Building Regulations

First what you need to do is get from the console permissions what you want to build

Planning Permission and Building Control/Regs are two different things. Put simply, planning permission is what you are allowed to do and Building Control is how you do it correctly. I would recommend speaking to your local council's Building Control department and running through what you want to do. Unlike the Planning Department, the Building Control department just want to make sure things have been done correctly and, more importantly, safely. On any building project, you should notify Building Control (and pay the fee) and they will inspect and make sure the work has been done to standard as the work progresses. They may be able to give retroactive building control approval, or tell you what you need to do to bring things up to code.

Your builder should have registered the work with Building Control who would then have visited at various stages to satisfy themselves that your build is to specification. Building Control may be prepared to approve your completed works but will want to see evidence of the specifications and both compliance to the specifications and BC regulations. If you have no documented or photographic evidence of the various stages of work (indicating compliance to fire proofing, drainage, structural etc), BC May either refuse to approve the works or could ask you to expose what is beneath your lovely new extension. In terms of legal standing, I suggest you speak with Citizens Advice.

If I understand correctly your extension from the 1980s which needed planning at the time has been rebuilt ? This should have been done under Building Control with your existing Council They would have come out at certain stages to look at the quality of build and wether it conformed to current building regs And at the end signed the work off If there is an issue with the standard of work you now have a legal situation