With hundreds of pricey home lots deemed illegal in recent years by Fairfax County development officials, the Board of Supervisors heard yesterday from angry homeowners who have been left with homes and lots that no longer can be sold or built upon.

"I need help now. I lived in my house for 36 years, and I didn't know anything was wrong until I decided to sell," said Mary J. Kearny of McLean, who said she and her husband can no longer afford the home because of rising taxes.

"My husband and I were stuck with the house," Kearney said. "We can't afford it. We can't sell it. We don't know what to do now."

"This has spiraled into the most expensive nightmare of my life," said Peter Berk, who has knocked down a house on a Great Falls lot and been prohibited from rebuilding it. "It defies logic."

County development officials in charge of land subdivisions estimated that as many as 5,000 such lots may be considered illegal, a fact that likely would come as a shock to the largely unsuspecting owners. The illegal lots typically were not flagged in past sales and mortgage transactions.

In response yesterday, the board revised the rules, legalizing lots created before 1947, as long as they met the zoning code of the time. The illegal lots created later could be validated under a cumbersome procedure that could cost homeowners several thousands of dollars.

"It still does not make sense," said Berk, whose lot was created in 1953. It was estimated that validating his lot under the approved procedures could cost as much as $6,000.

Kearny, whose lot was created in 1945, said she was satisfied.

"I got what I wanted," she said.

Exactly how so many lots created long ago and built upon might now be considered illegal is a matter of some dispute. The situation arises in part from shifting legal interpretations and changing legal descriptions for lots. It has become more of an issue as builders look to construct housing on available land or increase the size of older homes.

"To me, the county looks silly," said Supervisor Stuart Mendelsohn (R-Dranesville). "We created this issue out of whole cloth."

Board members indicated that, in future meetings, they would consider validating lots created after 1947.

In other action, the supervisors took issue with what they called a discriminatory plan by the city of Falls Church to increase water rates more for county users than for those in the city.

The Falls Church Department of Environmental Services supplies water to about 110,000 Fairfax County customers and to 10,000 customers in the city. The City Council gave preliminary approval to two rate increases last week. Falls Church users would be charged $1.89 per 1,000 gallons of water starting next year, up from the current rate of $1.64. County users would pay $1.97 for the same service. For every following year, the difference between the rates is projected to grow.

Falls Church gets its water from the Washington Aqueduct, which draws its water from the Potomac River. The city in turn sends most of it across its boundaries to county customers. Officials say that the aging system needs tens of millions of dollars of improvements in pumps, tanks and water lines.

Falls Church City Manager Daniel E. McKeever said yesterday that since most of the improvements are to benefit customers in Fairfax, the city's water authority is justified in asking them to shoulder a greater portion of the improvement costs. He also said the differential is common in other cities that supply water to neighboring counties. But Fairfax supervisors said they were outraged that their constituents would pay more.

"I'm shocked,'' said Supervisor Gerald E. Connolly (D-Providence), whose district is one of three affected. And he asked county officials to determine if Falls Church's intention is legal.

A final decision from Falls Church on water rates is expected in April.

Staff writer David Cho contributed to this report.

"To me, the county looks silly," said Stuart Mendelsohn. "We created this issue out of whole cloth."