Hello. The builder is liable for damage repairs or cleaning if you can show the damage and mess has been caused by his works, which seems likely. I suggest you put your complaint in writing, and say that he has caused this damage through his negligence in not mitigating agaist mess and dust etc. I suggest you get a quote for a commercial clean, andtell the developer the cost, saying you expect him to pay this within 14 days or you'll ,sue him in the small claims court. This should work. If you can afford it, get a solicitor to send this letter, as it will show you are serious. However, it will cost you a couple of hundred pounds for this which you won't be able to claim back.