After more than three months of work and behind-the-scenes wrangling, Arlington County officials, housing activists and developers reached a unanimous agreement last week on how much developers could be expected to contribute to the county's affordable-housing effort.

Most members of the county working group that approved the plan professed measured optimism about the agreement, which the County Board is likely to approve next month.

"This is as good a compromise as we can reasonably hope to achieve," said County Board Vice Chairman Chris Zimmerman (D).

The agreement comes at a critical time for the county. Progress on large-scale retail and residential projects has slowed to a crawl over the past year while details of the plan have been worked out, and the county continues to lose thousands of affordable apartments.

The county's affordable-housing program was upended in December when an Arlington County judge ruled that it was illegal for the county to ask developers to reserve 10 percent of floor space in projects built near Metro stations for affordable housing and to solicit cash contributions from commercial developers. The county's planning department had long allowed developers more density or height in their projects in exchange for cash contributions for affordable housing or actual affordable dwellings for lower-income residents.

In February, county officials staved off a movement by Republican state legislators to codify state restrictions against such requirements, in part by agreeing to form a working group with the development community and housing activists to see whether the sides could hammer out a deal locally.

Sen. William C. "Bill" Mims (R-Loudoun), chairman of the Virginia Housing Commission working group on affordable housing, which monitored the talks, said he was pleased with the result.

"This is what we hoped would occur," Mims said. "It's better for issues like this to be resolved by the local private sector and officials than the General Assembly."

Mims said legislators would not intervene again, except to approve the Arlington deal when it is introduced as legislation by state Sen. Mary Margaret Whipple (D) when the General Assembly convenes in January.

"I can say with the utmost confidence that the General Assembly will not step into this situation again, except to approve the agreement carried by Sen. Whipple," he said.

The plan gives developers seeking to build projects larger than zoning allows four options for contributing to affordable housing. Developers could choose to provide a small number of affordable apartments or townhouses on a given site; provide a number of affordable dwellings nearby (within a half-mile) or a slightly higher number of units anywhere else in the county; or make a cash contribution to the county's Affordable Housing Investment Fund, ranging from $1.50 per square foot to $8 per square foot, depending on the size of the project.

Millions of dollars are at stake, given the large-scale redevelopment in the county over the past several years, particularly along the Metro Orange Line corridor.

Since 2001, there has been more than 10 million square feet of commercial and residential development in Arlington, officials said.

"Now we'll be able to do business in the county again, and that's a good thing," said Andrew A. Viola, regional vice president of Bush Construction Corp.

But some affordable-housing advocates said that the agreement, although positive, would not be adequate in a county that is rapidly losing affordable housing. In the past year, nearly 2,000 affordable apartments have been torn down or remodeled to make way for luxury condominiums, officials estimate. They also fear that developers will simply opt to make cash contributions rather than create the low-cost housing that the county needs.

"I'm very glad they came up with something, but the crisis is so large I fear it is still inadequate," said Kristin Carbone, program director for Buyers and Renters Arlington Voice.