Unused Celadon City house refers to remnant data in Pokémon Red, Blue, and Yellow which suggests there was an additional house in Celadon City, but it is unknown what its purpose was.
The entrance warp leads to the fifth floor of Celadon Department Store (map 0x88). It is possible that this was a different map during development, as warping to the fifth floor from the ground appears to be unusual.
The exit warp leads to a building with no door (picture, right) possibly suggesting that it once had a door during development.
GameShark code 017C72C8 will add a door to the building, which likely would have been how the building would have appeared during development.
If you use GameShark codes 0108B1D3, 0108B5D3 and step out of Celadon City's Pokémon Center, you can also spawn where the unused warp is.
These codes are for Pokémon Red and Blue and may need their sixth digit adjusting by -1 in Pokémon Yellow.
- Torchickens, IIMarckus (documentation): Torchickens found the exit warp, IIMarckus found the unused (but accessible in a different way) entrance warp