Neighbour has been underpinning his extension without our knowledge

[kindredsp]

We live in a 1930’s semi. Our neighbour has a small extension running across the back of the back wall of his house (approx 2.5 meters deep) which has been there a long time. A long time before we moved into our home several years ago.
This summer cracks have started to appear between the extension wall and the existing wall of his house. It looks like the extension is starting to move away from the main wall which is right up next to our boundary with him.
He’s an older guy who’s pretty diy savvy and tends to do any work to his house himself. But he’s had eyesight problems recently which have forced him into early retirement and he admitted to my husband that he can see very little these days it’s all a blur.
My husband went over to his house last week to help him with something and was shocked by the extensive cracking on the inside of the extension. He then showed my husband that he’s been digging down into the foundations of the extension to try and underpin it and stop it moving any further. We had no idea he’s been doing this and it’s very close to our boundary and party wall. The neighbour seems to feel like he’s got it all in hand with his son popping over now and then to help.
But it’s got us worried and unsure of how to pursue this. We’re obviously worried about the detaching extension and his works causing damage to our own home. We feel like we have a good relationship with our neighbour but he doesn’t seem to think this situation is serious and needs expert opinion/work.
Has anyone got any advice as to what to do? And how to best proceed in order for experts to actually come and see what is happening and what he’s been doing? The extension is detaching from the main back wall of the property within 10cm of the party wall. I don’t want its movement to damage the foundations of the original wall (which will obviously affect us). Thank you

Underpinning requires planning permission and unless he is a qualified structural engineer you might want to ask WTF he is doing. The extension should (in most cases) have had planning permission as well. Both sets of permission should be viewable on the council website if in England & Wales. Not sure about the rules elsewhere.

What kind of soil/ground are you on?

Oh and my first port of call would be council planning, possibly then your own insurers. Anything affecting party walls used to require a report on impact to the shared party but I could be out of date.

We’re on clay soil. He told my husband that the extension was built 30 years ago by his then father in law who was a builder. I don’t think it ever got formal planning…
Ok, so council planning department first then? Do I need a party wall surveyor or something? Thank you

When our neighbours extended they had to get a full structural engineers report on the boundary area (no party wall even!) to advise on any likely issues - this was at their expense and required as part of the planning process.

We got to choose the structural engineer as effectively it was "our" report which they paid for. In our case the area is classic clay and Arts and Crafts era houses so mostly underpinned already so we used an structural engineer who specialised in that and in our area.

My guess would be he never got planning permission for the extension and skimped on the foundations or attaching the extension into the main building. On clay the requirements for foundations are deeper than on some soil types (but IANABuilder!). Check planning consents for the address on your council web site and then start with the planning office for advice. Stress your concern is specifically with the damage you can already see and future damage to the property, not a neighbour dispute in the normal way.

I've generally found that if you can get through to the right person in council offices they will be very helpful.

Ok will do that. Thank you!

all extensions need Building Control approval but not all need planning permission. They are seperate departments. You could can Building Control in your local council and ask for advise?

Or booj a structural engineer to give advise....maybe you could ask neighbour if he would share the cost and have the same engineer look at both houses at the same time? Surely better to find out the problem now....rather than when a wall collapses.

God, what a Loon. I wonder if his buildings insurance provider knows he is busily doing his own underpinning?

Anyway, I agree, this is a Building Control issue, not planning.

Underpinning does not need PP. A single storey extension built 30 years ago probably didn't need PP either. Both would require approval under the Building Regs which are not usually public documents.
You need to speak to Building Control.

Underpinning does not require PP
You dont seem to be suggesting that it is enroaching on your house- just close?

Yep, Building Control at your local authority will be the way to go. Don’t panic too much about the effect on your house - if the extension isn’t built against your party wall (it sounds like it extends rearwards away from his house and yours) then it’s unlikely to do you any damage. The issue sounds like it’s with the extension not having been tied in properly to the house, and built on insufficient foundations. Applying for building regs approval would have sorted this out at the time it was built. As it is, it’s likely to be much more expensive to remedy now than the minimal fees BC charged 30 years ago.

It may vary by area but it absolutely needs both planning and building consent in my area.

I can see that you are very concerned but I'm not sure you need to be overly worried. What damage do you fear this could have to your property?

Yes. It’s on his land. The extension stops just a few centimetres short of the party wall and there is a brick wall running down the boundary between our two homes. He can’t get down the side of the extension to dig (the gap between that and the garden wall is a couple of centimetres. But he’s dug down the side flanking his patio. I’m also aware that the extension has been built over a drain that runs across the back of the houses on our road. I only know that because it was flagged as something to know about if we ever extended ourself when we bought our house, as it would need extra care to extend over the drain. He also built a second roof over the original roof (apparently it leaked too much). I wondered if the weight of the extra layer of roof might not help the situation 🙈. God it all sounds so bodgy. But all done well before we moved in 5 years ago.

I’m worried it’ll damage the original back wall as it detaches itself which is connected to our back wall and cause us problems. Or that the collapsing foundations and/or his remedial work could impact on the foundations of the original structure which could again affect us.

Sorry, not good with the mumsnet site but have answered above ☝🏻