Building Owners' Protection Association and Expert as a Solution?

Okay, thank you all

Is this now about the formal execution (completion notification?) for you? You can also submit it alone, but then you have to live with possible consequences. Which amounts are still outstanding towards the [GU]? Why should / may the contract not be valid like this? Were defects excluded? Have you ever been to a lawyer?

Hm what will you do if he still sends you a final invoice? He has several years for that.... you said the contract is not valid, I hope that for you. Because if you have, for example, a contract according to VOB, you receive the final invoice and then do nothing for 2 weeks, then you're in deep trouble.