what do you understand by finishing the rooms? the plaster on the wall and the electrical installation are included anyway in a turnkey delivery. what is missing are a few wallpapers and the flooring. for the children's room with 15 sqm area, that is maybe 1000€ with parquet flooring, wallpaper, etc. done by yourself. that does not make a big difference. the interior doors are also not very expensive. you can save money on the exterior (carport/garage, garden, access path, etc.), which can be done later. or you can save on big expensive extras like underfloor heating, ventilation system, etc... or on building-technical items like size (about 1500€ less per sqm) or additions like dormer or basement. if it is only the basement because of 1 room, it would be cheaper to include this room on the ground floor. the dormer is certainly great in the house, but it must also serve a real purpose. often the stairway is there and the dormer is wasted space. the dormer is often a eye-catcher on these houses (also looks really great) and makes the house into a captain’s house (or with sharp angles into a Frisian house). maybe you should just tell what you want to build at all. size, important extras, plot? etc... more input