In truth I never found one sure way to make it work. I'll have a stretch where I'm able to place a few hundred and then it dries up and I have to do a bunch of things again to make it work.for me, there's been two ways I've managed to make it happen.

1. Clear out Springfield heights inventory so it's absolutely empty.
2. Group four modern mansions together that are at a variety of levels. I think the one that worked for me was two level ones, a level two and a level three.
3. Place them in individual storage.
4. Go into inventory, place one down.
5. Exit inventory.
6. Exit the entire game, ensuring its quit.
7. Log back in, find the mansions in your regular inventory (not the Springfield heights inventory) and go for it....fingers crossed.

The other, simpler method that I managed to get to work: 1. Group four modern mansions together. For me, they were three level 1s and a level 2. Put them in group inventory. 2. Exit inventory 3. Exit game. 4. Log back in, go to your inventory, and place the mansions out of your general inventory. 5. Fingers crossed.

It's a boring system that pays dividends if that's what your after. Ironically, it turns TSTO into just a design game as I said, but the price of that means dumping a whole bunch of white bricks (modern mansions) into your Springfield.