Bypass building boundary through land extension instead of division

my sister and I want to build a semi-detached house there. According to my current understanding, there is a building boundary of 3 meters on both sides of the property that must be observed (also at the front and back, but this is not relevant to the question and is therefore not marked). If a semi-detached house is then built in the middle, a garage may be placed on one of the two building boundaries, the other side must remain free.

Your current understanding will have to be expanded several times. I’ll start with the smallest misconception: You are allowed to place garages simultaneously on both side building lines. Then the popular belief about the three-meter flat rate is incorrect; usually, the correct rule is 0.4 times the building height or at least 3 meters (BW and BY deviate the most from this rule with their own regulations). But now to the specific problems for your property: First, you cannot simply make a division behind your parents’ garage, even if there is currently a parcel boundary there. The parental building must still comply with the floor area ratio (here 0.4). This is not entirely certain here; one difficulty lies in the currently assumed (and required for the buildability of 150/2) easement of 150/1 with a right of way, drive, and utility for 150/2 and 150/3 (which 150/2 would also have to take over for 150/2, unless 150/3 is accessed in the future via Bremthaler street). Even the access of 150/1 via the dead-end road 149 suggests that access of 150/1 to /3 via the former Goethestraße (Rudolf-Dietz-Straße) is not desired. At least, I see no indication that a semi-detached house on 150/2 would be opposed. Are you sure—given the degree of development this could well be—that the development plan from the time your parents’ house was built is still valid?

Oh, you mean that you want to build on your parents’ property? Briefly put: the owner of the house is the one who owns the land. No buts, no exceptions.

There is also the heritable building right, but we probably don’t need to open that can of worms here (for those interested see the pianists thread – warning, big can).

According to the information I have had so far, there is no such thing as "obtaining permission," but the regulations are fixed and cannot be circumvented.

Yes, "approval" is not enough; the keyword for forum search is "taking over of setback areas." However, I do not currently see this as necessary here; you can post a development draft for discussion.

I start with the smallest misconception:

Of course with the smallestn misconception.

In NRW, a [Vereinigungsbaulast] can be registered. Then two plots of land are treated as one in terms of building regulations.

Generally, a total of 15 meters of boundary construction with garage is allowed, maximum 9 meters on one side.

But that should mean something like a 7m long garage on each side is already possible, if I understand correctly.

First, you cannot simply make a cut behind the parents' garage, even if there is currently a parcel boundary there. The parents' building must still comply with the floor area ratio (here 0.4).

I don’t quite understand that at the moment. If I understood correctly, these are already two properties, only both belong to the parents? Then nothing changes for the property with the parents' building.

First of all, I join in asking about the floor area ratio and its compliance for the resulting parents' property? However, I (still) do not understand why this should be a question of development or easements.

2 questions would also torment me:

[*]Are semi-detached houses even allowed? and
[*]Is a perimeter development absolutely necessary with this size of the property? It seems to me that there would be enough space to do without it, if only to be freer in the size of the buildings.

even if a parcel boundary currently runs there. The parental development must continue to comply with the floor space index (here 0.4). This is not entirely certain here,

Floor space index and its compliance for the resulting parental property?

Why are you worrying about the parents' independent property? It is and remains. Nothing more and nothing less. The floor space index of their property is not needed at all.

to bequeath a property that borders directly on theirs.