You should definitely go to the architect, who of course will cost money. But right now, I don't see an optimal living value... You saw it yourself: in your plan, the kitchen unit would have to be moved forward in order to work there at all. The living room is more like a cramped seating area with empty space in between, you can't stand at the desks, the toilet would also have to be moved forward, but then the washbasin won't fit. The architect can calculate costs for you, maybe an extension with a bay window or a dormer could increase the living space. The question that arises for me: does it really make sense to convert an attic of about 65 sqm of living space (I counted the roof slopes at half), or should you maybe look for another (somewhat larger or without slopes) apartment before burning money here just because an attic exists in the family? Also the ownership question: who will own the attic, who will invest the money and labor? Legally it will still belong to the father-in-law. Who will pay for the materials? Him or you? And what role remains for you? Edit: It still has to be less than 65 sqm, since the walls and stairway still have to be deducted :(