The family office — a private firm that manages a single wealthy family's fortune — is proliferating in New York. As the ranks of the ultra-rich swell, more are building in-house investment shops rather than handing their money to outside managers.

The fictional office Rowan Holdings, which manages one family's assets from a discreet Midtown floor, is typical of the trend: small, private, and increasingly active in deals once dominated by big institutions. Together, these offices have become a quiet but significant force in the city's markets.

The growth has spawned a cottage industry of advisers, lawyers, and recruiters catering to them. It has also drawn scrutiny, as vast pools of lightly regulated capital take on a larger role in New York finance.