Some things are deceptively complex (foundations and roof trusses come to mind) and the cost if you get it wrong can easily double the total cost of your build

Some things are deceptively complex (foundations and roof trusses come to mind) and the cost if you get it wrong can easily double the total cost of your build 6 months after you think you are done. Walls, floors, ceilings and everything in between them are both less complex and cheaper to fix than foundations or trusses.