Decision for a heat pump

No, it is not a hairdresser. But it is true that at least 4 hours are spent bathing per day. And that cannot be changed.

I am currently in the process of calculating the heating load using the website heizreport.de and also calling master builders to get a heating load calculation from an independent professional as soon as possible.

Nevertheless, the question is, what does the term modulating mean in heat pumps?

The professional would slightly undersize and leave out the buffer.

Can it happen with undersizing that it then fails to become "as warm as desired" (layman's terms now) in winter?

One much more fundamental question:

Assuming that not only underfloor heating is supposed to heat constantly, but also radiators are supposed to guarantee frost protection and occasionally heat a room and maybe in the future also heat a room constantly,

is a heat pump then generally the means of choice?

As far as I know, a heat pump is designed for constant heat load and does not have fluctuations inside?

If not, which form of heating is then recommended?

Can it happen with undersizing that in winter it does not manage to get "as warm as desired" (expressed loosely now)?

No. The heating load is designed for your standard outdoor temperature - so it can achieve the desired 20-22 degrees indoor temperature at -13 degrees (?) without the use of an electric heater. If in doubt, even more. When is it actually continuously that cold - also during the day - at your place? If that should actually be the case, the installed electric heater would be available as support. So the place will always be warm.

The advantage, however, is that the heat pump fits better at normal temperatures than a larger device.

Under the condition that not only a underfloor heating should heat constantly, but also radiators should guarantee frost protection and occasionally heat up a room and maybe in the future also heat a room constantly

is a heat pump generally the means of choice?

No, it basically is not. But I would not connect the room that should be tempered or occasionally heated up to the heat pump anyway. Because it only delivers a low flow temperature and you gain nothing if you want the room warm now and it will only be warm in 2 hours (and with the small radiator maybe not at all). For these situations, there are other options. IR heating panel for quick heating, for example.

So, not connecting the outbuilding? Can I then basically guarantee frost protection using IR panels? Or would electric convectors be the better option?

Just for your information: I actually already have huge radiators in those rooms (30 cm deep, 2 m long, 1 m high) designed for low-temperature flow. Using these with a digital thermostat set to, for example, 5 degrees, doesn't make sense?

So, not connecting the outbuilding? Can I then basically guarantee frost protection with IR panels? Or would electric convectors be the better option?

If you want to do it with the heat pump, you need a heat pump with two circuits. To what extent the heat pump runs properly (you also have to consider the heating load of the outbuilding) with two such different room temperatures, I cannot judge. Just estimate – and I would say that it doesn't run efficiently. What I still haven’t quite understood is the type of room or outbuilding that needs to be kept frost-free. What exactly is it about, since you keep throwing around different formulations? Room or building? One room or several rooms? Frost-free or sometimes properly heated? What is in the building or the room or the rooms that makes you concerned about this? You could also install a mono air conditioning unit (pure indoor unit with two core drillings to the outside). Then you are independent of the heating system. But as I said, I still don’t quite understand exactly what you really want.