Buy the second half of a mansard - determine the price

Hello everyone,

we have a condominium in the family that I might want to buy.
It is a two-family house, and the apartment is located on the second floor.
Above this apartment is the attic, which was divided into two separate rooms with a partition wall (several years ago).

For me, the apartment is only interesting if I can use the entire attic. So I would like to buy the owner on the first floor her half.

Question: How do I determine the value of this room to have a rough benchmark before I start the negotiations?

Condition of the attic: Simple room without fittings. PVC floor, bare walls, no roof covering – you can see the roof insulation, no radiators, decommissioned water connection.

Size of the room to be purchased approx. 30 sqm. (Sloped walls have not yet been deducted)

Goal: To move the apartment door to the main staircase so that the second floor and attic are within the apartment and accessible.

I hope I was able to explain the situation reasonably well :-)

Regards from the Saarland

How do I determine the value of this room to have a rough benchmark before entering into the negotiations?

Difficult to say. However, the room is probably worth exactly what the owner wants for it. Supply and demand are relatively manageable here. Have you already dealt with the other factors? Such as building permit, change of use requiring approval, permissible floor area ratio, parking space requirements, co-ownership shares in the condominium owners’ association, approval of the change of use by the association, building regulations, second escape route...