I understand the problem differently. The owner of the apartment will want to use their garden gate, which is also practical to avoid having to carry plants and the like through the apartment. That would also bother me as a parking space owner because I would see a potential risk to my vehicle that would be difficult to prove.
Because I find the topic interesting, I did some googling, all based on the MBO (Model Building Code) and not the corresponding LBOs.
- It could actually be an emergency exit. The house only has 2 upper floors, and therefore climbing up with a "portable ladder" is permitted. For this, the fire department must be able to access the house if necessary. The gate must not be lockable or only opened with a triangular key (but a lock can be seen in the pictures). I could imagine that the middle upper floor/attic apartment is only accessible as a second emergency exit through the garden of the ground floor apartment below.
- If not already considered in the declaration of division, according to a ruling (OLG Hamm, decision of 03.08.2009 – I-15 Wx 288/08) "Individual apartment owners can, in specific cases, establish a usage regulation by a majority vote, according to which a common area standing under the special usage right of a single apartment owner can be used as an escape route in an emergency.".
- As an indication that a parking space is not suitable as access to an emergency exit and may not simply lead over it, the ruling of the Wuppertal District Court (judgment of 15.07.2015 – 91b C 25/15) can be considered. "The problem was that after the city imposed the requirement to keep an emergency exit constantly clear, a parking space was practically no longer usable. An architect was therefore commissioned to reallocate the parking spaces."
Good advice will probably only be obtained from a specialist lawyer for building law (and an expert, etc.). Or by notifying the city, with the probable consequence of losing this parking space (and then receiving an indefinite compensation). Alternatively, to get the developer to install a device so that this gate can only be used as an emergency exit (e.g., connected to an alarm). The neighbor probably does not use it very often to bring their soil into the garden. But it is your right that this is only (!) an emergency exit and is not used daily.