New foundation next to neighbour wall

[marsaday]

I am planning on doing a small single story extension on the back of my house.

 

Our house is a semi and i won't be building on the dividing walls of the houses or the neighbours garden wall which is 1.8m high.

I will be building about 200mm away from this wall, so the foundation will be touching this walls foundation i imagine (if i go to the same depth which is currently 700mm as done a trial pit).

 

I have spoken to BC and we are on a sandy soil in my area.

I understand the party wall agreement comes into play, but only if i go below the neighbours foundation i think.

 

So i need a bit of advise on the best way to do my foundations down this side.  Couple of questions:

 

1) Is it ok to excavate down the side of this wall the full way which will be 4.5m.  Going no deeper than the current footing.  I have done a trial pit and it seems the wall has been built on a thin concrete base and then the bricks have been used to build up the foundation.  I have a photo inc.  The extension comes to the end of the current conservatory which will be going.

 

2) Do i need to place a membrane between the wall foundations and my new ones ?

 

3) My total foundation length is about 21m.  If the depth of the foundation next to this wall is an issue could i lay a wider foundation which has some rebar in it.  I have been reading sandy soils work better with a wider strip foundation, rather than a deeper trench.  If i go wider, this means less worry at this wall side.

 

4) I don't want to use an engineer for this work and would rather just speak to BC about it all.  I am doing the work myself and i have already had a chat with BC about this issue and he said to cast the outer two thirds of the strip and a few days later dig out the centre third and cast.  I think i would need to use some rebar for this.  Reading a bit more it seems all the foundation wants to be done on the same day so not sure if this is an ok idea.

 

Thanks for any help and i can give more info if needed.
 

[Ferdinand]

Welcome to the forum and thank you for such a clear explanation.

 

I will leave others to comment on the PWA aspect, but I question that 200mm gap you are suggesting. That seems to me to be the wrong width because it is not easily maintainable:

 

1 - ISTM that there is not space for you or  your builders to build it tidily (bloke plus trowel and room to work is more than 200mm). For such a narrow gap you or your builders will be working overhand (ie from the inside) which will make your new wall a probable mess.

 

2 - Once it is built it will be impossible to maintain your wall and the ground in the gap. eg What will you do when it fills up with autumn leaves which bridge the damp proof course, or should building work ever be required down there,or if a child gets stuck, or if you or your neighbour get rats and the ratman needs access?

 

My suggestion is that the gap should either be zero, or at least 450mm (= 18 inches), which is just wide enough to get in to sweep out etc. 600mm seems to me to be more sensible, as you can then do something with it such as ladder storage etc. 

 

The only exception I would make would be where your gap is short enough that you can reach down it with your arm plus a sweeping brush, rake etc. and reach the end.

 

I would suggest reducing the width of the extension by 300mm to make the gap wider. You can recover much of any lost inside space by using a thinner design of wall ... eg by using Celotex rather than Rockwool insulation. Alternatively talk to your neighbour about butting right up - even if you are not needing the PWA it is good to talk unless there is a good reason not to do so.

Ferdinand

 

Edited April 28, 2017 by Ferdinand

[marsaday]

Hello Ferdinand

 

I am the builder !!  I will be doing everything myself.

 

No chance am i building on his wall, as party wall complications, fees etc.  He is really clued up about these issues and i don't want to go down that route.

 

I might look into widening the gap.  

 

I won't be sacrificing the wall width as this is a tiny amount anyway and false economy.  My wall is probably going to be 350mm in width as i want to use 100mm celotex in there as opposed to the minimum.

 

The utility could loose some space or the main room.  I do have some give on that as the area we create will be big.  

 

The issue then will be the change over at roof level where i break away from our shared tiled roof (original lean to).  Both our lean to's are slate roof and glass.  The middle section can't be seen very well, but it is slate.  My new flattish roof in fibreglass will be built over the tiled/glass roof i currently have and extend out to where the conservatory sits and also fill in to the right.

 

His slate roof will stay and run into my new brick wall which will be built upwards.  I will then use a flashing to connect into the wall.  The wider gap will mean i now need to provide some form of drainage for this very narrow section of roof as it will be draining into the gap.  Here is another photo to show you how the current drains work now.  I have a boxed in drain hidden by the conservatory currently, but this will come down obviously. My neighbour has an overflow drain if needed at the base of the brick coping. 

