From Longman Dictionary of Contemporary Englishspare roomˌspare ˈroom noun [countable] DHHa bedroom in your house for guests SYN guest room
Examples from the Corpus
spare room• Detectives believe they were about to be shipped to the manufacturing base in a spare room of the house in Highgate.• Friends offered to let me stay in a spare room or on a couch.• Every spare room became a coveted rental unit.• We must furnish our spare room at once if this sort of thing is to happen often.• He was lying in bed in Miles and Juliet's spare room.• Back in the spare room there were problems.• He had left the fire on in the spare room. 