Heat pump capacity and buffer tank size(s) for new build single-family house

You won't achieve gigantic temperature differences anyway in new buildings. A hydraulic balancing is done, i.e., through the flow rate and the control of the heat pump, the rooms are brought to the desired temperature level once and the setting is left like that.

What does the approach look like?
I only see floor plans instead of room-specific heating load calculations. ;-)

MH ok, so I do imagine different temperatures. Especially in a still unused children's room or the not always used but heated basement rooms.

What do you imagine? 2-3 degrees (realistic) or maybe 7-8 degrees (never ever)?

In a thermos flask, you also don’t have cold beer at the bottom and hot tea above it. Everything is equally warm because it is super well insulated from the outside. The temperatures just have to approach each other. Physically, it can’t be otherwise. In an uninsulated or poorly insulated old building, of course, it would be different.

A little is of course possible, but not door to door 5K difference. In addition, every (tightly) installed heating circuit is worth its weight in gold to bring the heat into the rooms with as low a flow temperature as possible (= cost-effective). Ultimately, the rooms with a running heating system heat the unheated ones as well.

Basement "cold", not in the new building. Moisture must be removed there, especially in the first years. Please heat, continuously!