The larger your house, the more amazing things it can hold. This is also true for how many tenants you can have which are friends that can recall to your house. Placing a house add-on to your house is rather simple - go into your house and double click the house add-on in your backpack. Some of the useful add-ons include a House Safe, certain crafting stations and Guest Room Furniture.
House add-ons cannot be removed once placed, but they can be replaced with others. Certain add-ons that get replaced will be lost, but you will get a confirmation popup which tells what happens to the old add-on.