Hello Archon, 

Once built, you don't have the option to delete buildings. 

You can add building locations by expanding your city, for that go to the city editor. 

The blue slots aren't yet unlocked, you can get them either through your friends or by buying them with drachmas. It should be noted that at a certain moment you will inevitably have to buy the slots at the price of 500 drachmas. For this price you will get 4 or 5 locations.

After purchase, the slots will become green and you will be able to build !