The D.C. government wants to require residential developers to include low-priced housing in most market-rate projects, an effort to use the seemingly insatiable demand for upscale housing in the city as a springboard for creating affordable places to live.

The Zoning Commission is weighing two proposals for the approach known as "inclusionary zoning," one crafted by a broad coalition of affordable housing advocates, the other proffered by Democratic Mayor Anthony A. Williams's Office of Planning.

Each plan would require developers to price some units -- generally between 10 and 20 percent of the project -- below market rate, and they would be compensated by being able to build projects as much as 20 percent larger than what zoning allows. The plans differ on the exact number of units of affordable housing that would be required.

Advocates concede that the approach would not eradicate the shortage of affordable housing. Tens of thousands of people are on waiting lists for vouchers or public housing in the District, and thousands more are being priced out of neighborhoods by rental and sales prices that have doubled or even tripled within the past decade.

But inclusionary zoning would create a vital stream of lower-priced homes without requiring public money, proponents say, and would ensure some economic diversity in even pricey projects. Had a policy been in place over the past five years, one study found, as many as 3,000 affordable units would have been created, many in affluent Capitol Hill and Wards 2 and 3.

"It's one of the few tools that we have that allows us to distribute affordable units in higher-income parts of the city," said D.C. planning director Ellen McCarthy, who is to appear before the zoning commission tomorrow night to testify on behalf of the city's proposal. "It makes that linkage and harnesses market forces to get there."

The commission held two public hearings on inclusionary zoning last week and scheduled an additional session for this week to accommodate the rest of the more than 100 people who asked to speak. Most are community activists and affordable housing experts who support the proposals. But several developers and representatives of the building industry, along with a few residents concerned about overbuilding in some neighborhoods, have lined up in opposition.

"You're pushing a subsidy onto the backs of homeowners, or landowners. We didn't cause the problem," developer Don Deutsch of Faison Associates, who testified last week, said in a later interview. "If affordable housing is a social goal, and we believe it is, why is the city not doing anything to address this issue?"

District officials say they are spending millions to subsidize affordable housing, mostly in less-expensive neighborhoods, but are eager for creative ways for the private sector to contribute as well.

Mandatory inclusionary zoning was pioneered in Montgomery County 31 years ago and later used in Fairfax County. A number of high-priced cities -- including San Francisco, San Diego and Boston -- have implemented it in recent years as local governments grapple with skyrocketing demand for affordable housing and a dearth of inexpensive places to build.

In the District, which as recently as a decade ago had essentially no housing under construction, the decision to pursue the program illustrates the extent to which the market has turned around.

"There's a large amount of building going on in places where no one's been building for 30 years," said Cheryl Cort, executive director of the Washington Regional Network for Livable Communities and part of the D.C. Campaign for Mandatory Inclusionary Zoning. "Developers are going to make their money. We think they can be part of the solution for creating and sustaining the economic diversity of our neighborhoods."

The coalition has pushed for inclusionary zoning for more than a year, and the concept was unanimously supported by the D.C. Council this month. After the coalition submitted its proposal to the Zoning Commission in November, the D.C. planning office consulted with developers, advocates and other land-use experts before submitting its proposal.

The proposals differ on such issues as how much affordable housing to build; how to measure it -- number of units or square footage; which parcels should be exempt; and whether developers should be able to fulfill some of their requirement by creating affordable housing at another site.

The city's proposal would exempt the lowest-density areas, where projects of 10 or more units are uncommon, and downtown, where land prices are highest and few sites are left to develop. The coalition would include projects of 10 or more units anywhere in the city but exempt downtown until 2008.

After the hearings, the Zoning Commission -- a five-member panel made up of three local and two federal appointees -- will schedule a session to debate the proposals and decide whether to propose one and how it should be structured. The proposal would be circulated for public comment before a commission vote.

Some developers have said the current proposals do not increase the size of new buildings enough to offset the cost of including low-priced units and questioned whether the sometimes slow-moving D.C. government could efficiently administer an inclusionary zoning program.

They warned that developers would stop building in the District if the program is too costly or burdensome -- a prediction that has been made in other jurisdictions but has not come to pass.

Christopher J. Donatelli, whose Bethesda-based firm is building several projects in the District, said balancing the concerns of developers and city residents is crucial.

"You can't really argue with the fact that there's a need" for lower-priced housing, he said. "There just need to be adequate trade-offs."