A former gas station on the corner of 16th Street and South Van Ness Avenue that was slated for market rate condos will now be affordable housing, Mayor Lee and District Supervisor David Campos announced today.

The 72-unit project at 490 South Van Ness originally included only 12 affordable housing units. Now, they will all be affordable.

The project took four years to get city approval and it was slated to break ground this year, but the turnaround means the city will purchase the land from J.C.N. Developers LCC for $18.5 million. It will then put the project up for bid.

J.C.N Developers originally paid $2.5 million for the property in 2009 or $180 per square foot, meanwhile the city will be paying $1,298 per square foot. Another way of meassuring it is per unit (at 72 units) which in this case it costs the city about $260,000 per unit.

“I know it seems like a lot, but that’s actually a pretty good deal,” said Louis Cornejo from the Urban Group. “A property in the Dogpatch sold for almost the exact same amout. I am actually surprised the city got it.”

The hard part, he said, is getting the city to approve the project. If it takes more than three years to get approved than the developer assumes more risks because the real estate market, like everything, is at the mercy of boom and busts cycles.

Paying for the construction of the building is contingent on voters approving a $310 million housing bond on the November ballot. A chunk of that bond — $50 million — will go towards building projects in the Mission, including the South Van Ness project.

Mayor Ed Lee and Campos said the effort is a way to tackle the ongoing housing crisis. The announcement comes on the heels of a Planning Department study showing that the city is losing as much affordable housing as it is building.

“A few weeks we did a walk through South Van Ness for the purpose of identifying land that we could, as a city, purchase to build affordable housing,” said Campos. “Whatever land is available we want to build housing.”

He called the deal, “a credit to the work that we have been doing in the Mission and a credit to the community.”

Campos pushed hard at the Board of Supervisors to impose a moratorium on market-rate housing in the Mission. That effort failed to get enough votes, but an initiative calling for a moratorium is likely to be on the November ballot.

The project would mostly likely be a combination of low and middle-income housing, though those details have not been worked out, Campos said.

This will be the fifth affordable housing project in the Mission that either the city or a non-profit are developing. The others include 1950 Mission St., the parking lot on 17th and Folsom Streets, and the property on Shotwell and Cesar Chavez streets.

The delays in those projects have been considerable.

What makes 490 South Van Ness. Ave. different is that it is a shovel-ready project because it has already been entitled by the city, said Olson Lee of the Mayor’s Office of Housing. That means it has gone through the necessary approvals to begin construction.

It too, however, will require a bid process and it’s unclear how quickly the housing office will get the bid requests out.