Construction description / additional costs - who has experience?

Note to you for this point. If you want additional things like electric shutters, finished attic, premium doors or anything else, then wait until you have signed the notary contract. All of that can be paid afterwards without any problems and you will save a few thousand euros in taxes as a result. Maybe discuss it once with your company beforehand.

Be careful: if these are more than "minor things" (in the eyes of the tax office) that are added after signing the contract for work, then one might be accused of tax evasion. It would be better to negotiate such things directly with the subcontractors—if possible. You will also have a hard time including such services in the financing, so you need to have equity in reserve for that (i.e., it was not officially shown to the bank beforehand or included in the financing).

Do the house connection costs have to be listed in the construction description? These are not necessary in advance to actually build the house, as is the case with development, for example.

Only in the notarized contract. It then states roughly: move-in ready piece of house including utility connections etc... on plot xy...

He is supposed to draft a separate letter where he certifies to you again that everything is clear. We did that too and nothing else came later. Better safe than sorry