Extension - shared wall with neighbours or not?

[BrightGoldenHazeintheMeadow]

We are planning an extension into our garden. Currently both neighbours do not have an extension and we have a fence on each side of our garden. Our architect's drawings have a wall on our side of our fence on each side. When we were speaking to a surveyor yesterday (about starting the process to get party wall agreements) he said it would be better to have shared walls - not least because that would give us up to an extra 6 inches each side our architect has said:
"In regards to the side walls your structure has to be independent of next doors. Supposing they knocked down their extension! you would have no wall!. I have set the wall as wide as it will go on an eccentric footing. You are not losing 600mm of space at all. If you want to come to a shared wall agreement with a neighbour then the footing can be in the middle of the boundary and normal blocks to be used for both leafs of the wall and a rockwool filled cavity for fire and sound protection”.

Any thoughts welcome. Thanks.

Was there a mistranslation between inches and mm when talking to the architect? 6 inches is 150mm, so 6 inches each side is 300mm. I'm not sure that's enough space to warrant the extra costs/hassle of shared walls/party wall agreement/etc.

I don't know why your neighbours would agree unless they are extending to a similar footprint.

Are either of the boundary fences yours out of interest? Strikes me if your neighbour did want an extension later and the fence was removed in between the buildings do you have enough space to do any essential maintenance eg guttering on that side. You are building very close to the boundaries.

Well he’s talking nonsense about them knocking down their extension and you having no wall. You would be creating a new party wall and nobody can just knock down a party wall!

Of course you will get extra space if you build on the boundary, unless you choose to have more insulation on a shared wall vs a non shared wall, but that’s your choice.

Tbh your architect sounds a bit defensive about having not gone to the boundary. Do you have/will you need planning permission?

The big danger IMO in building just to the side of the boundary rather than on it, is what if your neighbours do the same later. Then you end up with a tiny gap between the two extensions, no access to maintain your extension side wall, and the tiny gap gets full of crud and damp. For me this is the main reason to build on the boundary - plus the extra space, even if it’s not much.

By the way, if you build on the boundary and your neighbour later builds an extension against the wall you’ve built, they have to pay you half the cost of the wall.

We built on the boundary and had that agreed in our party wall agreement. Our neighbours were about to put their house on the market so weren't that bothered but it means the buyers get the advantage of a nicely built wall which they can use when they extend and we both end up with a bit more space

You have to have a party wall agreement regardless of whether you build up to the boundary if you’re digging within a set distance (I think it’s 3 metres) of your neighbour’s foundations.

We live in a semi and our neighbours’ house is already extended on the ground floor. Their extension wall is on the boundary. We had the choice of mirroring their extension size-wise and also building up to the boundary (with their consent) or leaving a gap, in which case we could build whatever size we wanted.

I kind of wish their house wasn’t extended as in some ways I’d rather have gone bigger but I had similar concerns about leaving a gap which was physically inaccessible for maintenance. So we have chosen to mirror their extension as they’re happy with us building up to and including their wall.

We've got a shared party wall on our extension with the neighbour. Not only did we have lower build costs as we shared the cost of the wall but the house is warmer because the wall is not external and there is no hard to reach debris (eg leaves) to clean up in a tiny gap.

If they agree to a shared wall then snap their hand off.