare to the case you have lots of land around it.

- We opted for a deep drainage system with natural fall without a sump pump, lucky we are on the sloping site. This is adding more cost compare to the sump pump, but there would be no maintenance cost every year.

 

 

Edited July 11, 2020 by Tony C

[puntloos]

1 hour ago, Tony C said:

Hi, my groundwork is starting next week with 25m2 basement (2m depth) and additional 130m2 ground floor slab. I think the cost for the basement will be significantly different with the method and restriction, i.e. you have lots of space or you are working in confined area.

The quotation from the groundwork contractor was certainly varied greatly. Its a lot of unknowns when you are digging basement I guess.

 

In my case

-  Building in London, backland site (more £ on muck away cost!)

-  Basement is on the boundary. The groundwork contractor will dig basement partially, then insert metal sheeting support, dig a bit more, lower the metal sheeting.

This is slower process compare to the case you have lots of land around it.

- We opted for a deep drainage system with natural fall without a sump pump, lucky we are on the sloping site. This is adding more cost compare to the sump pump, but there would be no maintenance cost every year.

 

 

You left out one key point

 

How much are you paying for this ?

[Thorfun]

6 hours ago, Conor said:

 

You could do a lot better with the ground works. Best looking for a local small firm or self employed digger driver. We got a place about 10miles away to take clay away at £150 per load. Think we ended up to 20 loads. We're on a sloping site with a fair amount of space... So we're able to store about half the excavated material on site. May end up having to get that carted off eventually!

 

Wee also a sloping site which makes things much easier.... I'd guess out £14k cost would double if it were a full basement.

 

Tanking is expensive.. makes up £15k of our £40k quote.

 

 

yeah, it's the first quote I've had back so there's plenty of shopping around to do. also I reckon we can store most of the spoil on our site and then when the existing building is demolished us it to fill and landscape so I can definitely get the costs down.

 

thanks for the costing of the tanking. if this guy hasn't added tanking and that's going to add a further £15k then it's getting to a level where we might have to ditch it. but I'd rather do a lot of the work myself and it just take longer than ditch the basement as it's not really something I can do later!

[Tony C]

3 hours ago, puntloos said:

You left out one key point

 

How much are you paying for this ?

PM me, I can share the quotation and spec.

[puntloos]

14 minutes ago, Tony C said:

PM me, I can share the quotation and spec.

Thanks, will do!

 

But.. as long as it's anonymous (in public) forum rules allow us to share things publically I think? Of course builders might not be big fans to know how cheap is possible (perhaps with shoddy work!)

 

Here's my quote - anonymized: https://docs.google.com/spreadsheets/d/1YXmQvNAT9tiMNJIrj_Ka0g-WVb1Hb0_eoEuw8_VqkPA/edit?usp=sharing

 

In short: about 100K for 25m2 insulated  basement + 130m2 foundation, but no ground works. (they expect a big hole)

Edited July 11, 2020 by puntloos

[Bitpipe]

Do you have a detailed SE design, based on ground investigation, to base these quotes on? I

 

I.e. do you know the bearing strength of the ground exactly where your basement will be, ground water levels, any potential soil contamination that would complicate muck away?

 

If not then you're getting 'best scenario' quotes but reality could be quite different - useful for budgetary considerations but risky to assume that they are not subject to change.

 

Both the basement design (amount and type of steel & concrete plus waterproofing strategy) and groundworks will be highly dependent on ground conditions. Different soil types bulk up by different ratios and this drives the muck away cost (as it's per 20t truck but also by volume).

 

Tricky access can also drive up costs as it just slows everything down. Some soil conditions can necessitate sheet piling instead of just battering back the ground and poor ground conditions can necessitate piling to support the slab.

 

Bottom line is you won't know what you're dealing with and what the cost is (and whether you can afford it) until you have that ground report and it's always best to get one specced by the SE so you get the data they need. You then shop around to get the best quote for that work and when you have the report the SE can do your design - you'll know quickly if you need under piling or sheet piling to support the ground.

 

When you have a full design, you can shop that around to get a comprehensive quote and can see if splitting the GW and build is cheaper.

 

So, sadly you need to commit some ££ before you know for certain you can afford it. You can take a chance but if you hit issues, you'll have one very expensive hole in the ground.

 

We went through all of this and had the pain of the GI company screwing up and having them to come back (at our cost) to get the necessary data, but we got through it ok in the end.

 

Friends nearby discovered before the dig they had a stream 3m below their plot and had to pay for expensive dewatering during the build, however would have been a disaster to find that out mid dig.

 

[puntloos]

4 minutes ago, Bitpipe said:

Do you have a detailed SE design, based on ground investigation, to base these quotes on? I

I.e. do you know the bearing strength of the ground exactly where your basement will be, ground water levels, any potential soil contamination that would complicate muck away?

 

Sadly (?) there is a house on the plot, so I think we can 'only do so much'? We can't drill (straight down) into the ground where the basement would be, at least not the entire area. Or would a SE not need to drill straight down and hiring a SE "now" could be a good idea (slightly pre planning permission)? 

