Advice on Foundations/Retaining Wall/Semi-basement

[jimmyharris80]

Hi all,

 

Recent joiner here looking for some advice on our foundations/retaining wall/semi-basement on our sloping site.  Here’s a pic of our site, I’ve marked on it approx where the new dwelling will be sited:

 

 

 

We have PP for a three story dwelling, the ground floor is less than half the area of the first floor, you can see a few design 3D’s on my intro post here of how the dwelling is situated on site and what we are proposing, here is a sectional diagram showing the split level:

 

 

The architect has added notional B&B flooring on both floors for BRs, I think we need to look at slab for ground floor as we will have UFH throughout (we're off grid so ASHP in the plans), first floor could be a little more complex as approx. half of it is over ground that slopes away.  I’m struggling to see how this will all hang together and want to be clear on possible options we have and requirements when I instruct our SE.

We only envisaged excavating for the ground floor, building up retaining walls and ground floor in blockwork with timber frame from 1st floor up.  If we excavated the whole house footprint there would be considerable additional muckaway, retaining walls, etc for what would effectively be a ground floor void with no light and reduced head height.

Any input you guys have would be hugely appreciated, if there's any other info, floorplans, etc that would help please shout and I can add them up.

Thanks
 

[jimmyharris80]

Here are the details of our soil tests:

 

Site plan:

 

Ground make-up:

 

Test result from TH2:

[Bitpipe]

Did you get an actual topological survey of the site that can be imported into a cad tool? Our architect insisted on one before starting any work and it was a useful tool thereafter.

 

Anyway, your GF looks over complicated. Why not push the retaining wall to the back of the build and have a contiguous slab and steal some extra space while you're at it? Could be a useful location for a TV/cinema room / plant / gym etc. The step down into dining room looks awkward and will restrict your usage if/when mobility becomes an issue. Easy enough to put ground level light wells back there too

 

We built a passive TF on-top of a basement and it worked well, we just mimicked the passive slab design with SE specced EPS under the slab and on the outside of the concrete walls. Essentially you'd be doing a 3 sided version of this in block, ICF or cast concrete then have TF to the front and for the first floor upwards.

 

Alternatively you could have the retaining wall discrete from the house structure and have a simple slab and TF construction from slab upwards.

[Conor]

We have a sloping site. Looked at all options, no basement, patrial basement, split level. Went for full basement with slab raft foundation. Worked out more expensive but we got much more floor area and a simpler build. And extra £19k for an additional 50m². Either do the same or at least get rid of the roasted bit adjacent to the dining. And speak to your SE sooner rather than later about foundation and retaining wall options. They will be able to advise on the best methods.

Edited March 22, 2022 by Conor

[jimmyharris80]

 

Hi Bitpipe,

 

54 minutes ago, Bitpipe said:

Did you get an actual topological survey of the site that can be imported into a cad tool? Our architect insisted on one before starting any work and it was a useful tool thereafter.

 

 

Yes, we did get a topo survey done and this was passed to our architect.

 

54 minutes ago, Bitpipe said:

Anyway, your GF looks over complicated. Why not push the retaining wall to the back of the build and have a contiguous slab and steal some extra space while you're at it? Could be a useful location for a TV/cinema room / plant / gym etc. The step down into dining room looks awkward and will restrict your usage if/when mobility becomes an issue. Easy enough to put ground level light wells back there too

 

This is what I was afraid of, even to my untrained eye it looks complicated.  We have discussed extending the basement as an option, just need to consider additional costs of muckaway, retaining walls and slab as we're doubling the size of it if we go down this route.

 

The step down was to increase head height in the kitchen/diner, I don't feel foregoing this would be a huge issue, the 1st floor would all then be on the same level with a ceiling height throughout of 2575mm which I think would be workable.

 

54 minutes ago, Bitpipe said:

Alternatively you could have the retaining wall discrete from the house structure and have a simple slab and TF construction from slab upwards.

 

This is what MBC suggested as the way to go when I spoke with them, it would mean a passageway around the basement level and bridges to access the garden from the 1st floor, we're getting into fairly significant redesign territory here I fear!

Edited March 22, 2022 by jimmyharris80

[Bitpipe]

32 minutes ago, jimmyharris80 said:

 

Hi Bitpipe,

 

 

Yes, we did get a topo survey done and this was passed to our architect.

