Could I get usable space out of my cellar?

[renovbod]

Hi I have an 1875 victorian mid-terrace in Surrey which has a 3 room cellar. I'm looking for any advice or knowledge as I have had very contradictory advice from people so far.

The house is 6 rooms over 2 floors so this would potentially add 50% again to the space. two of the rooms are aprox 4m by 4m, the other (the back room) is about 3m by 2m. However the celler is only about 5"8' and in the front room is lower due to an extra layer of concrete on the floor (I don't know why that is there but I guess it's a bad sign?). Although it's very dry so I'm hoping that is in my favour!

What I would like would be to get headroom so that, for instance, the back room can be used as a second toilet-come-laundry room. the middle room a sitting/cinema room and the front room a gym/wine cellar/spare room whatever.

I don't want to take stupid risks... anything like building or planning regs that are required I want. I know the builder who helped with the rest of the house renovations might do it but I'm not sure if that is a risk or good way to get it done. He mentioned that there might be an option to put in a concrete collar around the footings and just have a ledge at knee height aroudnthe rooms, which would be fine (my wording not his so might not be technically accurate).

People say it's expensive but it's all relative - if it was £10k-£15k then it would be a no-brainer... if it's £100k then I need to think about how I'm doing to make good what is there! and forget the idea of extra headroom!

Any knowlege would be useful! Thnaks.

 

[JohnD]

it would be useful to dig a small hole against a wall to see how deep the footings go. There is a chance you can dig out enough to increase the headroom and give an insulated, damp-proof floor.

It is still possible to do it if the footings are shallow, but you have to dig out and lay concrete in small bays against the wall to avoid undermining or weakening the foundations.

It an old house the walls are probably laid widening by one or two bricks with each course to form footings, there might or might not be concrete under the bottom course. Concrete and mortar may be very weak and eroded by water or washed away and filled with mud. this can he cleaned out and repointed in small sections but is very laborious.

You will have to involve Building Control who will want to see how deep the footings go and how you propose to deal with them.

BTW do not be tempted to dig a hole and put a sump pump in it, this will suck away the soil as mud and grit in the water, washing away the ground under your house and forming a cavity that will grow until the house falls into it It is possible to dig a pit lined with porous concrete though, and pump clean water out of that as it filter back the soil particles.

 

[Shytalkz]

It is still possible to do it if the footings are shallow, but you have to dig out and lay concrete in small bays against the wall to avoid undermining or weakening the foundations.

Crazy method imo.

How will that work, then? How thick is this concrete? What's to stop the top moving inwards under lateral pressure from the foundation and soil loads? How far down is this dug?

Why not just underpin the foundations down below the level of the floor that you want, which puts building load straight on the top of the underpin and gives loads of friction at the bottom to resist sliding from soil pressure, until you cast the floor slab.

 

[JohnD]

if you tell me it's wrong I'll believe you

but I have seen it done in the past where a new concrete floor was to be laid. the idea was that you did not dig out the whole floor in one go, but dug out and cast the bays beside the wall first. by laying in small alternating bays they avoided exposing a large area of unrestrained footings at one time. by subsequently excavating the rest of the floor and casting the remaining slab they restored the sideways restraint across the whole floor.

 

[Shytalkz]

I'm not saying it's wrong, it is but one method after all; it's a variation on sheet piling.

I understand the short lengths, as it would be done in the same way as underpinning.

But, to restrain the top of it and thus stop it trying to act like a vertical cantilever, it would be necessary to come up with some arrangement for propping before digging down; it would also require it to be taken up to ground floor level and a concrete slab or other form of permanent propping at floor level, in order to permanently restrain the top.

If it was designed to act as a vertical cantilever either permanently or temporarily while the basement was constructed, it would need digging in a long long way below the required floor level, to get the required resistance from the soil - and would need a lot of reinforcing to stop the concrete deflecting inwards as the soil was removed from in front of it.

