Newly built single-family house KfW55 standard - How to heat?

The layout of the technical room suggests that the designer did not consider your question, but assumed a gas connection. I would prefer an air-water heat pump with photovoltaics.

Air-to-water heat pump plus photovoltaic. Lower maintenance, more ecological (if powered by renewable energies), and with a properly designed underfloor heating system in a new building, also cheaper to operate than gas.

As an air-to-water heat pump user, I can only tell you to get one if you want to deal with optimization. Of course, it also runs with the basic settings, but then maybe your wallet will rattle. If air-to-water heat pump, then a modulating one with heating load calculation, but not because "we've always done it that way," rather calculated according to your wishes. Pay close attention that the spacing of the pipes in the bathroom is about 5cm and that a large area maybe even includes the walls. Because that is always the coldest room that needs the most temperature.

Alternatively, solar heat pump with cooling function costs a bit more.

What I definitely no longer want to miss is the controlled residential ventilation. With 10 rooms in the house, I do not want to ventilate manually.

We will also install a heat pump with photovoltaics. Which one it will be now depends on the total price of the house. I am leaning towards a ground-source heat pump + 10 kWp photovoltaics.

... I would prefer gas.

We will also install a gas condensing boiler in our new building (plus the required solar thermal system).

The basic problem we have, however, is the limited space in the technical room.

I quickly googled, I don't have practical experience yet: A wall-mounted gas condensing boiler requires 0.5 sqm of space (space under the unit). A floor-standing gas condensing boiler requires 2.0 sqm of space. How will and how do you want to insulate the attic? Apparently, gas condensing boilers can also be installed there.

-Wood stove is to be installed

We will be looking at gas stoves soon. Do you mean a wood stove by fireplace stove? We currently have that and might switch to gas, but we are not quite sure yet.