Still considering gas heating for the new building?

So you can design the underfloor heating so that you want 23° in the bathroom, 18° in the bedroom, 21° in the other rooms, and 20° in the kitchen? That would be very bad for resale because that is a very individual matter

You adjust all rooms once at the heating circuit distributor. The adjustment due to weather then takes place centrally at the heater via the parameters and heating curves set there.

As already mentioned, it is either direct sunlight or there is something wrong with the insulation. The underfloor heating is slower to warm up than radiators, but hardly slower to switch off. Off is off in both cases, and neither system provides active cooling. Both systems heat floors and walls, which then continue to emit heat afterwards, with the underfloor heating doing so just a bit more.
Insulation plays a much bigger role in the inertia towards cooling. A modern house hardly loses any heat and therefore takes ages to cool down. Even if it suddenly drops below freezing outside, the slowest underfloor heating still balances out faster than the house cools down. Anyone who suddenly loses a few degrees during the day has a problem with the insulation!
The same applies in summer. If it gets warmer outside, the outdoor sensor regulates this and the heating is off before the heat can enter the house. If heat comes directly as sunlight through the windows, that naturally warms the house strongly, independently of the heating. Shading is the solution here.

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It will be very difficult to understand a proper installation and correct planning of the underfloor heating. And in case of doubt, it will certainly not be easier to claim the warranty and provide the corresponding evidence.

Since I am just reading about the outdoor sensor. It is attached on the north side and therefore tends to perceive cooler temperatures.

I can confirm the 3 hours of inertia. It does not take a whole day or 24 hours. I see warming in the warming of the rooms by sunlight, not by the heating. It should rather switch off at higher temperatures.
In short, regards

how do you manage 18°C in the bedroom? we don’t even manage that in winter, when the heating is always set to frost protection there. new houses are simply too heavily insulated. 20°C in the hallway is enough for the bedroom to easily hit 18°C - of course with the door closed.

Yes, that is why the heating load should be calculated room by room and not estimated. Then the underfloor heating can be planned depending on the desired room temperature. Hardly feasible as a layperson. That’s what the professionals are for. An individual room control is required by the energy saving ordinance as far as I know, but it is not necessary if planned correctly. Everything here is designed and set up so that the heating always delivers the flow temperature needed to maintain the room temperature. It’s hard to explain in a post. By the way, I did the same with radiators at my place. The thermostats are only used when I light the fireplace in winter. Otherwise, everything runs smoothly. Cooling with a heat pump probably works best with a ceiling heating and of course is not comparable to air conditioning. You might be able to achieve different temperatures if you insulate the bedroom towards other rooms. High bathroom temperatures often require a wall heating.