Montgomery County Executive Douglas M. Duncan is vowing to help build an additional 1,000 affordable housing units over the next four years and to supply publicly owned land to make it happen.

Responding to a challenge from a coalition of 28 churches and religious organizations, Duncan (D) said the new units will be in addition to those already planned by the county to help lower- and middle-income residents.

"Our goal is very simple," Duncan said at a meeting of Action in Montgomery on Wednesday night. "If you're good enough to work here in Montgomery County, you're good enough to live in Montgomery County."

The numbers facing middle-income home buyers are daunting. According to county figures, the median price of a home in Montgomery is now more than $320,000. The average price of a new detached single-family home exceeds a half-million dollars. A worker must earn more than $19 an hour to rent an average two-bedroom apartment.

For the 30 percent of households in the county earning less than $50,000, it means homeownership is slipping out of reach.

"If your household earns $50,000 or less, you cannot even qualify for a mortgage on a resale condominium," said Derick Berlage, chairman of Montgomery County's Planning Board. "A townhouse is a pipe dream."

The County Council has been discussing changes to the Moderately Priced Dwelling Unit Program, which requires developers to sell about 12 percent of newly constructed units below market value. The program has made about 8,000 affordable units available over the last three years.

As available land grows scarce, the number of homes and apartments created under the program has declined. The shortage is compounded by the county's decision to allow some developers of high-rise projects to buy out of their obligations. Builders say the buyouts are necessary because they cannot turn a profit if they must sell some units for less than market value.

Residents who struggled to stay in the county shared their experiences at the AIM meeting, which drew about 1,000 people to Sligo Church of Seventh-day Adventists in Takoma Park.

Sonia Raices had to move from her two-bedroom Silver Spring apartment in August after her landlord increased the rent from $1,100 to $1,500. Raices, 36, a single mother who moved to the region three years ago, found a three-bedroom apartment in Columbia for $1,100.

"I didn't want to have to leave and take my kids away from school and new friends . . . but the truth is, the lack of affordable housing caused me to have to leave," said Raices, who earns $43,000 a year as an office manager.

Action in Montgomery has made construction of 1,000 affordable housing units -- broadly defined as homes affordable to families with a combined income of $30,000 to $100,000 a year -- in the next four years one of its top priorities.

It is asking officials to identify 10 county-owned sites that can be used for housing. Once the locations are agreed upon, Action in Montgomery, the county, nonprofit organizations and developers would probably work together to get the housing units built. The units would then be sold or rented at below-market value.

Although Duncan agreed to come up with a list of potential sites within four months, Berlage said the task will not be easy.

"Buildable sites in the county are at a premium. . . . And any site that we identify for affordable housing becomes subject to a great public debate and controversy," he said.

Montgomery County Executive Douglas M. Duncan wants an additional 1,000 units built in the next four years.