There are several approaches and several options. Are you thinking of a hidden system like in the USA where you only see ventilation grilles, or should it be a simple and common split system as used in Europe?
Cooling the floor via the heat pump is more of a placebo effect. That only works if the house is located in the woods and gets little sunlight. What is certain is that your 55er house will definitely get warm and the attic will be similar to the apartments you’ve lived in, provided you don’t have much shading around it.
I would definitely cool the attic, the bedrooms, and also the living area. In principle, though, it will probably be almost the entire house (I don’t know your floor plan).
Basically, the main part of our garden does not have a perfect orientation, since it faces northwest. Nevertheless, the house will not be particularly well protected. Of course, we will plan appropriate shading into the construction project, but I do not want to rely solely on that, just as little as on the cooling function of the heat pump.
It looks like I will have to weigh the decision between a multi-split and a hidden/central system.
Regarding the room layout:
- Ground floor: Large living room-kitchen-dining room area with presumably >70-90m2, utility room, WC
- Upper floor: Master bedroom with wardrobe and bathroom, 2 bedrooms, additional bathroom
- Attic: 2 office rooms, guest room, bathroom with shower
- Basement: typical standard rooms plus wellness/sauna room plus fitness room (probably >40m2)
- Separate apartment: I am excluding this from consideration, as a single split unit will suffice there.
Without deeper expertise, I expect for the large house two large split systems that cool several rooms – ideally as inconspicuous as possible. The cost will probably end up somewhere under €10,000.
I lack any sense or reference as to how sensible a central system would be instead – plus rough costs and pros/cons compared to split.