Non-pressurised basket
A traditional single-wall espresso basket that relies entirely on your grind and tamp to build pressure.
A non-pressurised (single-wall) basket has many fine holes and no restrictor, so the resistance comes only from the coffee bed. This rewards a fresh, even grind and good distribution.
Used well it produces better-tasting espresso than a pressurised basket; used badly it produces sour, fast, uneven shots.
Why it matters for small spaces
It is the basket to aim for once you have a decent grinder — but it makes a good grinder close to essential rather than optional.