Development plan found... opened it and... yuck! It's really fun to look for the street boundary line on a black-and-white drawing haha.
I would say you were extremely lucky to be able to build the garage in that size there. Elsewhere, you probably would have had to make the garage a bit smaller ^^
The height of the garage is a "problem" in Schleswig-Holstein, but now I know that it is in Lower Saxony, where nothing is permit-free from 30 sqm, regardless of the height.
The height now only matters because of the setback area - even if you own the neighboring property. With the joint building easement, the building authority now wants to achieve that both properties are legally considered as one - instead of an ideal separation, an ideal unification? Phew, interesting, with far-reaching consequences.
A setback area easement - if it exists in NI - would be the better way. You have a floor area ratio and it is not huge. Now your two properties are always considered together, which is nice for now.
If, for example, you do more on the existing property (parcel 30/155) in terms of sealing and someday something happens and you might have to sell a property, then you might no longer be able to separate the properties because the floor area ratio could be unevenly distributed.
That, in turn, leads to a significant depreciation - after all, the other is always dependent on how much has already been sealed on the other parcel. First come, first served. I know no one ever plans for that - I just unfortunately have some standing in the office from time to time.