 

Great, that will be useful for your SE also.

 

32 minutes ago, jimmyharris80 said:

 

This is what I was afraid of, even to my untrained eye it looks complicated.  We have discussed extending the basement as an option, just need to consider additional costs of muckaway, retaining walls and slab as we're doubling the size of it if we go down this route.

 

Get a few quotes, you may be surprised. There are often economies of scale in groundworks. You may also save by simplifying other aspects of the design.

 

Also, if you create extra space, you don't need to fit it out initially.

 

32 minutes ago, jimmyharris80 said:

The step down was to increase head height in the kitchen/diner, I don't feel foregoing this would be a huge issue, the 1st floor would all then be on the same level with a ceiling height throughout of 2575mm which I think would be workable.

 

Standard ceiling height is 2400mm (conveniently the size of a plasterboard sheet) our ground floor is just a smidge under this at 2350mm and it feels fine. First floor is 2300, also feels fine so 2575mm should feel spacious.

 

32 minutes ago, jimmyharris80 said:

 

This is what MBC suggested as the way to go when I spoke with them, it would mean a passageway around the basement level and bridges to access the garden from the 1st floor, we're getting into complete redesign territory here I fear!

 

Yes, works well for them as they don't do any concrete work other than the slab.

 

However you can should get a quote for your own passive style slab & 3/4 basement wall from a groundworker - it's really just a concrete wall/slab with appropriate EPS underneath and to the sides. We did our basement like this and it was straightforward for MBC to drop their frame on the top. They waited until it was in place and came to do final measurements before putting frame into production so no surprises.

 

You could have suspended timber floor for the dining / kitchen level also (would need some steel to support) and if you go for a low energy design then the UFH only needs to be in the ground slab - also that heating is in the slab itself, not on top, so you save on screed and on slab insulation.

 

Edited March 22, 2022 by Bitpipe

[Bitpipe]

33 minutes ago, jimmyharris80 said:

 

This is what MBC suggested as the way to go when I spoke with them, it would mean a passageway around the basement level and bridges to access the garden from the 1st floor, we're getting into complete redesign territory here I fear!

 

Not sure what value a passageway would add, you're effectively doubling up on the wall requirement (ok, in this scenario the rear wall is timber and the retaining wall more substantial).

[Thorfun]

I agree with @Bitpipe and @Conor here and you should think about pushing the retaining wall to the back. Makes it a lot simpler and the SE design for the load from the house to the existing planned basement retaining wall will be complicated and expensive and probably won’t be stupidly more expensive to just do a full dig out. 
 

that was the advice I was given by friends in the trade and others on this site and we don’t regret it. And like it was said above you don’t have to fit it out to start with but the space will be there when you have funds to do so. Exactly what we’re planning. 
 

(I go through a lot of this in our blog on here)

[jimmyharris80]

Thanks for the responses guys, pushing the retaining wall back does make sense, additional costs, yes, but these would be balanced by the simplification it brings and the increase in square m on the ground floor, cinema room, gym, plant room are options here (budget won't stretch to the former two, but can leave unfinished with that as an option in the future).  Plant room in here also frees up space in the utility which was doubling as our plant room.

 

We do have a large garden where we could lose some of the soil by landscaping and we will need to backfill the SE side elevation so we can reduce muckaway cost here.

 

A level change in the kitchen/diner would remove the need for steps/handrails and will reduce the height of the bifolds and windows, both will bring costs savings (not huge, but savings nonetheless!).  First floor can then have all timber joists throughout instead of mixed timber/concrete, whether this saves cost I'm not sure but certainly looks simpler from a design/build perspective.

 

The next logical question here is the impact on our planning permission, I would hope the change in window/bi-fold sizes would be classed a non-material amendment, but building out the basement and increasing the sq m on the ground floor and adding access to the basement rooms from the hall/bedroom 1, would that require a new planning app?  Fundamentally the super-structure remains as is in size and form so I would hope the planners would be sympathetic to that, but who knows with the planners, all a bit of a lottery.  Here's what we have approved at present with my amendment overlays in yellow as I see them:

 

 

Any views on planning implications?  I can stick a post on the planning forum to garner opinion before I have an initial chat with architect, just conscious of starting too many concurrent threads!

