In order to be able to build a set of houses or hotels, you must own a monopoly on the properties with one player. Then, once it is your turn, tap those those properties on the board to go into Manage Mode. The Improvements Screen will show up now. Drag a single green house down to each property in the monopoly. Then hit the green checkmark to confirm the purchase and unlock the "Climbing Your Way Up The Ladder" achievement. Do the same with the hotels to unlock "The View From The Top" achievement. Note that you must have enough money to purchase the house and hotel sets (price varies depending on property group).