Relaxing requirements to have parking lots at senior and affordable residences within reach of subways would free up space and money to build apartments, city officials testified Wednesday at a second day of hearings on Mayor Bill de Blasio’s housing plan.

“Three unnecessary parking spaces are the equivalent of two units of affordable housing,” Housing Commissioner Vicki Been said. Officials said spots in parking structures can cost $50,000 to build, taking into account design, materials and labor.

But City Council members from the outer reaches of Queens and Brooklyn challenged how so-called transit zones — where parking requirements for developers would be waived — were drawn. They said public transportation options and other amenities must be improved before the parking is taken away.

“Senior citizens and other residents are not sardines,” Mark Treyger, a Democrat representing parts of southern Brooklyn that have seen bus line cuts, told Newsday. “They need to be mobile, they need to get to doctors’ appointments, they need to live out the golden years of their lives.”

De Blasio’s aides are working to win the favor of the City Council, which will vote in about 50 days on two key components of the mayor’s plan to secure 200,000 affordable housing units within a decade.

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Wednesday’s hearing centered on proposed design changes, or “zoning for quality and affordability,” to make it easier to build more apartments on limited amounts of land.

Planning Commission Chairman Carl Weisbrod said parking wasn’t necessary for senior and low-income residents who are unlikely to own cars or drive

. Officials said transit zones could be negotiated and redrawn.

Though they said they support the blueprint’s end goal, City Council members and borough and community boards have raised concerns about sweeping changes they say the mayor’s plan would bring to their neighborhoods, from new development that can displace poor residents to taller buildings that could affect an area’s character.