Longman Dictionary of Contemporary English
spare room
noun
EXAMPLES FROM CORPUS
▪ Back in the spare room there were problems.
▪ Detectives believe they were about to be shipped to the manufacturing base in a spare room of the house in Highgate.
▪ Every spare room became a coveted rental unit.
▪ Friends offered to let me stay in a spare room or on a couch.
▪ He had left the fire on in the spare room.
▪ He was lying in bed in Miles and Juliet's spare room.
▪ We must furnish our spare room at once if this sort of thing is to happen often.
Wiktionary
spare room
n. guest room