 

My current designs are built on data we have from 2 nearbyish (0.6m away) boreholes.

 

 

 

 

[Conor]

1 hour ago, puntloos said:

 

Sadly (?) there is a house on the plot, so I think we can 'only do so much'? We can't drill (straight down) into the ground where the basement would be, at least not the entire area. Or would a SE not need to drill straight down and hiring a SE "now" could be a good idea (slightly pre planning permission)? 

 

My current designs are built on data we have from 2 nearbyish (0.6m away) boreholes.

 

 

 

 

 

Get an SE straight away - you'll need all your beams, slabs etc calculated anyway for building control drawings. I'm a designer, and it's never too soon to get data and surveys, and you can never have too much info. 

 

Get them on a fixed price basis. For ground investigation, you'll get a mini digger in and dig down a couple metres somewhere close to where you are building. The SE will climb down, grab a handful of muck and go "hmmmm" and take it from there. They'll then say if you need further investigations, boreholes etc. For basement design, they'll always assume worst case scenario, i.e saturated, low modulus soil and high hydrostatic pressure. I'm pretty sure our walls could be 50% thinner and have half the amount of steel... But I'm an environmental engineer, not a structural!

[puntloos]

35 minutes ago, Conor said:

 

Get an SE straight away - you'll need all your beams, slabs etc calculated anyway for building control drawings. I'm a designer, and it's never too soon to get data and surveys, and you can never have too much info. 

 

Get them on a fixed price basis. For ground investigation, you'll get a mini digger in and dig down a couple metres somewhere close to where you are building. The SE will climb down, grab a handful of muck and go "hmmmm" and take it from there. They'll then say if you need further investigations, boreholes etc. For basement design, they'll always assume worst case scenario, i.e saturated, low modulus soil and high hydrostatic pressure. I'm pretty sure our walls could be 50% thinner and have half the amount of steel... But I'm an environmental engineer, not a structural!

 

Just a quick detail but would the house design fundamentally (ha) impact the work the SE does? Meaning if we decide to not build our garden room, would the SE outcome change?

[Conor]

1 hour ago, puntloos said:

 

Just a quick detail but would the house design fundamentally (ha) impact the work the SE does? Meaning if we decide to not build our garden room, would the SE outcome change?

 

Not really. What an SE does is calculate loadings and forces on various parts of the building. They'll take the architect's drawings and, for example, calculate the loadings on the foundation and determine the slab and trench requirements. For a basement, the spec of the retaining walls are particularly critical.

 

Re payment, you normally pay based on the number the number of calculations and drawings required. Site visits not normally included and you'd pay extra for that along with SI works. If you have a garden room in your design, you'd pay for the calcs for that design. If you have later additions or changes, you'll have to get these done as well. It's not big money in the grand scheme of things..I think our total SE fees are £800, and that included a redesign for when we found a well right under the proposed slab.

 

 

[Thorfun]

27 minutes ago, Conor said:

 

It's not big money in the grand scheme of things..I think our total SE fees are £800, and that included a redesign for when we found a well right under the proposed slab.

 

 

damn! I only wish for such low SE fees. the quotes we've had range from £3.5k - £6k! 

[Bitpipe]

12 hours ago, Conor said:

For ground investigation, you'll get a mini digger in and dig down a couple metres somewhere close to where you are building. The SE will climb down, grab a handful of muck and go "hmmmm" and take it from there. They'll then say if you need further investigations, boreholes etc. For basement design, they'll always assume worst case scenario, i.e saturated, low modulus soil and high hydrostatic pressure. I'm pretty sure our walls could be 50% thinner and have half the amount of steel... But I'm an environmental engineer, not a structural!

 

We went straight to the SE specified comprehensive GI survey, boreholes & dynamic probes. Historical maps showed that we were on the edge of an old landfill site so potentially made ground (which would have meant piling).  Old house was also in situ so we needed to pick areas right next to it that were representative of the basement footprint.  Was not cheap and we could maybe have done better with another contractor but we got there in the end.

 

We were also clay over gravel over chalk the latter can be like Swiss cheese (as its soluble) and it bulks up crazy when excavated. 

 

When you understand your ground water conditions, you can decide on the waterproofing strategy.

 

3 options really,

1) External membrane - works but needs to be carefully applied and protected during backfill. If you get a leak then will be hard to find out where it is.

2) Waterproofed concrete, water bar on joints etc (we used this system) - reliable but should come with an insurance backed warranty, ours was Sika and their rep was onsite regularly checking quality of work. Works well with ICF but you can't see the quality of the pour due to the insulation.

3) Internal membrane with sump and pump - needs a good backup strategy if pump fails (sometimes dual pumps are used), power cuts etc. 

 

Best practice is to have two of the three but as our water was low (6m) we just went with option 2 and have had no issues.

 

Backfill is also key - we used clean stone (fist sized) with a land drain to soakaway at the bottom - means any rain or surface water won't hang around. 

[Bitpipe]

10 hours ago, Thorfun said:

 

damn! I only wish for such low SE fees. the quotes we've had range from £3.5k - £6k! 

