Expansion of the attic in a single-family house in the outer district

which laws apply to us

As with all builders, the Building Code (Baugesetzbuch) applies to you in terms of planning law. If it is actually an outer area, §35 must be applied. To be allowed to live there at all, you would have to meet some requirement mentioned there (e.g., running a horticultural business). An indication of whether it is an outer area can be given by the land use plan, which may be accessible online. If you want to be sure, you cannot avoid going to the building authority/planning office. Reading through the paragraph makes it clear that the permissibility under planning law depends on so many individual factors that can hardly be clarified here. In terms of building regulations, your project is, IMHO, unproblematic, since according to the state building code it should be free of procedure, so obtaining a building permit is not required: § 60 Procedural-free construction measures, demolition notification ... (2) Also free of procedure are ... 2. the change of use of rooms in the attic of a residential building with only one dwelling into living rooms belonging to this dwelling You might want to ask your architect friend if he sees it the same way.