Measure the room in three dimensions
A small room is rarely short of space; it is short of usable floor. The fix starts with a plan view and an elevation of every wall. Note window sills, radiator positions, sockets, door swing and skirting depth. In many UK and European homes the box room is around 2.0 × 2.5 m with a 2.4 m ceiling — roughly 12 m³ of volume of which most occupants use the bottom metre and a half.
Divide each wall into three bands: the reach zone (roughly 60–180 cm from the floor) for daily items, the low zone below for heavy or rarely used things, and the high zone above head height for seasonal storage. Once you see the room this way, the empty wall above the desk or door becomes obvious real estate.