Landscape conservation area
... (almost) always means outer area. Construction measures – including reconstruction – are then subject to planning law under §35 of the Building Code. Even the acquisition of property by non-privileged professional groups can be inadmissible. Destruction by fire is actually unproblematic (see paragraph 4, 3.), however, the application for a new building could raise concerns about the previous permissibility, which the owner must then dispel. The conclusive documents should then not have been destroyed.
Would that then be like a compulsory expropriation?
Possible, e.g. §35 paragraph 5 sentence 2 Building Code:
For projects according to paragraph 1 numbers 2 to 6, as a further requirement for permissibility, a declaration of obligation must be given to dismantle the project after permanent cessation of the permissible use and to remove soil sealing
Is it also true that the state always has a right of first refusal?
No. However, there are various legal bases that can establish a right of first refusal, e.g. §§24, 25 Building Code or §66 BNatSchG
Before purchasing a property in the outer area, one should thoroughly inform oneself about the requirements and conditions, i.e. at least visit the building permit authority, the nature conservation authority, and the land registry office.