A. "After the Second World War, rent control applied to all privately owned residential rentals, including even one-, two- and three-family houses," said Stephen Dobkin, a Manhattan landlord-tenant lawyer.

Ms. Sherr said she also found there was a market for short-term residential rentals.

Downtown buildings tend to have retail space on the ground floor, office space on the second and residential rentals on the third.

"We can't list them, because it's not legal and we're a public company," said Marge Pearson, the director of residential rentals at Douglas Elliman.

His company is the New York subsidiary of Garden Homes Development, a family company based in Short Hills, N.J., with large national holdings in residential rentals and shopping centers.

Unlike the Manhattan lofts, he explained, many were residential rentals before they were upgraded and sold as co-ops or condominiums.

If the condo hotel is used for non-primary residence or residential rental, owners may be able to accelerate the depreciation on their condo hotel unit from 39 years, down to 27.5, 15, 7, and even 5 years.

Florida is the only state to charge sales tax on the rental of real estate, commercial or residential, raising more than a billion dollars a year from the tax.

The hospital closed in the 1960s and in 1981 became residential rentals under Section 8.