Gas heating capacity selection 20kW instead of 14kW

Hello,
we have a 14 kW Wolf gas-solar heating system planned for our place (120sqm living area with underfloor heating, 60sqm basement area hallway with standalone heaters). We are considering installing a standalone heater in the basement, which is fully insulated, for two rooms. However, the plumbing company believes that the 14 kW heating system is not sufficient for this and wants a 20 kW system.
I find this a bit confusing and also think I have read that oversizing can lead to higher operating costs and wear (among other things due to the lower limit of performance in modulation).
I have currently rejected this approach. Presumably, one should have the necessary heating load calculated instead of "just doing it". But I think it is exaggerated to increase the heating capacity just because of two standalone heaters.
Is there an opinion on this here in the forum?

Yes, there is. Definitely install the smallest heater available. I guess maybe 7 kW heating load at NAT. Equally important is that it can modulate as low as possible.

If by stand-alone heater = radiator is meant, then with regard to the underfloor heating installed throughout the house, I would focus on ensuring that the supply temperature in the house does not need to be increased because of the two additional basement rooms with radiators.

To achieve this, it might be considered to size the radiators in the basement rooms and basement hallway larger so that they can be operated at lower supply temperatures. Then it might possibly be possible to do without a mixer for two different temperature levels radiator/underfloor heating.

What speaks against a uniform planning with underfloor heating also in the basement? Or alternatively wall heating?

Based on the information provided, I also see no reason why 14kW should not continue to be more than sufficient. I assume it is a single-family house in planning, so new construction? 20kW is enough for a modern multi-family house.

If by standby heater you mean radiators, then with regard to the underfloor heating installed throughout the house, I would focus on ensuring that the supply temperature in the house does not need to be increased due to the two additional basement rooms with radiators.

To achieve this, it might be considered to size the radiators in the basement rooms and basement hallway larger so that they can operate at lower supply temperatures. Then it might be possible to do without a mixer for two different temperature levels radiator/underfloor heating.

What speaks against a uniform design with underfloor heating also in the basement? Or alternatively wall heating?

Based on the information provided, I also see no reason why 14 kW should not continue to be more than sufficient. I assume that this is a single-family house in planning, so new construction? 20 kW is sufficient for a modern multi-family house.

Yes, I mean radiators. And yes, new construction and semi-detached house. The basement is completely insulated (under the slab and on the exterior walls).

The uniform design with underfloor heating is simply not included in the construction and performance description, as it is a utility basement. I don’t even want to know how expensive that would be... Only the hallway must be heated due to the energy saving regulation, which is why there is a radiator there. I had never heard of wall heating until now. Interesting. But we want a very cost-effective solution, as the rooms are at least currently not intended for permanent occupancy (but mostly only for a few hours).

If the radiator is only for the purpose of the energy saving regulation, then take an inexpensive one and that’s fine. If you want it warm in a basement room once in a while, you just have to use a more expensive heater in terms of operating costs.

Heating power greater than 14kW is absolute nonsense.

Just as a comparison:

I live in an old building, ceiling well insulated, walls moderately insulated, windows mostly from 1994. At -12 degrees outside temperature, my 130sqm house requires 7.5kW of heating power.