The city has stepped in to stop a tony Manhattan prep school from selling an affordable housing complex to a private developer.

The 199-unit Trinity House was built in 1969 and is the last Mitchell-Lama building on the Upper West Side.

Last year, Trinity School, which has owned and operated the building for the last 40 years, informed tenants that they were selling to Pembroke Properties, Inc., who intended to convert the units to condominiums.

“We are thrilled,” said Dina Heisler, who raised her two children in a two- bedroom apartment in Trinity House, at 100 W. 92nd St. She has lived there since 1976 and pays a little more than $1,000 a month for.

“This place was the only way I could have I could have raised my boys in the city. It made it possible for middle income tenants to stay in the city and have good quality housing.”

Representatives from Pembroke Properties and Trinity School both expressed surprise that the sale of the building—which had been projected for upwards of $30 million—had been rejected, and both said they had yet to be informed of the verdict.

Neill Coleman, a spokesman for the city’s department of Housing, Preservation, and Development, said that the high sale price made the agency realize that the developers would not have been able to charge affordable rents for the amount they were spending on the purchase.

Tenants are now looking to purchase the building themselves or find a nonprofit developer to do so.

Mitchell Lama advocates, concerned about losing the stock of affordable housing, hailed the decision.

“We see it as the beginning of a policy, hopefully,” said Amy Chan, an organizer with Tenants& Neighbors, an affordable housing group. “We can’t let these buildings fall into the hands of predatory buyers.”

--David Freedlander