Neighbours extension foundations (or lack of)

foundationer · 28 Aug 2015

The neighbours four year old extension starts on top of our shared (probably Victorian) garden wall. I've been digging a hole to see how far down it goes down / whether they underpinned it at all and... there's nothing there. It stops one course below ground level.

I'm supposed to be building an extension up against theirs so this is a problem isn't it? Anyone got any suggestions as to what to do about it/how big a problem? Pictures attached...

jeds · 28 Aug 2015

You already know this but there's a chance that digging your new foundation next to that will cause some movement, although how much and if any is impossible to predict. If it were me I would point out to your neighbour that their botch job has put you at a disadvantage (maybe choose some different wording) and the only safe option is to underpin. Really they should bear the cost but you might get an easier ride if you ask them to contribute rather than cough up the lot. See what they say. They might not even know about it and might be so grateful that you go straight to the top of the Christmas list and offer you a bottle of shampoo as compensation. You never know.

On the other hand they might say; so what, not causing us a problem, we're happy with it as it is, goodbye. That being the case, if the soil is reasonably cohesive I'd dig and fill as quick as possible (same day) and make sure the fill is at least above the bottom of that lowest brick. That shouldn't cause any more movement than they might get anyway and will probably improve their chance of movement in the future anyway.

PS. Somebody might say party wall act. I wouldn't bother myself - they didn't. Have a holiday instead.

^woody^ · 28 Aug 2015

Their choice or standard of construction methods is bluntly, none of your concern, and it does not affect the fact that you have a strict duty to provide support for their structure from your land.

They certainly are not obligated to underpin, or pay anything towards you underpinning it to suit your own extension build.

Use some sacrificial corregated steel sheets pushed in either before digging the trench, or as part of a bay excavation.

foundationer · 28 Aug 2015

Party wall stuff's all sorted already. They're right nice the neighbours. bit worried the BCO coming to visit Tuesday to have a commencement look about will have some expensive ideas for all of us.

Any idea what underpinning a 3m wall is likely to cost? Not too much, right?

What do you mean by this Woody?

^woody^ said:
Use some sacrificial corregated steel sheets pushed in either before digging the trench, or as part of a bay excavation.

theprinceofdarkness · 29 Aug 2015

A garden wall is normally built across the party line. Its an old wall, what have your neighbours done about the DPC (or lack of it)? Its that half of the wall is on your land so why not incorporate it into your build? I did this with my neighbours Victorian outside loo. I inserted a DPC.
You can "underpin" the wall for the cost of the concrete. Dig out a section 9" wide and 2' deep, tunnel under the wall. Fill hole with concrete, do another remote section the next day. repeat until wall is sitting on concrete blocks.
Only complication is to insert flashing as the height of your build goes over their roof level, to seal the gap at the top.
Frank

cjard · 29 Aug 2015

He means to use sheeting to keep their earth in place while you dig a trench, and the sheeting remains in place (sacrificed) when you pour your concrete foundation

mfarrow · 29 Aug 2015

How often does the sheeting refuse though? Especially around a house which is going to be full of all kinds of ****.

It is correct the neighbour has no obligation to pay anything. Imagine the situation was reversed.

JohnD · 29 Aug 2015

it looks to me like the neighbour is resting a shed or conservatory roof on top of the garden wall.

With no foundations, I presume no building regs or planning permission. He's lucky it hasn't fallen down already. Perhaps he poured a slab.

If it was me, I would dig a trench for my own foundation, but dig and pour it in bays, and if it happens to support the neighbour, good luck to him. Certainly replace any disturbed soil with concrete.

foundationer · 30 Aug 2015

It's a whole single storey kitchen extension - seems to be building regs free - I think they decided it was PD and then must have decided that meant they didn't need building regs either. I've not actually asked them...

JohnD · 30 Aug 2015

I might consider putting sacrificial shuttering between your own foundation, and the backfill under his. Then, when his extension falls down, they can build a new one without disturbing yours. You could still pour the bays, both sides of the shuttering, at the same time.

foundationer · 30 Aug 2015

What do you reckon the BCO's likely to suggest?

Also: I stuck a crowbar in till it hit something solid. 30cm from the rendered face of the wall. I guess they cut a trench and filled that side then?

jeds · 30 Aug 2015

John's idea of bays is a good solution. You can happily take out 700 or 800mm bays without causing any significant movement. Don't use anything sacrificial between your concrete and the soil though. All that will do is create space for the soil to move and settle.

foundationer · 2 Sep 2015

BCO had a nose and said 'dig it in bays and fill with concrete' so top marks there John!

Raises a question about detailing though - the concrete fill will have to come basically up to ground level to properly support the wall won't it? But there's supposed to be a layer of hardcore, sand, DPM and slab over the trenchfill - can the slab basically be built right on top of or even alongside this concrete support? Not sure if that explains it all that well - maybe I need a diagram?

JohnD · 2 Sep 2015

poured concrete will settle somewhat, so to support and underpin the wall you can leave a gap, and fill it later with bricks and rammed mortar. Otherwise the concrete will shrink away out of contact with the bottom of the wall. You might need a jetwasher or something to get the soil off the bottom of the wall.

I do like the idea of the neighbour's wall standing on its own foundation so that your house will not be affected if/when they demolish and rebuild. A few courses of brickwork on the strip might achieve that.

catlad · 3 Sep 2015

If its a standard lean to extension then there is not going to be much loading on the garden wall but you still need to take care while digging, Noseall's idea seems the easy'est solution to me.

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Neighbours extension foundations (or lack of)

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