I know you always want plans, pictures, attachments – the plot designated with the zoning should not be built on, it is not even building land, it is only about the setback distance on the building land, which has nothing to do with the designation.
Ownership structure? I am the owner of one plot and am considering becoming the owner of the other.
You don’t understand the problem in several respects. For plots in public hands, for example, there are different regulations regarding setback distances than for private plots.
The building authority rejected or did not accept a preliminary building inquiry on the grounds that the setback distance was undershot, since I am (not yet) the owner of the garden land.
That is not correct as stated there. Either you did not accurately report the statement of the building authority, or nonsense was told to you by the building authority. In any case, this is not a good basis for a possible property purchase, which may not bring you any or not the desired benefit.
Accordingly, I would have to buy first in order to get a statement from the building authority, which then ultimately results in “no, we do not approve a reduction of the setback distance because there is a designation on it.”
Which only proves that the (non-)registration of the building encumbrance obviously
does not depend on the respective owner of the plot. If your construction is basically approvable, then you can also register the encumbrance on the neighboring plot. If (for whatever reason) that is not possible, acquiring the plot will not help you either.
If you want reliable information on this, there is no way around either uploading plans here or sending them to me or consulting a publicly appointed surveyor and/or architect on site. You should definitely only acquire the garden land after thorough consultation.
Best regards
Dirk Grafe