'downstairs door' is a correct and usable phrase in written English.
You can use it when referring to the door on a lower floor of a building. For example, "I heard a loud crash coming from the downstairs door."
Agent Wong rang a downstairs door bell and a buzzer sounded, opening the door.
A few times the downstairs door slammed hard enough to jolt him awake.
She rushed to No. 18 and saw two grime-covered young women coming out of the downstairs door.
Stanley Twardowicz, an artist, said Kerouac threw pebbles at the window of his loft on Main Street when the downstairs door was locked.
Four days after the attack on the World Trade Center, F.B.I. agents crashed through the downstairs door at 6 Tonnelle Avenue, a small apartment complex in Jersey City near the entrance to Routes 1 and 9.
Often, the campus looks like a scene from "Meet Me in St . Louis" with young men and women talking to one another through open windows, or exchanging a chaste goodbye at the downstairs door — men and women are not allowed in the living areas of each others' dorms.
Downstairs, doors open onto a terrace that gives way to sprawling lawns.
Downstairs, the door was unlocked, and the second-floor hallway, long and lit like an E.R., was empty except for a janitor mopping a men's bathroom.
In Spenser's bedroom, which has been moved downstairs, a door behind his bed leads to a short ramp.
In what could have been totally throwaway stage business, Ms. Mackay is delicious as she examines the hinges of the downstairs bathroom door behind which Ryan is locked, clearly thinking about the possible use of a screwdriver.
Downstairs the doors of the house were open.