In other words, it's overly complicated compared to simple mass concrete underpinning of the walls down to below the required floor level, excavating to level and casting the basement slab!

 

[chessspy]

Cellar extensions are popular in Chelsea and Kensington, (where I used to work) and are darned expensive due to the excavations and sectional underpinning etc.
What you are proposing is not so dramatic as this, but you should commision a surveyors report to see what procedures you should employ to do it.
The extra concrete floor is interesting, I wonder who's underneath....

 

[gangman]

Last year we done one of these, and it happened to be in Kensington. It was on the High St., full of galleries and antique shops.

The cellar floor was lowered about 2ft. and we done it the way John has seen it done. Dug out in 1m strips to expose existing founds, we left the founds as was and cast 500mm of reinforced concrete in front, which was tied into the slab. It was done this way, rather than underpinning because the properies either side had very 'valuable' antiques on display and this method was deemed the least disruptive and less likely to cause any damage.

To be honest I prefer this method as it is much better than underpinning, less digging means less carrying out.

 

[chessspy]

Hi, gangman,
I used to work for Holloway White Allum, in Kensington. (Mick Gannon was the subby's name)
Ever done anything with them?
Alan D.

 

[gangman]

Hi, gangman,
I used to work for Holloway White Allum, in Kensington. (Mick Gannon was the subby's name)
Ever done anything with them?
Alan D.

Can't say I've heard of them. The job in Kensington was a one off, terrible place to work. Expensive to park, no skips, don't want to work there again.

 

[Shytalkz]

The cellar floor was lowered about 2ft. and we done it the way John has seen it done. Dug out in 1m strips to expose existing founds, we left the founds as was and cast 500mm of reinforced concrete in front, which was tied into the slab. It was done this way, rather than underpinning because the properies either side had very 'valuable' antiques on display and this method was deemed the least disruptive and less likely to cause any damage.

So if all four sides are done that's a metre off the useable space in each direction, whereas underpinning would lose you way less than 500mm.

There is no more probability of damage occurring if the foundations are underpinned; if anything it's far less.

The only advantage over underpinning to the method you've seen is as you say, namely less spoil removal.

 

[gangman]

This was an art gallery and the step was incoperated into the final design. It was only three sides as the rear wall was taken out. This method was a lot cheaper than under pinning and the basement was going to be left as just storage but the price was low enough to warrent the work being done.

Also this job had 16 PWAs which as you can imagine could get very expensive, and this method was more 'agreeable' with the neighbours.

 

[renovbod]

Thanks for the replies. The information is very useful and JohnDs method correlates with what the builder has said in the past (which is reassuring!) - I'll ask him about the underpinning when he is back in a couple of weeks to look at it. He did say about test holes to discover if it was viable or not (checking footings and water level) so I think I need to ask him to do that to answer my question.

I get the impression that if the builder's methodology passes Building regs then I am good to go - if either the builder says it's not possible or the building regs say no to what he proposes then I abandon the idea and make good what's there. However, I do note the comment saying I should commission a surveyors report - if the builder comes up with a methodology and the building regs say OK is this necessary? - or is it an essential step that I should go through to ensure peace of mind (or perhaps it's a requirement by Building Regs that someone with a certain professional certificate for surveying has looked at it?) - sorry for naive questions!

Also, I have read that Building regs will require 7ft min head clearance and 10% of floor covered by natural light (I don't know, but guess, this is a per room requirement) - are there any other fundamental requirements I haven't picked up on yet?. I guess 7ft clearance is in the hands of what the builder says is possible but there is only a window in the small room at the back, no way one could be added to the middle room and it would require exterior work to add one to the front where the whole room is below ground (at the back it is almost entirely above ground)...

I have also read that most cellar projects need building regs but not planning permission, but also that an external window requires planning permission... it seems a bit of a circle as I would have thought most cellars would need a window to fulfil 10% natural light building regs which would in turn require planning permission - which contradicts that most cellars jobs like this don't need planning permission. I must have something wrong here!

Many thanks again for your helpful comments!