You can check if a Home's construction is suitable with the Housing Menu.
A structure must meet numerous requirements in order for the game to recognize it as a proper Home suitable for spawning NPCs.
A Home needs to be at least 6 blocks tall and 10 blocks wide (or 4 blocks tall and 8 blocks wide not including walls, floor or ceiling).
Including the floor, side walls and ceiling, a Home must have at least 60 total tiles, but less than 1900.
A house must be fully enclosed:
- The side walls (vertical blocks) can be made of blocks, doors or wood platforms.
- The floor and ceiling (horizontal blocks) can be made of blocks or wood platforms.
 Empty floor space
A House requires at least three tiles of empty floor space.
- Furniture must be placed so that at least three tiles of space exists that is clear of any furniture or blocks. This includes the Comfort and Flat Surface items (see below), which in a minimum-sized house must be placed off-center in order to make the House suitable. This space can be left between two furniture items, or between an item and the House's framework wall.
- Some decorative items do not affect this requirement, such as Statues, which can be placed in the empty area without invalidating the house.
- One of these empty floor tiles must be a solid (non-platform) block, which the NPC needs in order to stand at night.
 Background Wall
A house must have a background wall in order to be suitable. Any walls crafted or mined by the player are valid, when placed.
- Walls not placed by a player (such as naturally-occurring dirt walls) do not count, with the exception of those generated in Floating Island structures, or the planked walls found in Underground Cabins.
- Background walls may contain holes if the holes are no taller or wider than four tiles. (e.g. 4 background tiles across or tall, so the maximum size is 4x4, 16 in total. The hole cannot exceed 4x4 or 16 tiles.).
- Walls may have multiple holes as long as they are separated by at least a single tile of background walls.
- Having background holes will sometimes allow monsters to spawn within the Home.
 Comfort items
At least one Comfort item must be placed in a Home. Currently the following items may function as Comfort items:
 Flat Surface items
At least one Flat Surface item must be placed in a Home, and the Flat Surface item can not be entirely placed on wooden platforms. Currently the following items may function as Flat Surface items:
 Light Source
If a Home is located near Corruption, it may become uninhabitable. A Home is corrupted based on its "Corruption Rating," which calculates all tiles in a 45-block "radius" around the house; a house's Corruption Rating must be 250 or less. This is about half the blocks in the radius. Each block of ebonstone, ebonsand and corrupted vines & plants increases a Home's Corruption Rating by 1. Conversely, each sunflower reduces a Home's Corruption Rating by 5; and each block of hallowed grass, pearlsand and hallowed plants reduces the rating by 1.
 Simplest valid option
- 25 Dirt Block for floor, wall and ceiling.
- 32 Dirt Wall (8 Dirt Block) for background wall.
- Wooden Door (6 Wood). Can be replaced with 2 Wood Platforms.
- Wooden Chair (4 Wood).
- Wooden Table (8 Wood).
- Torch (1 Wood, 1 Gel, results 3).
Summary raw materials:
This works, but if you don't want house to be occupied by vines it's recommended to replace dirt by wood, stone or mud, or just clear the walls from grass. While this is the simplest option, it's very susceptible to corruption.
- Homes may be attached together and share common walls, floors, ceilings, and doors.
- If you wish to prevent NPCs from wandering around, place a torch on the ground on each side of the door. It will not only count as a light source but also prevent the NPC from leaving. Keep in mind this will also complicate opening the door for the player, as you will need to face away from the door to open it or right-click the torch and re-place it when you want to enter. (Hint: Wooden Beams can work just as well, & don't have to sit on the Floor, the same goes for Wood Platforms)
- If an NPC arrives and there is a house you may simply quit and restart in single-player and the NPC will "teleport" to a house they claim. Altitude isn't a factor, even if an available house is in the air as long as it qualifies as a house and NPC claimed/was set to it. Depending on the time of day, a simpler way to force them to appear in their Home is to wait until night time, and walk 2-3 screen-lengths (so you cant see the house or the npc) away from the NPC/Home and they will immediately despawn and respawn in their room.
- In single player you can end up with 10 NPCs (and one more during the holiday season). It is suggested to build at least ten rooms that meet the requirements for NPCs when planning a headquarters.
- the side, top, and bottom of the world does NOT count as a wall.
- 1.1: There is now a built in interface for checking if a house is valid for being moved into and designation the specific house for the specific NPC.
- 1.0.6: Requirements modified; may now use a bed, toilet, throne or bench instead of a chair, and may use a dresser, bathtub or bookcase instead of a table or workbench.