House

Every town NPC needs an enclosed room of their own before they will move in, and the game runs the same three stage check on all of them: the shape of the room, the furniture inside it, then a set of properties that have nothing to do with either. The first two stages give you a readable error. The third often does not, and that is where most rooms actually fail.

The Old Man, Traveling Merchant and Skeleton Merchant never need one. Everyone who arrives unprompted, the Guide and Goblin Tinkerer included, still needs a room before they will come back after dying.

Open the housing menu from the small house icon above your armor slots, take the question mark cursor and click inside a room to run the check by hand.

The perimeter has to be sealed against outside air. Solid blocks count, and so do unactuated platforms, doors, trap doors and tall gates in their closed position. Background walls have to cover the interior, and only walls you placed count: natural dirt wall reads as unsafe, which is why a furnished cavern still reports a missing wall. Gaps are tolerated until they reach five tiles across in either direction, at which point the room fails and enemies can spawn inside anyway. Glass wall gives you a window that still seals.

Size is counted in tiles including the frame: 60 at the bottom, under 750 at the top. Shape is free. A dome or an irregular blob passes as long as the total lands in range and the outline never breaks. Liquid inside the room changes nothing.

A room needs at least one light source, one table, one chair and one door, and any variant of each fills its slot. The cheapest legal set is a work bench, any chair, a torch and one wood platform. The table family stretches from a bookcase to a piano to an alchemy table, and the chair slot accepts a bed, a toilet or a throne.

Two things trip people here. Platforms count as doors, so the entrance can be a single platform tile in the ceiling that the NPC will never touch. And light coming from a block, a wall or a platform does not register at all, however bright it looks. It has to come from a furniture object, anything from a candle to a lamp post.

The game then scans a box reaching roughly 45 tiles out to either side, 42 up and 45 down from the room's edge, and totals every Corruption, Crimson and Hallow tile inside it. Sunflowers subtract. Cross a low threshold and the room reports as corrupted; cross a much higher one and you stop getting even that hint. A room infected after someone moves in evicts them until you clean it up.

Because the box reaches so far past your walls, thin infection near a base can break it, while a room deep inside an evil biome can still pass if the tiles right around it are clean. That is the workaround for buying biome locked stock from a vendor like the Painter. A King Statue or Queen Statue solves the same problem by pulling the NPC to you instead.

The last check hunts for a floor tile the NPC can occupy at night. It has to be solid and unactuated, with solid tiles either side and clear air in the three by three above. The game scores the five by four area over that tile starting at 50 points: closed doors and gate hinges cost 20 each, ordinary solid blocks cost 5, chests cost 30 apiece, and blocks stacked directly overhead cost 15 each, while open space adds 5. Reach zero and the tile is thrown out. If nothing in the room survives, the check fails without telling you why.

In practice: keep a clear strip of floor, do not line the walls with chests at head height, and do not put the only open floor tile under the door. Any valid room holds one town NPC plus one town pet, though in a small room the two share a single standing tile.

Truffle adds one rule on top: the lowest tile of his room has to sit above the surface layer, and the same box used for the evil scan needs at least 100 glowing mushroom biome tiles in it. Mud, mushroom grass seeds and a room built on top is the standard answer.

• Bare wall area speeds up move ins. A room with no exposed background wall left can only be filled by hand, and sandstone brick walls slow the roll further.
• Build spares before you smash your first altar. Hardmode spread invalidates rooms with no warning, and spares give you somewhere to shuffle people when tuning happiness or keeping a pylon alive.
• Rooms can share walls, floors and doors. An apartment block reads to the game as separate houses that happen to touch.
• Do not build at the 60 tile floor. Crafting stations are commonly three tiles square, and later paintings run up to six across and four tall.
• Biome spread converting a wall counts as breaking it. Stone wall turning into pearlstone wall invalidates a room that has not visibly changed.