Interpretation of the 1957 development plan <-> possibilities for new construction

Of course not :)
That's why I'm asking here... I want to learn.

The problem, I think, is that those who are familiar here with building lines, floor area ratio, etc., cannot say anything definitive about your ancient development plan. Exceptions may play a role, which only your building authority can tell you or your architect for sure.
Even if the building authority is on vacation, that does not change this fact.
Basically, as long as you comply with all the requirements of your development plan, which remains unknown here, you can exhaust the floor area ratio on your property with your house, whether with a house alone or a house with an extension or a house with a subordinate outbuilding, etc., as long as it or details are not explicitly prohibited.
Also, read and understand your state building code again.
The "I know that..." leads to guessing, since one does not know what your knowledge refers to.
Given your uncertainty, I advise you to consult an architect who can professionally answer your questions and, if necessary, give you advice because he can (hopefully) recite the state building code by heart and also apply it to your property.
I can imagine that you can definitively clarify your questions with him within an hour, whereas here they can only be vaguely touched upon over days and weeks.

My inquiry was not about the building envelope, the development plan, or a boundary development. Rather, it concerned an outbuilding. It does not need to be privileged concerning boundary development. However, it would violate the setback area of the house.

My "knowledge" was based on the BayBO. 3) The setback areas must not overlap; this does not apply to [...] 3. Buildings and other structures that are permitted in the setback areas.

These buildings are also specified: (7) In the setback areas as well as without their own setback areas, even if they are not built on the property boundary, the following are permitted: 1. Garages [...] buildings without living rooms [...] 2. building-independent solar systems [...] 3. retaining walls [...]

No other buildings are mentioned there. Since I assume (!) that the workshop is a living space, this specification is missing in the BayBO setback area.

I also could not find any reference to an extension to a house. Therefore, I had hoped to find a hint here on how to classify a workshop building now.

However, I can also understand that due to the old development plan, too many question marks remain, and one cannot simply provide further assistance here. Nevertheless, I did not want to end the thread completely without comment.

Best regards FitoCari

Your hobby workshop would legally be considered a living space and not border-privileged. Setback areas, which must not overlap, usually refer to those of the "opposing" neighbor in the sense of boundary adjacency. For example, it does not have to be six meters between your house and your garage if you do not place them directly adjacent to each other. By the way, as a newbie, you cannot be held liable yet; under "Information" in my profile, you will find a way out of this.

Am I allowed to place a hobby workshop shed... etc. anywhere within the building area?

From a planning law perspective, yes.

As far as I know, distance areas are not allowed to overlap.

Correct, but
... this does not apply to
1. exterior walls that form an angle of more than 75 degrees to each other, ... (Art. 6 para. 3 BayBO)
therefore, I cannot infer from your

In the sketch, the house (blue) and the workshop (green) would slightly overlap.

your sketch implies that.

Does the annex have to have direct access to the house?

No.

Can it be completely independent?

Yes.

Does the annex have to be built together with the house?

No.

Could it also be built later on?

Yes.

New building permit?

Your decision.

What’s been bothering me a bit now is the basement question. Of course, I have already read the 11ant basement rule. But I don’t think I understand it. In the case of a hillside house, I can follow it. But does it mean that in the case of a (pot) flat plot you would never build a basement, right? But rather enlarge the footprint of the house and place the classic utility room on the ground floor? How would it be if there is already a hole from the demolished existing house? I don’t actually see the basement there as usable. Not deep enough. And the structure unfortunately isn’t that fresh anymore. But the area would roughly be what we imagine. [ / ] How is the calculation derived from this now, whether a full basement “pays off” compared to relocating the area to GF + UF? […] The soil is apparently very clayey. That means everywhere you want to build, the soil must be replaced. [ / ] The new building 7x11 would lie almost congruently top left/top plan with the existing building. That a) defines the building line on the left side of the plan and we want to go as far north as possible to have more garden area in the south.

Thanks for the impulse to expand the basement series by the aspects of existing basement and deep demolition. If I understand correctly, you have a basement with a typical basement height of that time, and also contemporary beneath a raised ground floor that lies about 80 cm above the ground. If you now wanted to avoid the deep demolition and still use the existing basement because of the house connections and as a storage room, but put a modern single-level ground floor on top, you would presumably create a crawl space about one-twenty (approx. 120 cm) “high” and probably wouldn’t reuse the old basement ceiling. In any case, the overall measure would be decisively too complex to sensibly apply the two 11ant basement rules universally. I would decidedly prefer to accompany this personally rather than by anonymous remote diagnosis. I don’t know when and where you read the rules; the Baul…ann no longer exists under the old name in any case, and “With or without basement: a rule as a decision tool” was only published in September 2023. You’d probably best choose the dialog approach there. How is the “new building 7x11” coming about? As far as one can vaguely speculate remotely here, I would probably tend toward a deep demolition but with a full basement only within the footprint framework of the existing basement and a non-basement “annex” / extension. But as said, this borders on clairvoyance and would require a serious diagnosis. Currently, I only have time for preliminary talks in Q1/25, but after more than three years of pause, a minute more or less probably no longer matters.

Hello 11ant,

please excuse the late response.
Thank you very much for your assessment of the deep demolition within the framework of the cellar rule.
You have helped us a lot with that.

You are right, it no longer matters down to the minute.
I hope that the "preparatory work" will be completed soon, and that we can enter into a dialogue with the construction company this year.

Best regards
Tobias