Bypass building boundary through land extension instead of division

I don’t quite understand that right now. If I understood correctly, these are already two plots, only both belong to the parents? Then nothing changes for the plot with the parental building.

First of all, I agree with @11ant on the question of the floor space index and its compliance for the resulting plot of the parents? However, I (still) don’t understand why this should be a question of development or easements.

We don’t know whether the non-exceedance of the floor space index of the parental plot (whose residential house including garage stands on parcel 150/1) has so far been assessed per plot (150/1 and /2 assessed together) or per parcel. In the worst case, this will be re-examined during a building application (for 150/2 or for a combined plot from 150/1 and /2). Apparently, the residential building on 150/1 complies with the floor space index – but does this also apply when counting the sealing of an access corridor of a floor space corridor? 150/1 and the workshop plot 150/2 apparently have no access roads to the main route of the former Goethestraße. At least with different owners, 150/2 could be denied building rights for lack of development, be required to grant a formal floor space right with the servient plot 150/1, or the question could arise whether to allow access from the main route of the former Goethestraße. We don’t know, neither do I, but the cul-de-sac 149 won’t be there for no reason.

Two questions would also torment me:
1. Are semi-detached houses even allowed? and
2. Is a border development absolutely necessary given the size of the plot?
It seems to me there would be enough space to manage without it, if only to have more freedom in the size of the buildings.

1. I see no exclusion of "D" or a specification of "E".
2. No, the original poster probably hypothetically wants to explore the maximum house building window and for this purpose assign the “lost” width of the side building line to use with privileged ancillary buildings. In this sense, his calculation is “plot width minus three meters on each side equals the sum of the widths of his own half and his sister’s half”.

apparently no access roads to the main route of the former Goethestraße.

It should be der former Goethestraße - what paradisical conditions those were when we no longer had an emperor for a long time, but at least could still correct a typo for a full ten minutes.

no, the OP probably wants to hypothetically explore the maximum house building window

I don't read that from the question. The OP wants two boundary constructions for the garages and is now raising a side question/problem that doesn't have to exist, because two garages would be permitted according to building law. open construction method, so 50 meters of house front are allowed, etc. The only thing that irritates me is that no one has taken advantage of it yet. Either it wasn't necessary and the residents planned well or there is the rule to generate a driveway on only one side. But I don't read about any such restriction. Therefore, I would make a simple preliminary building inquiry for the duplex: a garage on the left and right, possibly even with a bold extension to 2 x 9 meters for an additional building. The municipality may be keen to rejuvenate the old area a bit.

The only thing that confuses me is that no one has exploited it yet.

I correct myself: the development plan only shows suggestions on how to build between the existing buildings (grey). If you compare some areas on Google Maps with the site plan, you will notice that the owners/builders at that time took many liberties regarding the garages – among other things, carports were built facing the street or the garage was placed behind the building line. The development plan shows otherwise. The existing buildings even have garages built behind the building line. Nevertheless: The shed that will be demolished is already quite large and can be used as a visual placeholder when planning the semi-detached house. A boundary garage should also be possible for the neighbor (corner of Bremthaler Str).

The cul-de-sac 149 mentioned by has been extended, and the old building in the middle has been demolished and rebuilt.

It would also be nice if the OP would address his question and answer any open questions here.