San Francisco city leaders on Tuesday approved a decades-in-the-making affordable housing project for the Mission Rock neighborhood, near AT&T Park.

The ambitious plan comes with a big promise: a sprawling mixed-used development that the mayor says will be home to middle class workers such as nurses and teachers.

Asked if he believes the city can back up that promise, Mayor Mark Farrell said, "Absolutely."

"Look, we have 40 percent affordable housing on this project for low- and middle-income families and individuals here in San Francisco," Farrell said. "That’s exactly the type of housing we should be building."

Located at Third and Mission Rock streets in the city's Mission Bay neighborhood, the parking lot is on the waterfront and adjacent to Piers 48 and 50. A total of 21 acres will be developed, according to Farrell.

The new neighborhood will include an array of shops, cafes and offices, as well as 8 acres of parks and public open space, a community square, a wharf and public access to Pier 48.

With a stroke of the pen Tuesday, Farrell launched the legislation, which has been talked about for a while. The San Francisco Giants helped put the 1,500-unit project into motion several years ago.

"I believe Bruce Bochy was a rookie manager at the time for the Giants," team President and CEO Larry Baer said Tuesday. "What we had to do is to say, 'Look, the project is big enough that we will take less of a return on the housing in order to make this a diverse neighborhood.'"

The Port of San Francisco, the entity that controls the land, did the same. By taking less, the city and ballclub opened the door for a wider net of people to rent.

The average median income in San Francisco for a family of four is about $115,000. The affordable units will be available to those who make much less and also much more. The income range stretches between $57,000 and $173,000 a year for a family of four.

Translated into rent, that’s as low as about $1,400 and as much as $4,300 a month.

The promise of 40 percent affordable housing would be a record for any city project of this size. Of the roughly 600 or so affordable units, about half will go to families with an income of around $100,000.

Late Mayor Ed Lee spent much of his time in office helping to plan and promote the project, a point Farrell was quick to point out.

"We're now going to have police officers and firefighters and teachers living there and transitional age youth are going to be living there," Farrell said. "This is a big deal for the city of San Francisco and a huge part of what Mayor Lee was pushing for in all the new housing units. I think he'd be very proud."

Bay City News contributed to this report.