Garden box base plate directly on the boundary or better some distance?

Moin, do you have neighbors? Concrete slab=permanent installation=building permit?=3 meter distance from the boundary.

Yes, that is already approved and may be built on the boundary.

Your "garden box" will probably be a shed or garden house. Presumably, you will want to use the privilege of boundary construction and apparently have not yet used this with a garage. Then you have to build ON the boundary or keep a proper distance. However, rain gutters and roof overhang should still remain on your property.

Yes, exactly. It will be a garden house. As mentioned above, the whole thing has been approved or actually doesn’t need approval since nothing else is built on this boundary and the maximum length of 9m for boundary construction will not be a problem. Did I understand you correctly that with boundary construction you really have to build directly on the boundary and are not allowed to keep 30 cm or any other distance?

State?

In the thread

https://www.hausbau-forum.de/threads/bebauungsplan-garage-auf-grenze-ausserhalb-baufenster.27711

you wrote that you are already planning your garage as a border development - which sides and at what lengths did it end up being?

State?

In the thread

https://www.hausbau-forum.de/threads/bebauungsplan-garage-auf-grenze-ausserhalb-baufenster.27711

you wrote that you are already planning your garage as a boundary development – which sides and what lengths did it end up being?

Baden-Württemberg

The garage from the other thread is on a different boundary/different neighbor and does not concern these two boundaries. As I said, the house, garage, etc. are all already approved as shown on the plan above.

The question now is only whether the garden shed as shown on the plan is really a good idea or whether a few centimeters of distance would be better?

Baden-Württemberg

The garage from the other thread is at a different border/neighbor and does not affect these two borders.

Yes. State Building Code BW:

Setback areas in special cases
(1) Setback areas are not required in front of exterior walls of buildings or
building parts that have a wall height of no more than 1 m. Furthermore,
setback areas are not required in front of exterior walls of buildings or
building parts that contain only garages or ancillary rooms, serve local
supply, or are located on public traffic areas, provided that
1. the wall height does not exceed 3 m and
2. the wall area is not larger than 25 m².
If different heights result at a wall due to the terrain surface, the highest point of the terrain surface shall be used to determine the wall height according to sentence 2 no. 1. The boundary construction along the individual
neighbor borders may not exceed 9 m and a total of 15 m.

The 15 m in total apply to your entire property– and if you have already used more than 6 m for the garage, your shed must be smaller than the 9 m boundary length, no matter what.

As mentioned, the house, garage, etc. are all already approved as shown in the plan above.

Maybe the caseworker did not pay close attention to the measurements and thought of a mobile solution under "garden box," not a garden shed. Or is something else regulated differently in the development plan? This way, any neighbor could potentially cause trouble and report you to the district office.

In total, it is 13m on all boundaries (7m from the garden house already included)... I will have the base slab set directly on the boundary, then it is best/cleanest solved here. Whether the garden house then possibly has to be set slightly inward on the base slab or not, remains to be seen.

ok. If you have no overhang and gutter on the boundary side and position the drain together with the gutter on your inside, there is nothing to prevent you from placing the shed directly on the boundary.