Stock is on the property boundary. Consequences?

What don’t you understand? I can place a garage directly on the property boundary.

A house, even if the bb allows it, and what a building boundary is should be known.

I don't see anything about illegal construction. I see a copy of an official site plan - isn't that an indication that everything is lawful?

Was the house subsequently renovated on the exterior walls? Insulation added, so that the 3-meter distance was reduced by this additional thickness?

No, it has been like this for over 50 years and is marked as such on the city's site plan. So everything should be fine, I think. As far as I understand it now :)

I can place a garage directly on the property boundary.

That is true. Your first statement

Garages do not necessarily require building boundaries

does not say anything, therefore my question what you meant by that.

A house even if the building boundary allows it

But the house of the original poster is not located exactly on the boundary, but at a distance that is smaller than the minimum distance required by state law (which would still have to be proven). I have never seen such a determination.

and one should know what a building boundary is.

I know that and precisely for that reason I do not understand what you mean with

building boundaries may also be less than 3 meters.

I see a copy of an official site plan - isn’t that an indication that everything is legal?

No. A building can be recorded in the cadastre in different ways. This ranges from qualified building surveying through field comparison to aerial photo evaluation. Therefore, the true boundary distance cannot be extracted from the cadastral map without knowledge of the coordinate accuracy. And even if an undershoot of the minimum distance is determined during a building survey, usually no notification is sent to the building authority.

I do not read anything about an illegal building.

But the original poster asks the question

Or could the neighbor say I have to tear down my garage now?

This question would not make sense if he did not fear that the previous owner had done something illegal on the property.

So everything should be fine, I think.

I think so too. If the new owner has something to complain about or you plan changes to your house, you can open a new thread.

I do not know under what conditions the 3 meter building boundary may be undercut "municipal council resolution perhaps?". In Baden-Württemberg, for example, 2.5 meters are allowed in some new development areas. Therefore, he should check in the building code if there is one what applies to him and not worry about illegal construction right away.

well, since the neighbor doesn't respect his 3 meters either, there will never be any problems there. the 3 meters are ultimately a civil matter. as long as all relevant neighbors agree, you can basically do whatever you want on the boundary. you could even attach a garage of 30 by 30 meters if the neighbor agrees (building burden). the 3-meter boundary construction falls under a different category. however, the house is in the map, which means it was measured and everything is correct. otherwise: don't cause a fuss!