Foundation Advice for Party Wall

9 Apr 2021
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United Kingdom

I’m looking to build a conservatory on the back of my semi detached property. It’ll be a lean-to design with a 600mm dwarf wall.

One end will have a solid wall. This wall will come out 3m and slope down from approx 2750mm to 2500mm over the 3m. It will have the 600mm dwarf wall at the end of the 3m as a buttress.

Mu questions are firstly, for a wall of that height is it best to build a cavity wall or simple double skin? I was planning the latter for simplicity and I’m not overly concerned about insulation. However the height of the wall seems tall compared to the wall thickness and I don’t want it falling over. Approved doc A suggests a 1/16 min width, which, with a double skin or cavity wall is fine but I thought I’d check with the seasoned pros in this site.

Also, when it comes to foundations, I’m planning trench fill for simplicity and time. My neighbour has a single storey extension that comes out about 2m of the 3m I’m planning. Both mine and their walls will meet within 50mm of the boundary. So as to avoid any risk of subsidence to their property, I’m planning on digging down to their foundation depth a filling from there. Two questions on this; 1) does this strategy risk any damage to their property, and 2) if I made the foundations 600mm wide, I’d have to build my wall on one end of this width, do I do that or do I reduce the foundation width so the wall is central but risk having a narrower foundation than approved doc A recommends for a double skin if cavity walk?

Lots of questions I know but any help or advice from those who have done this before is gratefully received.

Thanks in advance.

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Building a cavity wall is no different takes no more time or is no harder than building a solid wall.

Walls don't need to be in the centre of a foundation strip

The depth of your foundation should not be dictated by the depth of the neighbour's foundation. It's dictated by now deep they need to be
Thanks Woody,

Pleased to hear there’s no problem building a wall on one side of the trench fill foundation. I will look at what depth footing I should have but is there any risk to my neighbours wall if I dig lower than their foundation?

I’m mainly concerned over the height of the end wall. 2750mm with a 600mm buttress, does that sound too high? Is there is any strength advantage at all over a solid wall vs a cavity. I was favouring the former as a cavity wall (as I understand it) needs to be built from the inner wall out so the insulation can be tied to the inner wall. Due to the neighbours extension I’m not sure how I could do this unless I build the cavity without insulation which somewhat defeats the purpose.

Thanks in advance

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