An individual can build fold-down work benches by installing two pipe flanges and hinges to a solid-core door and attaching it to a wall. A stand-alone work bench is buildable using a similar process, but by eliminating the hinges and adding extra pipes and pipe flanges to support four legs.
To build a fold-down work bench, purchase or repurpose a 28- to 30-inch wide solid-core door, and cut it to the desired workbench length with a circular saw. Screw two 1 1/4-inch pipe flanges under the door, placed 12 inches from the front and side edges, to be positioned furthest from the wall. Add two 3 1/2-inch gate hinges to the underside of the door, then attach the hinges to a 2-by-4 that is cut to the same length as the door. Find the studs in the wall to which you are attaching the workbench, and screw the 2-by-4 with workbench top to the wall.
To make legs for the workbench, measure the distance from the floor to the workbench when it is extended upward, and cut two lengths of 1 1/4-inch water pipe to serve as legs. If the pipes are not threaded, cut threads into one end of the pipes. Cover the other ends with plastic pipe thread protectors to rest against the floor. When you are ready to use the bench, screw the water pipes into the flanges to serve as legs.