Digging out under patio - have I missed anything ?

[SimonH2]

This isn't something I'll be doing any time soon, I'll be having to save up somewhat first - but I would welcome any comments. The house I've just bought is on sloping ground - so ground floor at the front is first floor at the back and the garage is underneath. Outside the back of the house itself is a raised patio area which is level with the ground at the front. I imagine most people look at it and think "patio" - I look at it and think "hmm, extra room".

The two pictures below show the view from the rear, and also from the side of the house looking in approximately the opposite direction. The tarmac area is about at the original ground (before the houses were built) level at the back.

Untitled

• SimonH2
• 16 Mar 2011
• 2

Untitled

• SimonH2
• 16 Mar 2011
• 2

So what I have in mind is lifting the patio and digging out the space underneath. I expect the outer leaf of the house wall will go down to ground level - are there any circumstances where the two leafs might not share the same foundations ? I assume the filling will be all the spoil dug out under the house (the garage goes all the way to the front). Once the space is opened up, add a roof (Bison beam ?) and re-lay the patio on top.

Add an outside door, and one between this new space and the garage, and Robert's your mother's brother.

I'd probably look at going part way round the side of the house, but not too far as the returns start diminishing. The width reduces (it's on a triangular plot) down to about 18" at the front of the house, and also the other side of the retaining wall is a drive that slopes upwards and so the difference in height would be increasing.

I know it's not going to be the best of spaces, but it would make a nice utility room and free up space in the garage.

I don't know what's supporting the steps - so I'd just have to figure those out when I got there. I'm also assuming there's no DPM in the outer leaf of the house wall at the bottom, so I'll have to add one (inject a chemical DPM ?). Also, I think it's safe to assume there's no DPM in the other walls that currently support the patio, and they probably aren't cavity (either add an inner leaf, or knock them down and rebuild them - digging out will be a lot harder without taking one of them down). I'm figuring that planning control shouldn't be involved since I'm not actually altering the exterior walls, but building control will be.

So have I missed anything ?

Assuming I was doing the bulk of the work myself (got friends with diggers, and know farmers with holes to fill, etc), does anyone have any hints as to the likely cost ?

 

[r896neo]

You will be frightened by some of the extra costs and headaches involved in doing work like this. The house wall acting as a retaining wall will have to be designed by an engineer in order to get through building control. It's likely you'll end up with a pretty serious wall (read expensive) Also you'll have to re-do the drainage etc behind the wall but the main headache will be getting at it and the associated equipment costs not to mention the various permissions and notices you will need due to the proximity of other buildings footings

The new room once created will need to be tanked as it will be largely underground, a dpc will stop water rising but not coming through the wall.

It might be finicially feasible to do if it were giving you extra habitable space but in reality it will cost you a lot and add almost nothing to the value of the house.

Sorry to be such a downer but you'd be much better off moving as i can see it costing you 10 grand easily and unless your settled there for the long term it's money down the drain.

 

[SimonH2]

You will be frightened by some of the extra costs and headaches involved in doing work like this. The house wall acting as a retaining wall will have to be designed by an engineer in order to get through building control.
...
The new room once created will need to be tanked as it will be largely underground, a dpc will stop water rising but not coming through the wall.

I think you must have misread my description. The house wall is a retaining wall at the moment (it stops the patio disappearing into the garage) - after digging out it won't be. Only a short section of wall round the side of the house would be buried and a retaining wall.

After digging out the space would have :
2 outside walls, those facing the Land Rover and the stone faced one nearest the camera in the first picture. I'm assuming these would need to be either rebuilt or have an inner leaf added.
1 wall dividing it from the garage, the one supporting the rear elevation where the patio doors are and a bit round the side.
1 Wall where the steps are. This is likely to be the most troublesome having the steps from the garage level up to house level, and over part of it's length being a retaining wall for up to (I'd estimate) it's bottom two feet at the highest point of the ground the other side.
And lastly, a short section (probably about 6 foot long) of wall bridging between the house wall (side) and the side boundary wall. This would be a retaining wall for the full high between garage and house levels.

I'm assuming it won't be cheap, but rather hoping that (if I save up a bit) it would be doable as long as I can do the bulk of the work myself. Just trying to get a feel for the issues I'll need to address, and how hard I've got to save

The extra space would be quite significant. The house isn't big, and neither is the garage - and a significant chunk of the garage is taken up by what is effectively a laundry/utility room. Just having a second route into the garage would be a benefit, as you can see the Land Rover fair fills the hole in front of the garage door, and makes access though the main door a bit of a squeeze - plus you have to park far enough back to allow for opening the "up-n-over" door.

 

[JohnD]

I think I used to live in that house. I moved afer my third wife went missing. Don't dig up that patio.

 

[SimonH2]

I think I used to live in that house. I moved afer my third wife went missing. Don't dig up that patio.

 

[r896neo]

ok apologies i completely misread and misunderstood.

 

[SimonH2]

ok apologies i completely misread and misunderstood.

Easily done, especially by me, so you're in good company

 

[r896neo]

Had a rethink about your actual plans rather than my fictional ones.


The main possible stumbling block i see would be the wall to the far side of the steps which you say is retaining a rising driveway behind it. It is possible that wall could have a stepped foundation and your excavating would undermine the foundations but unfortunately there is no easy way to find this out.

It seems unlikely but if the house that own that drive is older than yours it's not out of the question. what sort of height does it retain (in relation to the base of the steps) once it's level with the corner of your house.

 

[SimonH2]

Had a rethink about your actual plans rather than my fictional ones.
The main possible stumbling block i see would be the wall to the far side of the steps which you say is retaining a rising driveway behind it. It is possible that wall could have a stepped foundation and your excavating would undermine the foundations but unfortunately there is no easy way to find this out.

It seems unlikely but if the house that own that drive is older than yours it's not out of the question. what sort of height does it retain (in relation to the base of the steps) once it's level with the corner of your house.

Yes, that's something else I'd considered.

The house next door is considerably older, where my house is used to be a field. I can't remember what was originally along that boundary, but whatever it was got rebuilt - in fact part of that wall did get rebuild a few years ago (don't know why, I'd guess most likely the ground had settled and pushed the wall out). I'll have another look but from memory I think the drive would be 1 to 2 feet above the tarmac area by about where I think it would make sense to stop. Right at the back (the corner where the steps are) I think the neighbours drive may actually be slightly lower than my parking area.

I guess to a point I could extend the foundations downwards. My brother did this to his house to lower the floor level in the basement - so he could lower the living room floor to match the rest of the ground floor. Dug out in sections, shuttered, poured in concrete and vibro-compacted it to make sure there were no voids.
I think how far I went round the side of the house would be a case of seeing where I hit the foundations for that wall, and where the space starts getting so narrow that it's not actually that usable.

Now I actually own the house I'll have to go round doing some measurements and detailed drawings.

 

[r896neo]

The possibility of a stepped found on that wall would be my only real concern. It's a hard one as there is no way to know but if the drive behind is a matter of a few feet higher i can't see it becoming an issue. I was more thinking it could be 6 feet and a retaining wall holding back a few feet of driveway will be no major structure anyway.

It's one of those situations were there is no definate way to know until you get into the ground. Also are you certain there are no services/ drains in this proposed area as that would be a real and possibly expensive ball-ache