Floor space decides this one before drying capacity does. A flat with nowhere to leave an airer standing for six hours doesn't need a bigger airer — it needs one that disappears when it's not in use.
Start with where it's going to live
Before comparing rod counts or metres of line, work out where the airer actually sits while it's drying, and where it goes afterwards. A hallway or the end of a bedroom is common in a flat, which usually rules out anything with a wide footprint even if it folds flat for storage — it still needs to stand somewhere for several hours first.
Three-tier airer: smallest footprint
The three-tier concertina airer folds essentially flat, opens in one motion, and takes up a strip against a wall rather than a chunk of floor in the middle of the room. Drying space is modest — ideal for one or two loads a week rather than a full family wash — but for a one or two-person flat that's usually exactly the amount of washing there is. This is the one we'd point most small-flat renters towards.
Four-tier tower: more capacity, more footprint
A four-tier tower with rotating wings holds considerably more in one go and rolls out of the way on wheels when you're done, but it needs more floor space while it's actually drying, and the folded size is still bulkier than a concertina. Better suited to a flat with a bit more room, or a household doing a bigger wash less often rather than a small one more frequently.
Heated airer: when speed matters more than space
Where a heated airer earns its keep isn't the footprint, it's the time. In a small flat with no separate drying room and limited ventilation, cutting drying time from most of a day down to a few hours matters more than it does in a house with a spare room to shut the door on. It costs more upfront and a small amount to run, but if condensation on the windows is already a problem, faster drying genuinely helps.
What we'd actually pick
For most one or two-person flats: a three-tier concertina for the day-to-day wash, and if condensation or slow drying is a real issue, a heated airer instead of — not in addition to — the unheated one, rather than owning both. See the full range and measure your actual space before deciding, not just the airer.