The plumber postpones every appointment and no longer continues working.

How should I behave correctly if a plumber has been delaying a new construction for months? The plumber accepted the contract and said in front of witnesses that the whole thing would take 2 weeks. Meanwhile, it has been 14 weeks and only half is finished. The other work on the construction site can no longer be carried out, and the main trades are being postponed. Now he has had me informed that it should continue in about 5 weeks. But from experience, he will postpone this date again for some reason. How should I behave legally correctly?

What do you have in writing? Give him a written deadline. What are the reasons for the delays? Start looking for alternatives already…

How should I behave correctly when a plumber has been delaying a new building project for months? The plumber accepted the order and said in front of witnesses that the whole thing would take 2 weeks. Meanwhile, it has been 14 weeks and only half is finished. The other work on the construction site can no longer be carried out and the overall trades are being postponed. Now he has let me know that it should continue in about 5 weeks. But from experience, he will postpone this date again for some reason. How do I behave legally correctly?

Saying something in front of witnesses will not help much. Only written evidence is worth gold here...

Possibly [VOB §5] can help. Especially read paragraphs 3 and 4. We are currently experiencing something similar, that our plumbing company is working very slowly (we want to move next week and everything is still missing in the bathrooms). We are now escalating this always through our construction company, as even our site manager can no longer put pressure on the trades. They all have too much to do, so they leave half-finished construction sites and go where the loudest shouting occurs.

The plumber accepted the order and said in front of witnesses that the whole thing would take 2 weeks. Meanwhile, it has been 14 weeks and only half is finished.

looks like someone needs a nice call from a Serbian banker *LOL* Is the plumber perhaps related to the electrician from ?

What kind of contract do you have and with whom? With a [GU]? Directly with him? There are options – you don't have to accept that. Either have the site manager, site manager, or site manager handle it. Reduce the payment to the craftsman or cancel the contract. Have you already paid something? Set a deadline, and please do so in writing by registered mail.