Hastings Marketplace, a proposed village and shopping center, could be a gem for Manassas city planners: The 28-acre plot on Liberia Avenue would have a main street with shops abutting the curb and residences on the upper floors. With its shops anchored by a chain grocery store, the development would also include 240 residential units, including condos and single-family homes.

Planners across the metro area are pushing for these self-contained communities, and Manassas is trying to take the first step toward a more integrated city.

Playing a bit of catch-up with the rest of the metro area, the city is proposing a new type of zoning district: a planned mixed-use district.

The district would allow developers to build residential and commercial spaces on top of one another, embodying the city's vision of denser development with high pedestrian traffic and limited car use. Outside of Old Town, no such areas exist.

"The city's been a bit behind when it comes to more modern zoning districts," said Liz Via, director of community development for the city. Most metro counties and cities, including Prince William, already have these districts, which zoning administrators say are becoming increasingly desirable. Some, such as Fairfax County, have had them for more than 20 years.

The driving force behind this decision is the lack of space in the 10-square-mile city. Empty plots are hard to come by, so denser development is the only option for growth.

"If we're going to continue to compete economically, then we need to rethink how we do planning. We need to think about going up," Via said, adding that, without change, the city will be ceding jobs and housing to Gainesville and Haymarket.

Tomorrow, the City Council will have its first opportunity to vote on the development.

But the proposal has caused some unrest in several Manassas communities. Many residents are not opposed to the idea in principle, but most would rather not have the actual developments in their front yards. Hastings Marketplace, proposed by Opus East LLC for Liberia Avenue near the Prince William Parkway, is raising particular concern.

"You have economic concerns. We don't have those concerns," Oakenshaw neighborhood resident William Sebesky, 40, told city planners at a public meeting Thursday night. Residents' complaints included increased traffic, environmental issues and aesthetic considerations.

"I'm not against the idea. It's just the height of the buildings. It's just way too high," said Fred Koch, 48, who lives off Wellington Road.

Hastings Marketplace should not have buildings of more than four stories in height, said Scott Brody, vice president of real estate for Opus East. Residents are wary because city officials said some areas of the city are being targeted for buildings up to 96 feet tall. The Mathis Avenue corridor off Route 28 is a key area in the city's Comprehensive Plan.

The city said it is not rezoning any current property and that passing the mixed-use district would be done only to allow developers the opportunity to apply.

But some residents sensed an opportunistic bait-and-switch. Opus East approached the city in the fall to develop the land in Manassas, an area that is not the main focus of the city's dense development plans. Now those community members, remembering other commercial foibles such as gas stations next to single-family homes, feel targeted.

"My fear is that we're going to be locked into getting something that the residents don't want," said Jim Rogers, a Troutman Court resident who has advocated for his community on planning issues.

"Everything that we've seen so far, it doesn't sound reasonable," Rogers said.

But the new district is different from past commercial ventures in some crucial ways, city officials said. They emphasized that the mixed-use district is "performance-based," meaning developers have to meet certain conditions before getting approval to build. The city can then ask for proffers -- money that developers give to sweeten a deal -- to mitigate problems such as traffic, schools, public safety and environmental impact.

This is new for the city. "We have not been getting proffers," City Manager Lawrence D. Hughes said at last week's meeting. Other cities have received much more money for schools and roads from developers. But Manassas zoning ordinances have not had strings attached.

Some residents, Via said, have raised concerns about property values falling. But, she noted, several studies have shown that values often increase with denser development. And retailers, facing competition from the Internet and Wal-Mart, are now demanding that more residents live closer to their stores. So dense development will help Manassas woo these stores.

More important, the city is running out of road space. Although Manassas "will never be Europe," Via said, "you should be able to walk out of your townhouse and down the street and get your newspaper and your Starbucks.

"We are a society of automobile purchasers, so it's really a very unique American experience to create these integrated developments."

Pedestrian-friendly developments have been proposed across the country. Prince William County has seen an increase in these applications over the past six years, receiving very few before 1999 but 11 since 2001, according to the county's development office.