 

Edited March 23, 2022 by jimmyharris80

[Conor]

You'll need to go back to planning as you're increasing the habitable area. Same for building control.

 

I'd suggest you start looking at muck away options. You can save thousands of you can find a local farmer etc that can take the earth (assuming only inert soil and stone) and a local man with digger and dumper lorry. You do not want to be paying for landfill. We were looking at in initial £250 per load to landfill but found another guy with inert waste licence that took it for £130 per load. Saved us nearly £3k.

 

For perspective, our 115m² partial basement excavation, drainage, foundation hardcore and site stoning cost just under £15k

 

 

Edited March 23, 2022 by Conor

[Thorfun]

I know it's a kick in the teeth to have to go back to planning but, personally, I think it'll be worth it. but ultimately it is governed by your timelines and desires!

 

reducing windows/bifolds from 2.6m to 2.1m will have a bigger financial impact that you think I believe.

 

it's a tough choice and I don't envy you it as there are obviously costs and time delays involved in going back to planning when I can guarantee you're chomping at the bit to just get going! 😉 

[jimmyharris80]

2 minutes ago, Conor said:

You'll need to go back to planning as you're increasing the habitable area. Same for building control.

 

Thanks Conor. A whole new app or an amendment to existing, or is it only the planners who can advise on this really?

 

3 minutes ago, Conor said:

I'd suggest you start looking at much away options. You can save thousands of you can find a local farmer etc that can take the earth (assuming only inert soil and stone) and a local man with digger and dumper lorry. You do not want to be paying for landfill. We were looking at in initial £250 per load to landfill but found another guy with inert waste licence that took it for £130 per load. Saved us nearly £3k.

 

Will do, as we have a slopping site I think we could lose a fair bit here by levelling that out but it could need some retaining walls (sleepers maybe) to help with this, I'll investigate if there are any other local options.  The top soil we'll certainly use it's whether we can find a home for the silty clay underneath.

[Bitpipe]

9 minutes ago, Conor said:

You'll need to go back to planning as you're increasing the habitable area. Same for building control.

 

 

Correct, however given it won't change the elevations or massing I doubt you'll have much issue.

 

We got planning on the second go with a 1/2 basement  - original issues were ridge height and some other aesthetic details that we simplified. Once secured, we went back to planning with a full footprint basement, ground level light wells etc. Zero comments and a quick approval.

 

Having a dedicated plant room makes life easier also - frees up useful living space and makes your first fix easier. 

 

Also building into a slope will be less complex than digging a big hole like my & @Thorfun's basements and BC won't be as concerned about means of fire escape (we needed to choose between sprinkler or independent exit, going for the latter).

 

Also water management is easier as a good land drain to the rear plus either waterproof concrete or external tanking (or both) should suffice.

Edited March 23, 2022 by Bitpipe

[Bitpipe]

1 minute ago, jimmyharris80 said:

 

Thanks Conor. A whole new app or an amendment to existing, or is it only the planners who can advise on this really?

 

Will be a whole new app but should be straightforward.

[Thorfun]

1 minute ago, Bitpipe said:

Also building into a slope will be less complex than digging a big hole like my & @Thorfun's basements and BC won't be as concerned about means of fire escape (we needed to choose between sprinkler or independent exit, going for the latter).

I wasn't given the choice! the architects said BC wanted an independent exit so that's what we built, although we made it a bit bigger and have created a sunken courtyard as an outside space for the basement.

[jimmyharris80]

1 minute ago, Thorfun said:

I know it's a kick in the teeth to have to go back to planning but, personally, I think it'll be worth it. but ultimately it is governed by your timelines and desires!

 

it's a tough choice and I don't envy you it as there are obviously costs and time delays involved in going back to planning when I can guarantee you're chomping at the bit to just get going! 😉 

 

Yes, you're right we are keen to get going, but only when we are comfortable with the approach and the ultimate outcome.  I can put up with a few months of delay but psychologically we really want to make a start this year with demo and groundworks.

 

We also have the small matter of a bat survey that will need to be 'refreshed' if we go through another bat season, that survey was complete in last years season and found we had a single bat roosting in the original bungalow (don't get me started on that!), this doesn't impact our ability to demolish the garage to make way for the new build, but a considerable delay could potentially mean more expensive surveys.

