The Floating Seahorse home on the waters of The Dubai World in Dubai, United Arab Emirates.

Amid New York city’s luxury penthouses and sprawling duplexes is a stash of secret rooms: one-bedroom and studio apartments that will never appear on public listings or the open market.

These bijoux crash-pads — known as accessory apartments — are available only to elite families who have already bought (much larger) homes in the building.

Realtor.com reports that among one development’s 200 units are six small apartments — starting at around 500 square feet for $1.26 million (Sh126 million) — and offered for sale solely to existing buyers; four are already spoken for.

So what’s the appeal of these tiny, secret apartments? Quite simple actually: They are mainly for the help. “The competition to hire, and keep, the best staff — a personal assistant, nanny, butler or house manager — has got way more intense over the last 10 years,” explains Caspar Harvard-Walls, a partner at Black Brick in London (where the trend first gained traction). “So the way they are housed has changed enormously.”