boundary foundation

[Kur]

Hello all experts
We wants to build rear extension but our neighbour who already have one with side wall being on his side but very close to the boundary line wants to know how our foundation will be in relation to his one. I think his foundation is slightly on our side of the land because it`s wider than the wall. Can I jus adjoin my foundation to his one? So for example instead of having 600mm width we would build 450mm or less and use part of his foundation. I hope you understand what I mean. Thanks

 

[DevilDamo]

Subject to structural engineers/building regulation approval, you can go for a 450mm wide foundation, which are normally specified for single storey extensions. Two-storey extensions usually utilise 600mm wide foundations or single storey that "may" in the future have potential to go up an additional storey.

Just out of interest, how far is your neighbours wall from the boundary? And likewise, how far would your proposed wall be from the boundary?

 

[Static]

When you have the foundations designed you should get a party wall agreement in place and a sketch section through the two foundations will show your neighbour all that is happening..

Chances are the foundations will both be at the same founding level and a 25mm flexible joint put between the footings..

 

[Kur]

the neighbour`s is exactly at the boundary line. I`m attaching some detail drawing, do you think it is correct?

Untitled

• Kur
• 2 Aug 2010
• 1

Untitled

• Kur
• 2 Aug 2010
• 1

 

[freddiemercurystwin]

You won't be able to build off their foundation, your foundation needs to be separate as mentioned by Static with a 25mm soft joint and your wall needs to come off the centre of the new foundation. Your neighbour should never have built their foundation over the boundary however hindsight's great isn't it! So you'll end up with a gap between the two extensions. Are you doing the drawings yourself? I hope a 'professional' hasn't drawn them!

 

[Kur]

no, I drawn this, I know CAD a bit. So you are saying I would need to leave a gap? But we have permission granted for full width. Or maybe I would need a structural engineer to design a foundation?

 

[freddiemercurystwin]

no, I drawn this, I know CAD a bit. So you are saying I would need to leave a gap? But we have permission granted for full width. Or maybe I would need a structural engineer to design a foundation?

Planning is entirely separate to Building Regs as in they do not pay attention to any structural implications. It may be possible to do as you wish but you will need a structural engineer to design the foundations and permissions from your neighbours. It would also be advisable getting the deeds amended as you would be sharing a foundation otherwise a future house sale by you or your neighbour could be very problematic. Are you getting a Party Wall Agreement?

 

[jeds]

There are several possibles here but in my opinion the best/safest option is to bring your foundation back over your side by half a metre or so and cantilever the floor slab to the boundary.

 

[RonnyRaygun]

You won't be able to build off their foundation, your foundation needs to be separate as mentioned by Static with a 25mm soft joint and your wall needs to come off the centre of the new foundation. Your neighbour should never have built their foundation over the boundary however hindsight's great isn't it! So you'll end up with a gap between the two extensions. Are you doing the drawings yourself? I hope a 'professional' hasn't drawn them!

Under the Party Wall Act, you are permitted to cast foundations over a neighbour's boundary, as long as the wall itself isn't over the boundary line.
Although, this does cause problems in this instance. Seems it would have been better all round for your neighbour to have built his wall astride the party boundary (with your agreement). You both could have used the wall and both would have ended up with more space.

However, if you have room (and it looks like you have) you could cast your foundation, then cantilever an RC slab over the top of your footing, allowing space between so that you don't bear onto his footing. Then build your wall off the slab. You could probably even just cantilever the top of the foundation over slightly...we've done this in the past.

 

[maltaron]

Is it worth trying to get your neighbour to agree to you using their wall. It seems pointless to build another wall against the existing. You would probably need a party wall agreement but should be no problem as you are not actually going on his land, just plastering his wall on your side.

 

[simonjay]

Hello all experts
We wants to build rear extension but our neighbour who already have one with side wall being on his side but very close to the boundary line wants to know how our foundation will be in relation to his one. I think his foundation is slightly on our side of the land because it`s wider than the wall. Can I jus adjoin my foundation to his one? So for example instead of having 600mm width we would build 450mm or less and use part of his foundation. I hope you understand what I mean. Thanks

In that situation, I overpoured my foundation (overlap the neighbour's) about 4" higher and the BCO was happy with that. This effectively locks the foundations together. I then built a new inner skin to form a cavity wall (their's was solid wall). Also, you should be able to build right up to the edge - the neighbours foundation gives yours lateral stability.
Simon.