A potential lawsuit against my construction project is looming! What now?

As mentioned, only legal advice from a lawyer who is familiar with this can help.

I would expect the least help from the building authority now. They are excellent at sitting such things out.

Why should the building authority act as a mediation office? They have enough to do with their actual tasks, especially in urban areas. In my experience, neighborhood relationships do not differ so much city vs. countryside, but more single-family house settlement vs. multi-family house.

Update: After today's appointment at the tree building authority. Maybe also interesting for anyone who finds themselves in a similar situation.

Fact: A building preliminary permit (development application) was set up last year by the former owner of the property so that he could develop the property and sell it as building land or a buildable property. The positive decision (building preliminary permit) or permission to develop the property came about 6 months later. This decision stated that against this, after legal instruction, neighbors could file an objection within 4 weeks. The neighbors were already opposed to construction in advance but would have been rejected in case of an objection. This legal instruction was forgotten to be communicated to the neighbors by the building authority, so the 4-week deadline was automatically extended to 12 months for the objection against the preliminary building permit of the previous year. The then owner subsequently had the property surveyed and developed after the positive decision. A sale of the property followed, which I acquired about 3 months after the positive building preliminary permit from the then owner via a real estate agent and my current developer (as intermediary).

After that, the usual procedure followed: signatures from the developer, notary, property acquisition tax, etc. pp.

After becoming aware of the still possible objection to the preliminary building permit (notably before my ownership), I contacted the building authority to clarify the matter, which I have just described. An objection is therefore still possible; however, currently, there is none and no reason that could cause any form of delay of the construction project or the upcoming building application.

Theoretically, after receiving the building permit, there is again a chance to revoke it (same scheme).

The building authority, however, gave me a kind of all-clear here, since in the case of real problems, everything would proceed retroactively – from the original owner, through the building authority, notary, real estate agent, and so on.

If a neighbor really intends to oppose it and it leads to any (which theoretically is not possible) delay or other cost-related processing, it will get really expensive for the person or persons involved in the event of a counterclaim for damages.

I also learned that it is supposedly an "old dilemma" between the two original neighbors, and that I am essentially carrying it on their behalf.

I will now also introduce myself personally in order to cultivate a healthy and not immediately bad neighborhood relationship, so to speak. Also to contain remaining concerns and possible further complications that could worsen the whole situation.

Regards

A sale of the property followed, which I acquired about 3 months after the positive building permit, with the then owner through a broker and my current developer (as intermediary).

Oh... this too... a tied sale. What about the real estate transfer tax?

The building authority gave me something like an all-clear here, since in the case of real problems, everything would be handled retroactively - from the original owner, through the building authority, notary, realtor, and so on.

You mean a reversal of the purchase contract? I don't believe in that. However, I also do not believe in any reasons that could prevent construction within the framework of the development plan. Because even if there are still some objection periods open, who cares? What matters is whether justified objections can be raised, and I do not see those in a development plan area.

However, the building authority gave me a kind of all-clear here, since in the case of real problems, everything would be done retrospectively – from the original owner, through the building authority, notary, real estate agent, and so on.

*laugh*

There is a development plan and this was also approved by the building authority.

So there actually is no development plan that the property is part of. Otherwise, your previous owner would not have had to apply for a building permit in the first place.