Extension of garage & workshop - tips?

I have been running it for a long time. But so far as a mix in different locations (since rental apartment with basement and garage). Both stationary machines and handheld devices are currently being used. I think the room has to be about 5x5x2.2 m. Preferably also two rooms, so that material storage and work area can be separated. 400 V and several 230 V outlets (separate fuses). It is a hobby (non-commercial), but I already do a lot and would rather do more than less or expand with further stationary machines.

Exactly that is the problem: very few advisors recall which individual questions belong to which questioner. Then each individual question is handled separately. Therefore, the squirrel tactic is an own goal. A whole always lives from the interrelationships that make it a structure in the first place.

Yes, I understand that, but the previous questions were independent of each other. Road law/right of way, general financing, garage/shed, home automation basics. They are not yet connected but each is intended to provide basics for the purchase decision or help to realistically confront it early on.

Are there really properties where a basement is prohibited?

Is there really land where a basement is forbidden?

Yes! We are currently buying a plot of land in a new development area. According to the development plan, basements are not allowed in this new development area.

Depends on what you mean. A real trailer?

Yes, a trailer that can be towed by a motor vehicle and is roadworthy. You are allowed to park a trailer on your property without a building permit. It can be a construction trailer, shepherd's wagon, logger's wagon, caravan, or anything else – the main thing is that it is a mobile unit. Of course, you will be able to move your mobile workshop and, if necessary, remove it from the property.

I was wondering whether a shipping container would require approval and if not, how close it could be to the property boundary.

A container is basically also a mobile unit – I consider the chances that such a solution would be approved to be low.

I also don’t want to provoke the neighbors (there might not be any yet), but I don’t want someone to move in eventually who immediately threatens with a lawyer instead of talking to us. We are very easy-going people after all

Good attitude!

Due to lack of definition, you are with the term

Hobbywerkstatt

in a building law gray area. And depending on the brightness setting the caseworker has made on their monitor, the gray appears lighter or darker.
Building law distinguishes between planning law (Building Code) and building regulations law. Since planning law is plot-related, the relevant aspects of your question cannot generally be answered. A specific plot must be known for that. The workshop in the basement is likely unproblematic in terms of both planning and building regulations law.

An "annex house" is probably not a structurally correct term.

From a planning law perspective, it is an ancillary structure according to §14 Land Use Ordinance. From a building regulations law perspective, according to Art.2, paragraph 2, BayBO, a building.

In Bavaria, to my knowledge, garages up to a maximum of 50 sqm do not require building permission.

Correct.

Furthermore, they may be built up to the property boundary with a maximum wall length of 9 m. Is this correct?

Correct, provided the other conditions of Art. 6, paragraph 9 are also met.

However, one is not allowed to build a second one of the same size; if you build two, then the 50 sqm and 9 m apply to both together.

I cannot confirm this sentence because it presupposes many unspecified conditions.

What else may one build directly on the property boundary?

Art. 6, paragraph 9
covered underground garage driveways, elevators to underground garages and buildings without living spaces and fireplaces with an average wall height up to 3m ...

What else would not require a permit?

Art. 57 BayBO

Is a garage allowed to be used as a workshop if you were to build a second one?

That depends... A border garage is not allowed, unless you manage to convince the building authority that the workshop is not a living space. But I consider that hopeless.

whether a shipping container would require a permit

That depends... Planning law, dimensions
Without a development plan, in town, 40-foot container => No; development plan outside building area => Yes, outer area => Yes; 40-foot High Cube => Yes

how close it may be to the property boundary

That depends... Planning law, dimensions, use
Without development plan, 40-foot container, storage room => 0 m; Without development plan, 40-foot container, workshop => 3 m; development plan closed construction, 40-foot container, workshop => 0 m ...

But what is allowed and where could there be trouble?

If a container is permanently installed, it is subject to the same regulations as a solid building.