 

Do keep in mind that the SE need never come to site and can be based anywhere.

 

They just need the necessary data - detailed site survey, ground conditions report (best to ask them in advance what they need to see in it) and the detailed design of the above ground structure, point loading etc. and the below ground layout from your architect.

 

You can use one SE do do the whole house design or use one for above and one for below ground. If you're doing a timber frame package, they will provide their own SE for that bit - in the MBC case, they also use the same one for their slab design.

 

The challenge with basements (and groundworks in general to be fair) is that you don't know the actual cost and whether you can afford it until you know what's under your feet and that will involve spending money, some of which may be a bit speculative if the final numbers don't work in your favour.

 

However that is much cheaper than the alternative of committing to the project and having an unpleasant surprise mid construction.

[Bitpipe]

14 hours ago, puntloos said:

 

Sadly (?) there is a house on the plot, so I think we can 'only do so much'? We can't drill (straight down) into the ground where the basement would be, at least not the entire area. Or would a SE not need to drill straight down and hiring a SE "now" could be a good idea (slightly pre planning permission)? 

 

My current designs are built on data we have from 2 nearbyish (0.6m away) boreholes.

 

That may be good enough for the SE providing they are deep enough and don't show wild variation.

 

Boreholes don't usually measure the load bearing strength of the ground though, we had lots of dynamic probing - basically a small rig progressively whacks a metal rod into the ground up to 10 m deep and measures the resistance for each whack. Some of these are then bored to about 4m to get the soil analysis.

 

You need both sets of data to really know what you have going on, plus a desk survey to understand what's been there before.

 

We waited until after planning was achieved before commissioning SEs etc and we got quite a few related conditions that were addressed in the subsequent GI report (mostly relating to soil contamination reporting etc.).

[puntloos]

Wow @Bitpipe really sharing the knowledge.

 

I've been speaking to a prefab basement team where .. well.. I should clarify this with them but I think basically prefab basements that are waterproof by design but I think they still have to do additional work, a membrane etc.

 

SE and boreholes etc, so would you wait until after planning, or get going with this as soon as possible once the basic house design is done? As @Conor noted the probing done shouldn't materially change with the design so it sounds like I can do this sooner rather than later? The main difference between intention and reality that I am foreseeing is we might not be able to afford the basement. 

[Thorfun]

2 hours ago, Bitpipe said:

 

Do keep in mind that the SE need never come to site and can be based anywhere.

 

They just need the necessary data - detailed site survey, ground conditions report (best to ask them in advance what they need to see in it) and the detailed design of the above ground structure, point loading etc. and the below ground layout from your architect.

 

You can use one SE do do the whole house design or use one for above and one for below ground. If you're doing a timber frame package, they will provide their own SE for that bit - in the MBC case, they also use the same one for their slab design.

 

The challenge with basements (and groundworks in general to be fair) is that you don't know the actual cost and whether you can afford it until you know what's under your feet and that will involve spending money, some of which may be a bit speculative if the final numbers don't work in your favour.

 

However that is much cheaper than the alternative of committing to the project and having an unpleasant surprise mid construction.

 

good points. I might contact the oft spoken about SE on this site. my GI report should be with me next week so I'll have the exact details of what's underground then.

[Bitpipe]

On 12/07/2020 at 11:43, puntloos said:

Wow @Bitpipe really sharing the knowledge.

 

I've been speaking to a prefab basement team where .. well.. I should clarify this with them but I think basically prefab basements that are waterproof by design but I think they still have to do additional work, a membrane etc.

 

SE and boreholes etc, so would you wait until after planning, or get going with this as soon as possible once the basic house design is done? As @Conor noted the probing done shouldn't materially change with the design so it sounds like I can do this sooner rather than later? The main difference between intention and reality that I am foreseeing is we might not be able to afford the basement. 

 

Is it the one with the German sounding name? That's the firm our underwater friends used and they were very high quality but not cheap and they don't do groundworks. They do use a precast waterproof panel, seal the joints and then apply a very robust membrane. Our friends have had no issues.

 

Their economics did not work for us and for the price of the basement only they quoted us we got the whole job done inc. demolition, excavation and re-instatement.

[Bitpipe]

On 12/07/2020 at 12:10, Thorfun said:

 

good points. I might contact the oft spoken about SE on this site. my GI report should be with me next week so I'll have the exact details of what's underground then.

 

The SE who did my basement now works here https://www.buildcollective.co.uk looks like they cover a range of services so maybe worth a call also.

 

[puntloos]

Quick update, I received a ballpark groundworks quote for 14,000 to effectively prep a 130sqm site and dig the hole of the basement. Demo of the previous house not included since they need to see the site. Seems fairly reasonable.

 

So.. that gives me "100,000" for effectively everything that's underground. 

 

When it comes to the famous 'rule of thumb' - 1500-3000 per sqm, let's say 2250 for now - how much does that drop if the entire foundation can be assumed to be there? Putting it differently- how much would 'just groundworks+foundation' normally cost for 130sqm?

 

(very much in general, yes I'm aware this can all change based on what SE finds etc..)

Edited July 15, 2020 by puntloos