Hi secretlysims
at which point do you get the save error ? Every time you try to save the game ?
No just that one time I tried to save after assigning all units to the yellow one, bulldozing the lot and changing lot type to generic. I couldn't save before going back to map view - got the error, so saved using ALT F4 and the when opening the game got a white screen for britechester.
I haven't tried to save unless I thought the lot was okay and reset as residential.
I've also gone back and am now trying to assign all units to the pink one (that you couldn't move into or see the household - if there even was one). Demolished lot, made residential and tried to put a new residential down from the gallery. You can see after I did that i went back to the map view and it is overlapped with my build (the tall red brick).
Even if the lot is bulldozed and empty residential it still shows my build in the map view (below). I have saved the game now (to answer your question and check I can (I can). But the lot is not right.
Sent you my save file in case that is useful in any way too.
