Most small flats are not short of storage space. They are short of storage in the four places nobody thinks to look.
The 20 cm behind a door
A door swings through 90 degrees and needs about 90 cm of clear floor to do it. Everything outside that arc — the 20–30 cm strip along the hinge wall — is dead space. A shallow shelf unit fits there and is invisible from the room.
Above the door
The 40 cm between the top of a door frame and the ceiling is where the things you need twice a year should live. Boxes, not shelves: you cannot see up there anyway.
Under the sink, around the plumbing
A standard sink cabinet loses a third of its volume to a U-bend. Two sliding drawers that step around the pipe recover most of it, and you stop kneeling on the floor to find the washing-up liquid.
The corner nobody uses
A corner takes two walls of linear space and gives back a triangle. A corner unit turns that triangle into shelf area — usually the difference between a bookcase and no bookcase in a one-bedroom.
Then stop buying boxes
Once things have a place, the instinct to buy more storage goes away. That is the actual goal: not more furniture, but fewer decisions about where things go.