So I am quite confused. The son has already started a 3-year training, right? Until your house is ready to move in, at least another 1.5 years will probably pass. So you are planning a (pretty dark and probably also rather uncomfortable due to the size) huge room for the son, who might then only live with you for another 1.5 years? That is quite a waste of money. Better use the money elsewhere or at least make a proper granny flat that either the son or daughter with partner could live in or that can be rented out to strangers... And if you want a kitchen island, then plan the kitchen so that you can put in a kitchen island... that's the main advantage of a new build: I can plan the rooms according to my wishes!