Union construction obligation for building a garage

You have to know the exact circumstances to give useful advice here. Although I don't understand why you are asking here if you have already spoken with the [Bauamt].

Although I don’t understand why you’re asking here when you’ve already spoken to the building authority.

Well, just because it doesn’t work with a setback easement doesn’t mean it can’t work at all ... I only had a relatively short consultation with the building authority and am now trying to research a bit myself before the next appointment.

And now?
What are the exact circumstances?
What exactly do you intend to do?
A garage does not have to comply with setback requirements at first.
Is the maximum permissible wall length exceeded?
Or the average wall height?
Is a roof terrace supposed to be 'on top' or a workshop inside?
Why is your project not privileged in terms of setback areas?
What have you presented to the building authority?
What did they respond or suggest?
Where does the idea of the joint building encumbrance come from?
What does the existing development look like?
The fire protection requirements for small garages are very low.
Why is this a problem here?
Why is it no longer a problem once the boundary is gone?
Is there a development plan?
Are there already building encumbrances?
Site plan?

If you want to do anything with the property, you cannot avoid determining the heirs. Now you even want to enhance it through redevelopment so that the potential heirs might assert higher claims. Or damages. You also cannot sell it. The 6% will always cause you problems.

Ok, then I'll start from a bit further back. So, I already have an approved building permit in principle. The only condition is a setback easement on the property 2), as the 15-meter border construction limit is exceeded. The garage ("simple" garage without terrace or workshop) is supposed to be about 6mx7m in size, and the distances of both side walls as well as the rear wall to the respective nearest property boundary are less than 3 meters, with one of the side walls adjoining property 2). The side entrance door and the window were not part of the building permit inquiry, and the question now is only whether they can basically be installed on the boundary between 1) and 2) or not. There is no development plan for the region, and no further easements are registered either. The proposal for the joint easement came from an architect.

The 6% issue is known to me. However, I can sell my shares separately; this is possible in a tenancy in common.

The only requirement is a set-back easement

So it is after all.

whether you are basically allowed to build this on the boundary between 1) and 2) or not.

If you cannot maintain the 5m fire protection distance to the next protected building, no. Otherwise, free space easement.

The proposal with the unification easement came from an architect.

The unification easement legitimizes the construction of a building across a boundary (§4, paragraph 2, Building Code NRW). It is not suitable for remedying distance violations.

However, I can sell my shares separately; that is possible in a fractional ownership community.

Yes, sure. I can also sell a car without a TÜV.