Edith McKinnon was a technical writer for the Department of Defense in 1953 when she and her husband decided to escape the noise, pollution and crowds of Washington and move to small farmhouse on 10 acres in Great Falls.

Now, like so many other elderly people living on fixed incomes in Northern Virginia, McKinnon's dream house is anything but on the day her property tax bill arrives.

"When I moved here, it was in the boondocks. Now it's the place to be," said McKinnon, 83. The increased desirability is reflected in her tax bill: $94 in 1953, $5,900 this year.

Then McKinnon learned of a little-known provision in state law that enabled her to lower the assessed value of her land by promising not to develop it.

Two weeks ago, the Fairfax County Tax Assessor's Office handed her a new bill that cut her property taxes by $949.

Board of Supervisors Chairman Audrey Moore, one of several people who advised McKinnon of the tax relief plan, says she believes that many other Fairfax residents could benefit from it, too, and has asked county staff to develop a program to encourage participation.

"If homeowners are willing to keep their land undeveloped, we can reduce their real estate taxes," Moore said in a statement.

Other officials are not as optimistic but said the idea deserves study.

Thus far, only two people -- McKinnon and state Sen. Clive L. DuVal 2d, who owns 50 acres in McLean -- have taken advantage of the law, according to county officials.

It is unclear how much land one needs to own to be eligible for the tax break: one land-use lawyer said it was up to the Board of Supervisors, which must approve the so-called "open space easement" before the county assessor decides whether to reduce the tax bill.

A county staff report in April said each case would be looked at individually and cautioned that agreeing not to develop a parcel "does not necessarily reduce the value of the property."

Most landowners would need a lawyer to draft the agreement not to develop their property, according to Minerva Andrews, a lawyer who assisted McKinnon. Andrews said she did not charge her friend for the service, but that the legal bill would have totaled at least $1,850.

The law used by McKinnon and DuVal is similar to a state law that provides tax relief for landowners with more than 25 acres who agree to preserve their property for agricultural uses.

In Virginia, real estate taxes are based on a property's market value, and one of the key considerations in determining how much a parcel is worth is whether it can be developed to a denser use.

For instance, a family may live in a house on 20 acres, with no thought of developing the property. Nonetheless, if the land is zoned for one house per acre, the property is assessed based on that higher value.

Using the law as McKinnon did, the family can sign an agreement, which becomes part of the property deed, not to develop the parcel for at least five years, thus lowering the land's value and the family's tax bill.