Heating surfaces for a room with significantly increased heating load

Is the heating load for your rooms in the existing building then also smaller?

Partly extremely lower. My extension consists largely only of windows – so poor insulation values.

For example, the heating load for my living/dining area with over 50 sqm is like this.

That’s exactly why you have the heating system correctly designed. It might cost about 200€ from the engineering office, and you get a heating plan calculated according to your wishes and specifications, with a layout plan for the heating pipes (which you can also specify – from 14/2 to 20/2 pipe) and the respective flow rates to set.

My heating load is required at -11.6°C (97456)

For me, it’s a bit warmer. But that doesn’t make much difference. My calculation was done for 22 degrees room temperature, yours only for 20 degrees (I find that too cold!). That more than compensates.