Two adults need six to eight cubic metres. Here is where it comes from, in the order of how much each place gives back.
Under the bed, first and by a distance
A standard double platform holds about 1.5 cubic metres — a fifth of everything you need, in the one place that costs no floor area at all. Drawers on runners beat a lift-up base, which needs the mattress moved every time.
Divide it: one drawer for out-of-season clothing, one for bedding, one for the things you use twice a year. An undivided volume becomes a place things disappear into.
Full height beats wide
A 600mm wide cupboard that runs floor to ceiling holds more than a 1200mm one that stops at head height, and it takes half the wall. In a compact plan, the top 600mm of every wall is the most under-used volume in the building.
Put the things you reach for daily between knee and shoulder, and everything else above and below. A step stool is a smaller sacrifice than a second cupboard.
The stairs are furniture
Each tread of a loft stair can be a drawer, and a run of eight treads gives roughly half a cubic metre in a space that otherwise holds nothing. It has to be designed that way from the start; a built stair cannot be hollowed out later.
The volume beside the stair, under the lowest treads, takes the awkward tall things — the vacuum, the ironing board, the broom.
Outside takes what does not belong inside
Bikes, garden tools, firewood, sports equipment and the seasonal awning have no business inside a compact envelope. A weatherproof store on the terrace or against the north elevation is the cheapest cubic metre in the whole project.
Plan it with the house rather than buying it afterwards. Its position affects the terrace layout and, on a small plot, the neighbour's view.
The places people forget
Above the door openings, the void behind the bathroom door, the depth of the wall between two rooms, the kick space under the kitchen units, and the plenum under a raised loft floor. None is large; together they are another cubic metre.
None of them can be added later without opening a finished surface. Ask for them at the drawing stage and take the ones that suit how you live.