Is it possible to work on a DIY basis in parallel with craftsmen?

You have to look into the contract or then draw up an additional agreement. For example, despite [GÜ] and a theoretical complete ban on access until final acceptance, I already laid additional empty conduits during the pouring of the base slab, of course after consultation with all those responsible.

This is legally a problem. As soon as the house is contracted as a trade (usually BT, GU or GÜ), you are not allowed to do any work on it before the acceptance of that trade. Otherwise, you risk the warranty that you have purchased at a high cost with the trade.

In practice, these are often disputes about dents on the stairs or the front door. The contractor will understandably release himself from responsibility here. He may also argue away certain later damages if necessary.

From my point of view, most contractors will only let you into the house before acceptance if you accept significant cuts in defect liability as a result.

This happened to a good friend. They were also very happy because they "managed" to work in the house even before acceptance. Some dents on the windows and the front door were then not accepted as defects by the contractor during acceptance.

Hi, a friend of ours built with a developer near Hanover back then, and he wasn’t even allowed to enter the construction site or the property. Only at handover after almost everything was paid. It was more relaxed with us; with our general contractor (H.v.H.), we were allowed to do various things ourselves (tiles, interior doors, painting, sanitary fixtures, etc.). The services were credited to us, but in the end the savings weren’t that big... Also, he sometimes put quite a bit of pressure on us to keep the schedule. Today, I wouldn’t do that much myself anymore. Regards, Olli

Many thanks for your contributions. I think then we will simply get started as soon as the house is finished and has been approved.

Do it like this, it will probably be the easiest solution, saves stress.