lean-to (n.) Look up lean-to at Dictionary.com
"building whose rafters lean against another building or wall," mid-15c., from lean (v.) + to (adv.). Compare penthouse. "An addition made to a house behind, or at the end of it, chiefly for domestic offices, of one story or more, lower than the main building, and the roof of it leaning against the wall of the house" [Bartlett].