Awful extension next door to my house ...

I can't picture this at all. Is his wall higher than yours? Is your garden wall a wall that separates your garden from his, and one end of it now has his extension 'growing out of it'? Or something else altogether?

I think you need to look into things like being overlooked and whether your light is being blocked, then see if there is anything you (or he) can do to mitigate that. Presumably he had planning permission for the extension? If so, did you see them and agree?

If you can give us a bit more information, we should be able to help more.

I too cannot picture this, it does sound horrendous, however you firstly need to go back to your deeds and check that, secondly take a look at council planning portal and check what’s he done is what he has permission to do.

As the previous poster said, check light etc.

Good luck

If it’s a party wall to a house, then he would have needed a Party Wall Agreement. You imply that it’s to a garden wall not the house which could be quite different. Is the wall located as yours on your deeds? If it is then I think you will have to get a solicitor involved. Do you have legal representation with your house insurance, the service should be free. If so then a quick consultation with them might be the best way forward.

Do you mean the side of your house?

Any construction needs to be at least 1m away. You are going to need a surveyor and/or solicitor.