[Brickie]

7 minutes ago, marsaday said:

My wall is probably going to be 350mm in width as i want to use 100mm celotex in there as opposed to the minimum.

Rigid board insulation systems don't work in this scenario,where you have to build the outer leaf first. The insulation is pinned back to the inner leaf to allow a ventilation void-this is just impossible when the outer leaf is built first,as it would be in your situation. 

[marsaday]

Ok, but if i do go for a 450mm void do you think i could build the outer leaf ok ?  Working in that gap.

 

If i can't manage to do this can you mix the insulation types up.  So do this wall with the soft insulation and the rest with rigid ?

[Ferdinand]

5 hours ago, marsaday said:

Ok, but if i do go for a 450mm void do you think i could build the outer leaf ok ?  Working in that gap.

 

If i can't manage to do this can you mix the insulation types up.  So do this wall with the soft insulation and the rest with rigid ?

 

On your first point I think this is a question for you and your bricklaying skills.

 

Perhaps build something like a BBQ or garden seat from brick close to a fence (or put a temporary fence up 450mm behind where you are building it) to see how much space you need?

 

The other point on yours assuming the photo is the right way round is that you will be working left-handed, which may be more difficult.

 

Mine .. pic later .. is actually just under 400mm between the (done by a conservatory company) conservatory 1.5m wall and the boundary wall, and I wish I had a little more width. You can see that the brickwork is OK if not perfect. But if I become like Friar Tuck I can see problems later.

 

I applaud your descision to do 100mm celotex .. that was something I had done in the walls and floor of our conservatory over the standard 50mm and It makes a huge difference.

 

F

 

Edited April 28, 2017 by Ferdinand

[Simplysimon]

if you build with a timber kit you would be able to make the wall thinner and have the same amount of insulation. i notice your neighbour has attached a glass roof to party wall to cover the area between wall and extension.

[MikeSharp01]

9 hours ago, marsaday said:

 

No chance am i building on his wall, as party wall complications, fees etc.  He is really clued up about these issues and i don't want to go down that route.

Have you confirmed that you dont need a party wall agreement?

[Ferdinand]

One other point on distances - I take it that you have ascertained the distance you can go back under Permitted Development, including that to go beyond the 4m backwards to the 8m relaxation there are certain things you need to have done. My thought is that it relates to the original wall of your dwelling, rather than to the SS extension already existing.

https://www.planningportal.co.uk/info/200130/common_projects/17/extensions

 

An informed and engaged neighbour is a very good way to encourage us to follow the rules :-) . When i feel slightly hit over the head with a planning requirement, perhaps by a Krummelkacker, I try to think of it as helping me (the elephant) avoid the elephant traps before I fall into one, which is usually the cheaper option.

 

Best of luck with your extension.

 

Ferdinand

 

[marsaday]

The extension measurement has to be from the rear wall of the house, not the lean-to.

 

I don't want to build with a timber wall.  I can loose the space if needed.

 

I haven't confirmed to the neighbour about the PWA yet, but i have said no work is taking place on the lean-to party wall so it will not apply here.  Where it applies will be to the depth of my foundations.  He is asking for one, but i am not in a position to say if it is needed at this moment in time.

 

I will wait for BC to say how deep the foundation needs to be.  Then i will let the neighbour know what is what.

 

I have asked 4 specific questions in my OP so wondering if anyone can answer these please.

[Ferdinand]

Hi again @marsaday

 

I do not think that anyone here will give you specific comments on question 1, 3, 4. There are too many variables (soil conditions, neighbour foundation condition etc), and it would need a site visit by a qualified professional.

 

I will comment on 2 that I personally cannot see a reason for such a membrane.

 

And that I personally agree with your interpretation of the PWA, but note that appointing a Surveyor who is qualified for PWA may be a good way to keep your neighbour happy by professional confirmation that the PWA does not apply, and to get an authoritative opinion on your Qs for a relatively small sum as part of the bundle.

 

Perhaps you may get more by chatting to your BCO, and then confirming your chat in writing as a preserved record.

 

Ferdinand