It is not the square metres. It is that there is nowhere to be out of earshot, and that everything one person leaves out is in the other person's line of sight.
Somewhere to be alone matters more than space
The single most common reason couples leave a compact house is that neither can be unobserved. A door, a level change or a closable loft is what solves it, and a curtain is not. This is a plan decision, made once.
Even where a second room is impossible, a seat facing away in a different part of the house, with its own light, does more than another two square metres of open floor.
Sound separation is half the answer
One person on a phone call occupies the whole house unless the partition has mass. Filling the cavity and doubling the board on one side is a few centimetres and turns a visual divider into a real one.
The bathroom door matters as much. In a compact plan it opens onto the living space, and a hollow leaf makes every use of the room a shared event.
Two of everything, or a rule
One wardrobe rail, one worktop, one desk: shared resources in a small house need either duplication or an agreement. Duplication costs volume; the agreement costs nothing and has to be actually made rather than assumed.
The practical version is dedicated zones — your shelf, my drawer, this half of the rail. It sounds petty and it is what makes a compact house sustainable over years.
Cooking and cleaning happen in the living room
In an open plan the person cooking is in the room with the person relaxing. Quiet appliances, a hood under 50 dB and enough worktop to avoid clutter are not luxuries here — they are what stops one person's activity dominating the evening.
The same is true of the washing machine. Run it while you are out, or specify one quiet enough to run while you are in.
Outdoor space doubles the house
A covered terrace is the second room a compact plan cannot contain, and for two people it is where the extra distance comes from. Six months of usable outdoor space changes the experience more than any interior change.
Give it two seating positions, not one. That way the same trick — being in the same place without being on top of each other — works outside as well.