Question about expanding a single-family house into a two-family house

Expansion is approved by the building authority

So ... the last few weeks have been a bit stressful. We have gathered some offers (developer, installer, electrician, windows, etc.) I have attached a screenshot of the current approximate plan of what we want (red = new building, white = old building) The entrance area will be done by ourselves. It will be on the north side by the staircase (landing including stairs) The attic of the new building should be connected with and usable with the old building. Tomorrow we have another appointment regarding an offer from a developer. After that, we will decide fairly quickly with whom we want to build. We are currently leaning strongly towards a solid wood construction (not timber frame) Currently the costs look roughly as follows Electrician 10k Installer 16k (including solar, new wiring for the old area as well as new boiler) 2x bathroom (complete renovation of upper floor as well as new "bathroom" -ahem- on the ground floor instead of pantry/partly wardrobe) 12k Solid wood building with garage (garage however either with bricks or wood), roof truss and slab ~ 100k Windows & doors ~10k That’s about 150k so far Various smaller additional costs are still missing as well as exterior and interior plaster, floors, possible renovations on the upper floor, (floors, door frames and doors), excavation work, paving for the driveway, land extension (~200 sqm) etc. In general, the house is otherwise in very good condition. (thanks to the retired master carpenter and passionate craftsman) All windows in the old area are new (2 years old), the heating system is 4 years old and also designed for the size it will have. I’m curious what costs will still come our way. :D Somehow I have the feeling that we will probably even exceed our self-set budget (~ 250k).