Karen and I laid a floor in the office (plasterboard for fire protection, 50mm polystyrene for sound insulations, and floorboards) then moved everything in.  It’s not quite finished yet (door needs hanging, undecided about wall and floor finishes yet, etc.), but at least we have a space to work in.  Next move is to clean the old office and move the boys in (temporarily) so work can be done t their bedroom.