Next week, the Cambridge City Council will hold a public hearing on a plan to nearly double the amount of affordable units included in new housing projects. The proposal would require developers to set aside 20 percent of units at affordable rents — $1,766 a month for a family of three that earns about $70,000 a year, for example. The current requirement is 11.5 percent of units.

But if most New Year's resolutions are feel-good affairs, this one is shaping up to be a fight.

Some of us start the new year with a plan to hit the gym more, or eat better. In Cambridge, officials are planning, once again, to try to tackle the city's gaping shortage of affordable housing.


Proponents argue it will help middle- and working-class residents remain in what's become one of the region's most expensive housing markets. But some builders warn the new rules will backfire and only discourage new housing.

Sensing momentum within Cambridge for the higher set-aside, the business community is pressing to have some exceptions to the new requirement, said Sarah Kennedy, director of government affairs at the Cambridge Chamber of Commerce. One idea would allow large developments that already have permits from the city but haven't begun construction yet to remain at the current 11.5 percent set-aside unless they make big changes to their plans.

Kennedy expects those talks will continue as the measure moves towards a final council vote over the coming weeks.

"We're glad to continue the conversation on this," she said. "That's how the details will get fleshed out."

At least one city councilor acknowledged the difficulty of finding the right balance.

"Of course the development community is going to be nervous about this. I don't blame them," said Councilor Marc McGovern, who is also Cambridge's vice mayor.


He acknowledged that Cambridge has to be careful in selecting a requirement that does not deter builders, but also noted the 20-percent requirement has strong support within the council. "If we can produce more affordable housing, we need to take that chance and do it," McGovern said.

In the region's current building boom, these so-called "inclusionary housing" requirements have become a main driver of affordable-housing production, especially for middle-class renters who earn too much to qualify for low-income developments. With more building underway, cities have been revamping their programs to ensure more of those units are affordable.

A year ago, Boston amended its program to require larger cash contributions from developers who choose to pay into a city housing fund instead of building the affordable units themselves. Somerville recently boosted its requirement to 20 percent in larger buildings. Cambridge has experimented with requirements as high as 25 percent in pockets of Kendall Square.

The proposed new Cambridge rule would be in force citywide. In December the Cambridge Planning Board approved a version that requires a 20 percent set-aside and encourages more family-friendly three-bedrooms units. Up next is the public hearing Jan. 4.

Lee Farris, vice president of the Cambridge Residents Alliance, said the addition of new apartments across the city hasn't been enough to curb rising housing prices.

"We're having a serious hollowing-out of the middle" class, Farris said. "People are worried about our city becoming a city of lower-income people and very upper-income people. Inclusionary housing is one of the best ways to keep a better balance."


But some builders warn that requiring too much affordable housing could make it impossible for them to recover their costs and would essentially stop new development in its tracks. In testimony at a hearing in August, for instance, Tom Sullivan, president of development at DivcoWest, said the 20 percent requirement "would be devastating" to the company's massive NorthPoint project in East Cambridge.

The developer spent $291 million last year to buy the 42-acre site, which is permitted for more than 2,000 new units of housing. Keeping 20 percent of those units at lower rents would reduce his land value by nearly 40 percent, Sullivan told the City Council, according to minutes of the meeting. He declined to comment this week.

Several smaller builders also testified their projects would be difficult to finance if they had to set aside more units at the lower rents. Jack Englert, a principal at Criterion Development Partners, said projects he has built in West Cambridge just wouldn't be profitable enough to attract investors under the higher set-aside.

"We are right on the margins when we do these projects now," Englert testified. "If we move beyond 11.5 percent, we'll be unable to do them."

Under city rules, the council has until April to vote on the measure.

Tim Logan can be reached at tim.logan@globe.com. Follow him on Twitter @bytimlogan.