Terraria – Can the Terraria building be above ground for NPCs to come

During the daytime of 15 minutes, only 1 NPC can spawn every 2 minutes, the NPC must not exist yet and the necessary conditions (separate houses, …) must be met. If multiple NPCs are eligible, then only the first eligible will spawn. It is checked in the following order:

• Guide: No conditions.
• Merchant: House, players have more than 50 silver together (in their inventory).
• Nurse: House, used a crystal heart.
• Arms Dealer: House, carry a gun in your inventory.
• Dryad: House, must have killed a boss.
• Demolitionist: House, Merchant, explosive material in inventory.
• Clothier: House, Skeletron killed.

Exceptions to the single NPC rule, which spawn before the order given above:

• Second Merchant: House, at least 3 players having more than 10 gold together.
• Second Nurse: House, at least 3 players having 20 hearts (i.e., 400 combined health or more).
• Third Merchant: House, at least 5 players having more than 5 platinum together.

There is no order in which they spawn over time. But they can't spawn together, thus first in order…

In an answer* to a related question, it is stated that:

[A meteor] may not intersect with a Player, an NPC or a Chest.

If your house is only large enough for your NPCs, it's quite likely you're already safe. If not, as an alternative to making a skybridge barrier, you might instead make an "attic" full of chests. This will cause the game to look for a different location entirely rather than leave only a thin layer of meteorite, as a skybridge would.

**My understanding is that the rules listed in the linked post were determined by examining the game's code, but it wasn't done by me, so caveat emptor. ;-)*