-
FWIW the house we got fucked over on had building work like that (chimney breasts removed) and there were no building regs. Our solicitor suggested either we or the sellers could get indemnity insurance for a grand total of £100 that would allow us to sell the house - not sure if that is an option.
-
Some lawyers and lenders are OK with this, others not.
If I was buying a house that had structural alterations with no building regs I wouldn't be worried about insuring against the council making a fuss, I'd be worried that the work wasn't done properly. I would want an engineer to inspect the work, which would involved smashing things about to see what sort of metal had been put in.
Depends on how honest you are...
If you don't own up to having made any structural changes no one will ever know it wasn't there when you moved in. But the right/legal/safe thing to do is get building control to approve it. You can ask a structural engineer to spec it as your builder suggests and building control will accept their calculations, or most [all?] building control departments will do the calcs and spec it for you for a fee. Give your local authority a ring and ask for a price.
£550 sounds insane to a non-Londoner. Even in Cambridge's famous Cambridge, the least affordable place outside London, I'd want to pay half that.