15 minutes ago, Bitpipe said:

 

Will be a whole new app but should be straightforward.

 

Ok, thanks, will pick up with the architect then and establish what required from them from a time and cost perspective.

 

5 minutes ago, Thorfun said:

reducing windows/bifolds from 2.6m to 2.1m will have a bigger financial impact that you think I believe.

 

That's one positive!

 

19 minutes ago, Bitpipe said:

We got planning on the second go with a 1/2 basement  - original issues were ridge height and some other aesthetic details that we simplified. Once secured, we went back to planning with a full footprint basement, ground level light wells etc. Zero comments and a quick approval.

 

Fingers crossed we have a similar experience.

 

Now my head is whirring as to whether there are any other simplifications/amendments we should take a look at in the overall design 😣

[Thorfun]

3 minutes ago, jimmyharris80 said:

We also have the small matter of a bat survey that will need to be 'refreshed' if we go through another bat season, that survey was complete in last years season and found we had a single bat roosting in the original bungalow (don't get me started on that!), this doesn't impact our ability to demolish the garage to make way for the new build, but a considerable delay could potentially mean more expensive surveys.

we bought our bungalow with planning and they'd already done a bat survey. when it came to us putting in our planning application the planning department wanted a refreshed bat survey. we managed to persuade the ecologists to simply reissue the previous surveys for a nominal fee as nothing had really changed. if planning request new surveys it might be worth a chat with the ecologists? 

 

5 minutes ago, jimmyharris80 said:

I can put up with a few months of delay but psychologically we really want to make a start this year with demo and groundworks.

completely understand this as you don't want to be digging foundations or building in the winter! we were ready to start last Jan/Feb but the groundworker said 'no' and he wanted to wait for spring. I'm glad he did tbh as the clay got really soggy and wouldn't have fancied them digging the basement in the winter months. as it worked out we had really good weather and very few rain delays for our basement dig and build. if you can plan it to be watertight before winter then you're on to a winner in my opinion!

[Bitpipe]

13 minutes ago, jimmyharris80 said:

 

Yes, you're right we are keen to get going, but only when we are comfortable with the approach and the ultimate outcome.  I can put up with a few months of delay but psychologically we really want to make a start this year with demo and groundworks.

 

We also have the small matter of a bat survey that will need to be 'refreshed' if we go through another bat season, that survey was complete in last years season and found we had a single bat roosting in the original bungalow (don't get me started on that!), this doesn't impact our ability to demolish the garage to make way for the new build, but a considerable delay could potentially mean more expensive surveys.

 

Look, you have PP which you can start to move on - go ahead with some ground works i.e.  limited demo works, site clearance and (drainage etc) and get your BC (would always recommend private vs LA) to acknowledge this as a formal start.

 

We did similar bits and bobs - demo of garage, some extension of fouls to connect caravan (but they were part of final services plan) to formally commence some 8 months ahead of the proper works themselves.

 

[Thorfun]

24 minutes ago, Bitpipe said:

 

Look, you have PP which you can start to move on - go ahead with some ground works i.e.  limited demo works, site clearance and (drainage etc) and get your BC (would always recommend private vs LA) to acknowledge this as a formal start.

 

We did similar bits and bobs - demo of garage, some extension of fouls to connect caravan (but they were part of final services plan) to formally commence some 8 months ahead of the proper works themselves.

 

great idea. but obviously after ensuring that the CIL exemption paperwork has been received! 😉 

[jimmyharris80]

30 minutes ago, Bitpipe said:

 

Look, you have PP which you can start to move on - go ahead with some ground works i.e.  limited demo works, site clearance and (drainage etc) and get your BC (would always recommend private vs LA) to acknowledge this as a formal start.

 

We did similar bits and bobs - demo of garage, some extension of fouls to connect caravan (but they were part of final services plan) to formally commence some 8 months ahead of the proper works themselves.

 

 

Yes, good shout, thanks.

 

4 minutes ago, Thorfun said:

great idea. but obviously after ensuring that the CIL exemption paperwork has been received! 😉 

 

East Herts doesn't have CIL in place from the research that I've done, I will categorically confirm this before we start as that would be one expensive mistake!

Edited March 23, 2022 by jimmyharris80