Architecture

Architecture is the art and practice of designing buildings. Architects decide how a building's spaces are arranged, how big each one is, and where its walls, floors, and roof go.

An architect's floor plans are often the first place a home's size appears. Plan area and gross living area can differ, though, because plans may count garages, porches, or unfinished space that ANSI Z765 leaves out.

Design choices also decide what counts later. A sloped ceiling, a two-story foyer, or a room reached only through the garage can each shrink the gross living area of a house even when the plans show plenty of floor.

Further reading: Architecture on Wikipedia.