House construction in the Ruhr area on existing land

does not calculate 350K€,
but what is the current rent,
how much do you easily save per year,
what is the monthly payment after financing when living in the new house.
Whether the number XXXK€ is there only causes sleepless nights

350000, and the existing plot.. with possible 120/130sqm you can’t build the impossible 160sqm. A double garage is also not possible, so there is the affordable carport. Parking in your street for the second car? It looks pretty sparse there at your place, or is there a parking ban?
You also save on plastering one side, that’s great :)
You get construction power from your parents, saving the expensive commercial electricity...
Room sizes you can see at the parents’. However, I would certainly plan some things more modern, because inside you are quite free.
We are still here too ;)

don’t calculate 350K€,
but what is the current rent,
how much do you easily save per year,
what is the monthly payment for living in the new house after financing.
Whether the figure XXXK€ is there only causes sleepless nights

We currently live in a condominium. The financing is basically arranged for us as well (we had always been searching for existing properties). We know what we can pay monthly. So these points are largely clarified.

350,000, and the existing plot... with possible 120/130 sqm you can't build an impossible 160 sqm. A double garage is also not possible, so there's the affordable carport. Parking on your street for the second car? It looks quite sparse there at your place, or is there a parking ban?
You also save on plastering one side, that's great :)
You get construction power from your parents, saving the expensive commercial electricity...
You see room sizes at the parents'. But I would certainly plan some things more modernly, because inside you're quite free.
We're still around too ;)

So officially you're not supposed to park on the side because of garbage collection. But we certainly haven't had any problems there for over 10 years and the neighborhood has lived there for several years longer and there were never any problems. So even if we didn't have a garage or something similar, I wouldn't worry at all about parking at least.

I understood it to mean that the house including the terrace must end at the blue line

You understood that correctly.

Two row houses would still fit on it

... if there were no development plan.

whether the terrace has to be within the building window

Yes, it must.

to me it sounded more like the house basically has to look the same in principle as my parents' house.

I also consider that very likely. Unfortunately, the supplementary provisions mentioned in the development plan are not published.

How large the remaining building window actually is.

9.5m * 12m

9.5m * 12m

Nice that you are here :) With 9.5 x 12 I would try to build 7.5 or 8 x 12, creating a recess on the ground floor on the west side of the building volume by one meter (1.5 meters), then a 3-meter terrace on the west there. Possibly extend this a bit into the garden behind bushes. That's how I would start planning. Whether it turns out like that is another matter.