In a nutshell, you can't.
There are 58 possible slots for buildings. Of these, 30 are reserved for mayoral public work projects. 10 more are reserved for villager houses, and 4 more are reserved for player houses. Then you have 8 slots taken up by permanent town fixtures (town hall, event plaza, train station, etc.). Then you can have four buildings that don't contribute to the 30 PWP total: the police station, the reset center, the cafe, and one bridge (your town always must have one bridge). Then you need to reserve two slots for town events and visitors (most take up just one building slot, but some like the countdown take two). Add 'em all up and you have 58.
So unless you want to delete your town hall or something, the 30 PWP limit can't be broken.