Some Northern Virginia residents who hope to add a back-yard swimming pool or build on wooded property could face new environmental restrictions soon under a state law designed to protect the Chesapeake Bay.

The changes come as localities begin to implement the Chesapeake Bay Preservation Act, which requires 89 jurisdictions from Tidewater to the Washington suburbs to ban construction on sensitive land and tighten pollution controls over new development that potentially could pollute the waterway. The intent is to curb runoff of dirt, storm water and other pollution into rivers and streams that feed the bay, many miles away.

In Northern Virginia, where construction on wetlands, flood plains and other fragile areas already is heavily regulated, officials describe the bay law as a tighter version of current practice, although they say that for the first time it will subject some small property owners to the same rules as developers.

The strictness of local plans affecting developers varies. But area planners say the law's biggest impact is likely to be on homeowners or people who own undeveloped property they hoped to subdivide, only to find that it falls in an environmentally sensitive area where construction is banned or tightly controlled.

"The act is mom and apple pie -- you can't not embrace it," said Roger Snyder, executive director of the Northern Virginia Building Industry Association. "But it's also like embracing a porcupine -- there are a lot of potential quills."

Only 31 localities have filed required rules with the state, more than two months after a Sept. 20 deadline to do so, according to the Chesapeake Bay Local Assistance Board, which must approve local plans.

Locally, Prince William County, Falls Church and Fairfax City have approved regulations, and Prince William already is requiring new development applications to meet its rules. Other local governments expect to enact their rules by spring. Loudoun County is exempt from the law.

"For the development industry in Fairfax County, it is not going to make a radical change in the way we do business," said Patrick Hanlon, vice chairman of the Planning Commission, which will hold a public hearing Wednesday on proposed local rules. "It may for individuals."

Environmentalists contend that the law will cause only minor hardship for a good purpose, and some are concerned that local rules allow too many exemptions. "People are getting excited about something that's really not going to make a difference," said Mary Nightlinger, land-use chairman of the Fairfax League of Women Voters, which endorsed the proposed rules.

"Everybody's guessing," said David Bomgardner, a local land-use lawyer. "You hear things from various interest groups that paint a horror story or say the impacts are going to be negligible . . . . It's going to take years" to know.

In rural areas, where development controls are looser and a greater percentage of land borders the water, the impact is expected to be much greater.

The law requires governments to give land one of three labels: "resource protection area," where construction is severely restricted; "resource management area," where new pollution controls would be required; or "intensely developed," where less stringent standards would be applied.

Most local jurisdictions plan to designate few or no intensely developed areas. In Prince William, the exceptions are built-up areas such as Montclair, Dale City and Lake Ridge. In Alexandria, officials are considering the Old Town waterfront. But Fairfax's plan does not contain any intensely developed areas, not even Tysons Corner.

In the area's two largest counties -- Fairfax and Prince William -- the proposed or approved plans include generous grandfather and exception clauses. No existing recorded lot would be rendered unbuildable in either jurisdiction, although the allowable density could be reduced. Fairfax County's proposal could allow some government buildings to be constructed in areas where a private landowner would not be allowed to build.

Developers are angry because Fairfax County's proposed plan designates a wider area as resource protection -- subject to the strictest controls -- than the state minimum. But county officials and environmentalists say the proposed boundaries simply put into law the kinds of negotiated environmental protection benefits in effect for the last decade.

The most protected areas, including streambanks, flood plains, steep slopes, wetlands and a buffer zone around them, constitute only a small amount of local acreage -- 15 percent under the Fairfax County proposal and 8 percent in Prince William County.

Homeowners who live in the most protected areas of Fairfax County could add no more than 1,000 square feet of impervious surface to an existing house. That could exempt a wooden deck that allows water to flow through, but not a large addition or swimming pool with patio.

But the owner of a large undeveloped lot in a protected area may not be able to subdivide that land into a dozen lots and retire on the proceeds, Fairfax officials say. The new law could reduce the number of homes allowed on any given property.

In the rest of the county, projects larger than 2,500 square feet must have tighter pollution controls over runoff and septic systems -- most of which already are installed in subdivisions but which would be new for some individual homeowners.