A building maintenance unit (BMU) is an automatic, remote-controlled, or mechanical device, usually suspended from the roof, which moves systematically over some surface of a structure (up and down and/or side to side) while carrying human window washers or mechanical robots to maintain and/or clean the covered surfaces.

BMUs are almost always positioned over the exterior of a structure, but can also be used on interior surfaces such as large ceilings (e.g. in stadiums or train stations) or atrium walls.

The unit carries workers or robots while moving across the surface to be maintained. The carried equipment need not be robotic: robotic facade maintenance identifies automated cleaning or maintenance, while the BMU provides the moving support for either workers or machines.