Callback: House construction check
House construction check (17)
This callback is used to determine if the given building type can be built on a position. It should return 1 to allow building and 0 to cancel it.
Since the building is not created yet, you can't use variables 40 and 41 (they return zero). You can use variable 42, 43, 44, 45 and all town variables, though. Since OpenTTD r19744 you can also access the random bits via RandomAction2 or variable 5F.
Property 0A and 13 are checked even if the callback returns a nonzero value; they can cancel building as well.
For multi-tile buildings, this callback is always called for the north tile type, but the position you can access through variables 40, 42 and 43 is not guaranteed to be the final position of the north tile. It is guaranteed, though, that if the building really gets built, one of its tiles will occupy that tile. So, for example, if you return zero for desert tiles, this neither means that your building won't occupy any desert tiles, nor that its north tile will be on a non-desert tile; it only means that at least one tile of the building will be on a non-desert tile. (e.g. it can still be built on the very edge of a desert) This restriction was removed in OpenTTD r13489, the callback is now called for the final position of the north tile.
GRFv≤7 Note that GRF versions 7 and lower only distinguished zero and non-zero return values.