Hello Passive House,
go to the building authority and ask whether the defect can be remedied by a waiver of building obligations and/or neighbor approval. Alternatively, you can ask an architect/surveyor who is authorized to create the relevant plans in Saarland.
Basically, the situation should be such that you still have space within the building envelope, on which you relinquish outside for the benefit of the conservatory. Hence the name waiver of building obligations.
In NRW, I could give you a reliable answer, but in Saarland the regulations may differ, so ask an architect, surveyor and/or building authority. You can save yourself a paid construction preliminary inquiry. If a building application is required for the conservatory, you will need an architect anyway.
Best regards Dirk Grafe