Is border construction over 9 meters allowed in Saxony?

legally secured means building encumbrance. Because in the event of the neighbor's sale, your "contract" would suddenly be worth nothing.

Maybe a grandfathering clause could apply in such cases?

Would it be advisable to consult the building authority?

Yes.

legally secured means building encumbrance

Do you have evidence for that? On the website of the Saxon State Chancellery it says: This happens, for example, through the registration of a corresponding building encumbrance in the register of building encumbrances. The word "example" implies IMHO that there are other possibilities as well. Besides, it would be incomprehensible why Saxony has not adopted the wording from the MBO - as most other federal states have.

Maybe legal protection of existing rights could apply in such cases?

No.

legally secured means building encumbrance. Because in the event of a sale by the neighbor, your "contract" would suddenly be worth nothing.

why? you can write something like: I, Egon, and all my legal successors agree. That means the one who buys from Egon buys his consent as well.

The word "example" implies IMHO that there are other possibilities as well.

alternatively, I could imagine a security in the land register.

why? one can simply write something like: I, egon, and all my legal successors agree. i.e. whoever buys from egon buys his consent as well

But the legal successor does not have to care about that, so it is not a legal security (i.e. effective against unspecified third parties). The legal successor has no contract with the original poster. Of course, the neighbor could make that a condition of the sale and have it included in the notarial contract. But even then, he cannot be forced to do so. That would be unfair and possibly liable for damages, but it destroys the legal security of the construction-law-compliant condition of the garage.

The legal successor does not have a contract with the original poster.

He doesn’t need one either, because the legal successor encounters the excessively long boundary construction - according to the motto: bought as seen. However, the builder should not lose the proof, so that he can always demonstrate the legality. In NRW there was a change in 2006 concerning the attachment security. At that time, the word "öffentlich" was also removed. In the explanations of the changes it says: No more requirements are made as to how the mutual boundary attachment must be secured. The security can be made at the choice of the builder by registering a building encumbrance, entry in the land register, or also by contractual agreement between neighbors. Corresponding declarations of obligation by the owners of both properties on the building documents are sufficient. Maybe the Saxons have something similar in mind with their wording.

He doesn’t need to, because the legal successor finds the excessively long boundary construction as is – following the motto: bought as seen. However, the builder should not neglect the proof, so that he can always demonstrate legality.

In NRW there was a change in 2006 regarding the securing of attachments. At that time, the word "öffentlich" was also removed. In the explanations regarding the changes it states: No more requirements are made as to how the mutual boundary attachment is to be secured. The security can be carried out at the choice of the builder through the registration of a building encumbrance, land register entry, but also through a contractual agreement between neighbors. Corresponding declarations of obligation by the owners of both properties on the building documents are sufficient. Maybe the Saxons have something similar in mind with their wording.

I cannot contribute anything about Saxony, but here in Hesse it would have sufficed if the neighbors had signed the exceedance on the drawings/plans/elevations attached to the building application. The only important thing here would have been that ALL neighbors must sign who would have had buildings of ours standing on the boundary. Because ultimately, it cannot be said who has the relevant meters too much standing on their boundary.

In the end it did not come to that, one neighbor was opposed and refused to sign. So we planned the garage one meter shorter and with that it was approvable without signatures.