Legally speaking, when you buy a condo you’re buying title to a subdivided, three-dimensional space within a “common interest development.” A condo unit can be bought, sold, financed, and willed like a single-family house — which is why most lenders are comfortable lending on one. (Contrast that with a tenancy-in-common interest, which most lenders find incomprehensible.)
But you’re not only buying the unit. You’re also buying into a homeowners association that holds title to the common areas: in an urban building, that usually means exterior walls, windows (sometimes), roofs, yards, and garages — and in larger buildings, hallways, elevators, gyms, and roof decks. Your share of the common area generally tracks your unit’s square footage, which in turn drives the percentage of dues you pay.