Is my house subsiding

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Underpinning is a last resort. It's highly unlikely to be necessary. The insurance company will do all they can to avoid paying out to have it done, and it's not what you want either, even if they pay, due to the future issues with selling and remortgaging.

The crack at the moment is minor. No-one will be underpinning your house for such minor movement. As I said, most likely the insurance company will send out an assessor, who may also require further advice from a structural engineer.

I guarantee you, having worked on such issues on many occasions, the structural engineer will report on the crack, mention that it's wider at the top than the bottom, indicating potential foundation rotation caused by downwards movement at the far end of the extension. They will certainly mention the tree, and they will ask whether the extension predates the tree.

They will then ask for a trial pit to be dug to determine the depth of the existing foundation.
They will get soil samples taken from the underside of the foundation and have them sent away for analysis, to determine soil type (shrinkable or non-shrinkable), and if shrinkable, to determine volume change potential, modified plasticity index and desiccation. The first two will help to determine whether the foundations are deep enough and the last one will determine whether the soil is already so dry that it will have potentially shrunk.

They will get root samples analysed from the base of the foundation if there are any, to confirm what tree the roots belong to (probably obvious in this instance).

They will then make recommendations: If the extension predates the tree, they will recommend that the tree is removed. This will allow the soil to recover over an extended period of time, allowing the crack to close up.
If the tree predates the extension, particularly if it was mature or semi-mature, it's not quite so simple. If the tree was there before the foundation was built, your foundations should be deep enough to allow for a mature Ash 1m away. The engineer will be able to tell you if that's the case. They would need to be far in excess of the usual 1m though, and if high volume change soil then your extension should have been put on piles.
If the tree was already mature then a root protection zone should have been considered which would most likely have meant that the extension would have been put on piles.

For many reasons I suspect that the extension is probably older than the tree, but you need to confirm that??
 
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Certainly an ash tree shouldn't be 1m from the extension on clay. I would get the insurance co involved, but you might also, if you have legal expenses insurance with your house, have a chat to them about the tree and potential issues and see what they say. It may be a long road, and the tree may have a TPO, but ideally the tree should probably be removed. However, there is the caveat that removing a tree can cause "heave" - the opposite of subsidence. You may well need input from a tree expert.

Don't rush - take it a step at a time - do you get on with the neighbours? how close is the tree to their house? Big trees - ash, oak, lime, sycamore etc really should not be very close to buildings. It would be better to solve the problem through cooperation and dialogue, but if that isn't possible, potentially it could go legal.
No reason for the tree to not be 1m away from the extension, but the foundations would need to be designed to accommodate the tree.

If the tree was planted after the extension was built this is more of an issue, but then there is no issue of resulting heave should the tree be removed, as the result will simply be that the soil will rehydrate back to equilibrium, where it was when the extension was built and before the tree was planted.

There won't be a TPO if the tree isn't particularly old. If it is mature and predates the extension, with or without a TPO, the foundations should have been designed and formed to accommodate the tree, so 1: the tree shouldn't need to be removed, and 2: resultant heave shouldn't be an issue. I suspect this isn't the case though simply due to the movement noticed by the OP.
If the foundations haven't been designed or formed to accommodate the tree, there may be some recourse against the engineer or builder.

If the tree was mature when the extension was built, at 1m away, I would expect there to be some pretty large roots within close proximity to the trenches though, so I'd also expect some sort of root protection zone if the tree was properly considered. That alone would probably have led to a piled foundation.

If the tree predates the extension and the foundations aren't deep enough, underpinning is still a last resort. The potential heave resulting from tree removal would have to be considered, so it would probably be more appropriate to undertake regular pollarding to reduce influence on the nearby soil. This is where an arborealist would be able to advise.
 
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RonnyRaygun thank you so much for taking the time out of your day to write such detailed responses; the knowledge you have provided us with has certainly helped to calm our nerves. We are both in our early 30s and saved enormously hard to afford the home we purchased 3 years ago. I am currently on maternity leave, so the prospect of a huge bill terrifies us. Although this is a very uncertain situation, just understanding what the different courses of action may be have helped us to feel a little better. Still a long road ahead- we will let you know how we navigate it!

Thank you once again.
 
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Here you go HERTS
 

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Definitely quite a young tree - if younger than the extension get rid of it if possible. Hope you get on well with your neighbours ;)
 
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Tell your neighbour that the tree is affecting your foundation and will in time affect theirs. Ask them to have it removed and offer to pay half.

Andy
 
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Any cracking inside the house. Some minor cracking like that could be shrinkage of the clay but the extension is not going to fall down. Rake out the joints, re point and report back in 5 years
 
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Any cracking inside the house. Some minor cracking like that could be shrinkage of the clay but the extension is not going to fall down. Rake out the joints, re point and report back in 5 years
Thank you- yes a crack has appeared in the doorframe that splits the extension in two.
 
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Here is the picture.
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