Property division - house sale + house construction on a new plot

That is merely a shed and not living space.

Regardless of whether it is a garage or shed. It is an outbuilding – I said nothing about a living area and the state building code also does not have a 9-/12-meter regulation.

If construction in the second row is permitted, then in my opinion it is acceptable.

I have adjusted the spacing rule according to the state building code BW. BY 3m BW 2.5m

I have once adapted to the spacing rule of the Landesbauordnung BW. BY 3m BW 2.5m

Thank you very much! Basically, I could imagine it like this. Then I will inquire with the municipality whether development in the second row is permitted.

I have adapted it once to the distance rule of the state building code BW. BY 3m BW 2.5m

One more question: Would it be allowed for me to move the property boundary to the right of the old house slightly to the left, so that I have a somewhat larger garden on my "new" property?

One more question: Would it be allowed for me to move the property boundary to the left of the old house a bit, so that I have a slightly larger garden on my "new" property?

The State Building Code BW says a minimum distance of 2.5 m. You can ask the building authority about that. But I see no reason why the distance should not be maintained.