Delayed house completion - withhold money?

You have either made a formal or tacit acceptance. At the latest, the construction period ends with your move-in, and no delay occurs. Anything reported as a defect afterwards is, if at all, a warranty defect. You might be able to withhold the 5,000 euros if corresponding outstanding defects from the acceptance protocol have not yet been resolved. From your builder's perspective, it is pleasing that the remaining payment is so minimal. For 5,000 euros, I would not stir any more than absolutely necessary.

Here one must – I would like to place this tip at this point – indeed be a little careful not to lump the different relevant aspects together too much (retention for defects, assertion of delay damages, etc.), the risk is of course high with such questions. One can only advise against generalizing too much in this respect, because a reliable statement is always only possible based on the entire background (not least also with regard to acceptance, etc.) and above all also with regard to the individual (and complete) contractual provisions.