How to change the building type

Wherethe Project settings gear in the editor headerfirst item Building type

Steps

Click the Project settings gear.
In the Building type list, choose Apartment or Private house. Heating and plumbing calculations are updated immediately.

Good to know

Projects created before the New project window have no type set, and it is determined from the plan: if there is a roof or more than one floor, it is a private house, otherwise an apartment. The list shows what is currently assumed. Choosing from the list fixes the type: after that it no longer changes depending on whether a roof is drawn.
Changing the type counts as an ordinary edit: Undo (Ctrl+Z) reverts it.