I did this on a designer map. No enemy team and starting cash of $500000 helps. For every property I had, I upgraded all rooms and ordered the team in to do the work. Once I covered the 20th property it unlocked.
I had all 20 in the one game. Unsure if that is needed or not, but it may be.
To upgrade a room, select a house, press L2 and Triangle and then hit triangle repeatedly on a room until you have it at the max, press up or down on the d-pad to select the next room and repeat until all 4 rooms are due to hit maximum. Also press Square to call a team to work on the house, if there isn't one there.