Single Storey Extension - Garden Wall Interface

Hey All,

I am building a single storey rear extension on a semi-detached property. It is fully within my rear garden, however on one side there is an exisitng garden wall with small butresses (100mm) to support every 3m along the wall (down the garden).

I intend to build the extension wall up against the butresses leaving circa 100mm gaps from the exisitng wall, which I am fine with from an asthetic / interior space perspective. My neighbour doesnt have an extension and doesnt want us to take the wall down and rebuild it - party wall has been agreed on this basis.

My question is around the gap between butresses (garden wall to extension wall)

What to do? Fill with concrete carefully (in sections to avoid any potential buckling of the garden wall)? Leave an airgap and cap at the top and side to prevent dirt build up? Fill with pea single?

How to prevent damp through the garden wall effecting my new extension? membrane against the extension wall? would that cause the wall to sweat?

Thoughts/suggestions welcome. Experience of similar situations even more welcome.

well gap between your new extension will fill with all sorts of wind born detritus between the wall. will coarse you damp problem eventually so somehow you need to agree with your neighbour how to prevent this happening, because either you will have damp problem or wall will fall down.

Best idea would be to incorporate with necessary foundation to incorporate the wall into your new extension, if you or a building inspector explains to him / her what future implications might be.

tonyVent said:

Best idea would be to incorporate with necessary foundation to incorporate the wall into your new extension, if you or a building inspector explains to him / her what future implications might be.

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Thanks Tony, what specifically do you mean by "incorporate into the wall"? Tie in? There is no DPM on this garden wall and inserting one isn't feasible from a neighbor relation/party wall point of view.

Your roof edge detail will determine the extension wall offset from the boundary wall and boundary line.

It's foolish to even think of incorporating a garden wall into an external house wall, or sharing a foundation.

Design or fit a suitable cover or capping to keep things from collecting in the gap between the walls.

All this is part of your plan-drawer's job. An important part, why has he not done it?

Ive done this a few times with orangeries, there are various solutions -there isnt enough detail to know for your situation

sometimes we have done it with concrete pads and steels to form a load bearing wall -an option if digging a foundation along the garden wall will seriously weaken it.
It can be done in some circumstances with timber studwork, built onto concrete oversite -in which case it isnt load bearing so will need a steel beam or timber beam to support the roof, and probably a steel post set in the foundation at the front.

I have built a conservatory with a studwall, that had a renderboard external face -so no timber in the cavity, as it was against an old flint garden wall.

If you want to do in masonry, then as Woody says -ignore the existing wall and build a new one with its own foundation -the detail all depends on the existing wall and the roof detail.

Whatever you do, it needs to be capped at the top and detailing of damp proofing needs to be effective. Sometimes a new cavity wall, block in and out is the simplest arrangement, externally change to brick if it is higher than the garden wall.

Woody of course it can be done, you take down wall put your foundations in then rebuild wall and then continue with your own extension wall. If the neighbour wants to be really difficult then there is little option other than to just build with what you have. Is the dividing wall yours, the neighbours or jointly owned?

Hi all, thanks for your replies.

The foundations are in and the wall position is fixed as per my original post. The garden wall is a Party Wall, which is lower than the new wall (about 0.6m). Roof design is also compatible.

Currently the design is to have an DPM against the outer side of the outer leaf of the new wall and backfill concrete Inbetween (in sections) upto the height of the garden wall. The DPM is then cut back in above this into the outer leaf of the extension wall.

The neighbor is defficult and I am trying to avoid any further issues.

As per my original post, I'm keen to understand if the current option for the "gap" is the best, seeking others experience.