Old building renovation, underfloor heating, electricity, water, possible pitfalls, etc.

We are currently measuring the house

If the house is from the NINETEEN sixties, there was no war in between and the files should still be recoverable. That is by no means Napoleonic history yet.

and the order is also influenced by the older faction... stubbornness due to age, basically.

Are the seniors hoping for the mercy of the Grim Reaper, so that the stress of the second construction phase will not affect them anymore?

Pitfalls due to underfloor heating: Higher floor construction due to underfloor heating may require an extra step, ramp, landing, or similar at the staircase. If the floor rises too much, the interior doors lose their standard dimensions and become extra expensive. So pay attention to door height. What system should be used? Removing screed, installing underfloor heating, and putting in new screed creates a lot of mess. How is the lower floor heated? Mixed operation of underfloor heating and radiators is very unfavorable. New pipes, new bathrooms, etc. are also very complex. Am I reading correctly that the lower floor is supposed to remain occupied during this? If yes, it won’t be fun for weeks and months...

Am I reading it correctly that the lower floor is supposed to remain inhabited during that time?

I read it as if both generations still lived elsewhere, but the seniors wanted to lock the door right after wallpapering and have their peace; such frills like modern heating and new light switches are supposed to be done by the young people only for themselves.

I actually understand it as the parents currently living upstairs and then moving downstairs so that the upstairs can be renovated.

Anyway... as long as the house remains occupied, I strongly recommend hiring an architect to ensure that all measures are properly and, above all, timely scheduled. Only in this way will the construction site be completed quickly and the interventions on the ground floor minimized.

But if the ground floor is also to be done, then it's better to do everything at the same time and rent a holiday home for a few months. You can get that quite cheaply in times of Corona.

Since the family situation apparently is so incredibly important: Yes, I initially wanted to build new (das Grundstück wäre gross genug) and then completely gut and comprehensively renovate the old building. But for logistical, financial, and various other reasons, I decided against it after long and careful consideration.

I think it will only become really clear when you introduce the plans and pictures into the discussion. Are you actually following the thread by : ?