Buy land with cash, construction through KfW/NRW Bank

What does the property belong to? I would say: WA 5. Means 0.5 floor area ratio, terraced housing, one residential unit.

It is WA1 (visible to the right next to the continuing row) - WA5 is a multi-family house

It is WA1 (seen to the right next to the continuing row) - WA5 is a multi-family house

If that is the case, you need to consider whether 100 sqm will be enough. Personally, that would be too little for me, my open space is already over 50 sqm.

I have to correct myself by correcting you

The maximum development is 0.4 floor space index.

It is the site coverage ratio according to the legend. The site coverage ratio. I also always have to think twice because of the F in Fläche.

So you can build 102 sqm. Net, that is about 85 sqm of living space. But the 102 sqm also includes the terrace.
For garage and co you are allowed to exceed.

If that's the case, you need to consider whether 100sqm will be enough for you. Personally, that would be too little for me, my open space already has over 50sqm.

But the legend says floor area ratio, not gross floor area ratio. So theoretically, shouldn't it be a maximum of 204sqm if you don't consider the rest? Realistically, it would probably be 120sqm living space (so about ~68sqm floor area?), if the garage (+ parking space in front) takes up about 11x3.5m of these 14x9m. Currently, we are at 180sqm, but 100sqm of that is open space living room/kitchen/dining area alone and most of it is empty. We actually want 2 separate offices, not very large, a "guest room" (+ multipurpose storage), 1 bathroom with a large walk-in shower, no bathtubs, 1 toilet downstairs, bedroom and living room + kitchen can be open, with a dining table for 4 people in between. In the garden, a 2m fence (only allowed on the side), later a pool (no construction, but normal freestanding finished) and possibly an electric sauna, that's all the wishes.

We actually want 2 separate offices, not particularly large, a "guest room" (+ multipurpose storage), 1 bathroom with a large walk-in shower, no bathtubs, 1 toilet downstairs, bedroom and living room + kitchen can be open plan, with a dining table for 4 in between. In the garden, a 2m fence (allowed only on the side), later a pool (not built, but a normal free-standing one) and possibly an electric sauna, that's all the wishes.

The classic townhouse/semi-detached house variant comes into consideration: the constantly planned 2 children's rooms will be your offices. I don't see an annex apartment at all and won't go into that topic anymore. How to finance it now, I'm out. This is a financing question here.

If we calculate with 450k,

Realistically, it would probably be 120 sqm living space

Yes, because 120 sqm living space costs about €360k plus additional construction costs, garage and outdoor facilities.