Stairs have a top and a bottom, so you can always say which you want:
Go up (to the top of) the stairs and ask your mother to come down.
Go to the bottom of the stairs and ask your mother to come down.
You don't have to specify, but then you leave the listener to guess from context. An alternate solution is to modify the direction of the action, which implies the location of the speaker:
My son shouted up the stairs, "Mom, Dad wants to you to come down!" (your son is at the bottom of the stairs)
My wife shouted down the stairs, "I'll be there in a minute!" (your wife is at the top of the stairs)
This only works with certain verbs, though. "Shout", "yell", "call" and other loud vocalizations all work. For some reason "whisper up/down" is also idiomatic, but not "speak", "ask" and other normal vocalizations.