I am aware "before" can be used as a preposition of place. Is the meaning of this sentence clear?
There is a puddle of water before the stairs.
Yes, a preposition of place.
before
1 a (1) : forward of : in front of
So, there is a puddle of water in front of the stairs.
Before the stairs is ambiguous. It could mean there is something else in between the puddle and the stairs. Use in front of or right before the stairs.