Amazon has a sturdy wall-mounted fold-out desk for $215 that also becomes a full shelving unit when not in use so you can ...

This works especially well for desks that don't have built-in drawers. For even more storage space, consider an open cubby that functions similarly to an under-shelf basket. These slide onto the ...

For a desk with ample storage underneath, use three pallets that have been modified to form a base, shelves, and a desk surface by cutting down, ripping apart, and reassembling the wood planks.