Still considering gas heating for the new building?

Probably yes. So based on what I've read so far, I tend to go with gas.

However, that probably doesn't work without [Sonnenkollektoren], does it?

But it is possible; we even build according to KfW55 with gas and without solar. For this, well insulated and with controlled residential ventilation, so there should be no problem without the KfW requirement.

It takes forever to counteract temperature-wise. Especially in weeks with large external temperature fluctuations, it is very uncomfortable. The underfloor heating is always lagging behind.

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If the outside temperatures penetrate inside faster than the underfloor heating can compensate, there is something wrong with the building envelope.

We also want controlled residential ventilation.

Whether the building envelope is good or not, we will only see when the contract is in front of us and we have it checked.

Currently, we are in an apartment in a two-family architect-designed house, about 5 years old. There we have these problems with the heating.

Complete nonsense. IMHO, a radiator belongs in a child's room and not underfloor heating!

Example:

The daughter does her homework in her room (21 degrees) and then wants to go directly to sleep... then she needs 17 degrees... This is not achievable with underfloor heating! It is too slow!

We don't even get 17 degrees here in the middle of winter when the underfloor heating is completely off. In the bedroom, it is off all year round but it always has more than 17°. Is the combination of heat pump and radiators actually possible?