Lean-to

Lean-to, French: Appentis, built against the walls of Meaux Cathedral
A typical free-standing Adirondack-style lean-to shelter

A lean-to is a type of simple structure originally added to an existing building with the rafters "leaning" against another wall. Free-standing structures open on one or more sides (colloquially referred to as lean-tos in spite of being unattached to anything) are generally used as shelters.

A lean-to addition is an appendix to an existing structure constructed to fulfill a new need. Sometimes, it covers an external staircase, as in a 15th century addition against one of the walls of the large chapter room of the cathedral of Meaux. Other uses include protecting entrances, or establishing covered markets outside existing buildings.[1]