Ok this is just plain weird. I did some testing in game. I tried:
a) building on the same community lot I've been building on all this time, then playing in live mode a little, then saving - crashed
b) building on my current home lot (just four walls and a roof), then saving - no crash
c) building on a different community lot (just four walls and a roof), then saving - no crash
d) building on the original community lot, but in a different spot (separate four walls and roof on ground level) - no crash
e) building exactly where I had been when it originally started crashing - building a third story on the lot and adding roof - it crashed.
So, this is weird. I am going in to have one more try at just putting the room that was to go on top, in the basement and see if that works. If it does, I'll move on with my building in edit town as I'd planned and hope for the best - hope it's just a glitch with a third story on that particular lot.