Foundations for extension

29 Oct 2012
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United Kingdom
we're planning to build single storey extension next to the boundary. Our neighbours (semi-detached) already have extension on the other side - with the wall 4'' from the boundary and the foundations on the boundary.

We'll be digging our trench right up to their foundations and probably to similar depth. I'm a bit worried that this will remove the lateral support for their foundations and may allow movement.

We'll be aiming to dig & fill quickly, but I'm not sure if we should be supporting their foundations while the trench is open to reduce the risk (may be some sort of lateral bracing). The plans (approved by building control) don not have such provisions.
I'd appreciate your advise.
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You might have approached your neighbors and portrayed your fear to them, that if your builders went ahead there is risk their foundations may move, so to avoid this risk, you may suggest using their wall and offer some money in return, they may then give you permission to use their wall, in long term it would be better as there won't be that ugly narrow gap between two extensions that can trap animals, attract little adventurous kids to dare walk through the tight gaps and cause injury to themselves and waste rescuers time. it can also collect rubbish and fallen tree leaves and narrow gap would be hard to get in if there ever was a problem with external brickwork. Further advantage would be less heat loss, and any damage to their roof caused by your builders would be put right at your expense, that wall though in strict sense is in their property, all depends on them agreeing. Though in strictest sense, your fears are just fears, foundations are dug deep enough until they reach hard soil, even if the plans dictate 1m deep foundations, it is the BC who will inspect of hard soil has been reached, if not they can make your builders dig more deeper, so any movement is unlikely and you would not need any braces, or you could built your foundations in small sections. You may give BC a call for their advice. The risk would increase if you were to dig your foundations deeper than theirs.
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You will only risk causing movement if you undermine theirs, i.e. go (a lot) deeper than those next door. Even then, as long as their walls are adequately bonded and the foundations done in one pour and are without any shutter breaks, then the risk is minute.
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thank you for the replies - we'll make sure our trench is no deeper than their foundations. Their extension was built 6 yrs ago so the foundations depth should comply with the current building regulations.
We won't be able to use their wall as our extension will have different height & roof type. We'll try to think of some clever way to seal the small gap (probably around 8'') between the two extensions.

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