Behind-Closet-Door Storage

Measure your closet door's width and subtract 5-1/2" to get the shelf unit's overall width — that clearance lets it swing without hitting the jamb. For a standard 24" door that's an 18-1/2" unit, so shelves get cut to 17" (the unit width minus the two 3/4" sides). Measure the door height and subtract 2"; cut the two 1x6 sides to that. Cut as many shelves as you want, plus one 1x2 lip per shelf and one extra for the top.

Build a simple shelf unit out of the 1x6s — sides and shelves — keeping it 5-1/2" narrower than the door so it clears when the door opens and closes. Pocket screws give the cleanest joint; 2" countersunk screws and glue also work. Check it stays square as you add shelves.

Attach a 1x2 across the front of each shelf as a retaining lip, and one at the very top. Glue and screw or nail them on. These keep everything on the shelves from sliding off each time the door swings.

Fasten two casters to the bottom so the unit rolls along the floor and doesn't dangle its full weight from the door. Then bolt the unit to the door with hollow-core anchors, checking it's plumb with a level so the shelves sit flat. Load it and confirm the casters take the weight and the door still swings freely.