When Charlotte James bought the tiny, 14-foot wide wooden frame building that houses the post office in the Loudoun County hamlet of Hillsboro, she paid $2,000.

Today - 18 months later - the county has said the buildings is worth $20,090. "When they said '$20,090,' a still disbelieving James recalled last week, "I said, they've got to be kidding."

But the county assessor wasn't kidding as not only James, but thousands of other Loudoun property owners hit by sharply higher property assessments recently discovered.

Angered by assessments that in some instances have jumped as much as 100 per cent this year and a higher tax rate, many of the county's 50,000 residents are seething. On Monday a crowd of 300 gathered on the lawn of the county courthouse in Leesburg, calling for a 10 percent cut in the county spending and denouncing the governing Board of Supervisors as spend-shifts.

The protesters, many of whom have taken to wearing teabags as a symbol of their movement, have presented the county board with a petition calling for reduced government spending that has been signed by 5,000 people. Since that represents one out of every 10 county residents, protest leaders say that the signatures indicate they have mounted one of the strongest antispending drives in the Washington area.

The Loudoun board has called for a study of what a 10 percent spending cut would do to the largely rural county and has created a committee that will "examine the methodology" of the county assessor.

Until this month hardly anyone paid attention to the assessor's office. In fact the position was considered so unimportant that the county did not even pay a salary to the chief assessor, Lee T. Keyes.

Keyes, who is also the county revenue commissioner, draws only $500 a year in expenses for holding the assessor's job.

What has happened in the county is not just a product of the assessor's work.

For the increasing number o people who want an authentic slice of country life within commuting reach of Washington. Loudoun is the next best thing to Walton Mountain.

There are a number of old houses, in picturesque and secluded locations. There are real farms that produce corn and milk. And until recently, there were low taxes.

In a sense, Loudoun's blessings have become its curs. As newcomers snap up aging country houses, prices escalate. Assessor Keyes said that old houses used to be depreciated for their drafty windows and cracking plaster, but now, "there are buyers who welcome the challenge of these problems."

For some old residents who sold homes, the surging prices have given them a sudden nest egg for their retirement. But there are others including many of the protesters, who say they are caught in the middle: They do not want to leave Loudoun, but they cannot afford to stay.

While the latest reassessments were probably highest in Hillsboro - the average is "close to 80 percent," according to assessor Keyes - there have been big one elsewhere in the county.

Alma Atwill, a 78-year-old widow who support herself on three pensions totaling $2,400 annually, live in a big white Victorian house in Ashburn, which is between Leesburg and Sterling Park. Her house hasn't had central heating since the boiler burst in 1925, and "the land won't perk," meaning that it is unsuitable for septic tanks.

Nevertherless Atwell's house and three-quarters lot were valued at $88,730 doubel the appraisal last year. "I know very little about real estate." Atwell said the other day, "but this place would not bring that amount of money."

Keyes acknowledges that his office has "made some mistakes," but overall, he maintains, the assessments represent fair market value in an incredibly hot - some people say overheated - real estate market.

In some cases, as a check of county records shows, the newest assessments are far below market value, or at least the selling price.

For example, Jina Alexander, first chairman of the Loudoun Taxpayers Association - the main group behind last Monday's courthouse-square protest -said the assessment on her house went up from $70,000 to $92,000, but a potential purchaser has offered to buy it for $166.000.

Most of the protesters come from the still largely rural western part of the county. In fact, only 150 of the 5,000 signatures on the antispending petition belong to residents from eastern Loudoun, where the large subdivisions of Sterling Park and Sugarland Run are located.

Western residents complain that too much money is spent on frills, especially in recreation and schools. "It is not a function of government to provide entertainment," said John C. Larson, a member of the Taxpayers Association Board.

The Loundoun supervisors are trapped in a dilemma. While western residents, many of whom live on 5 and 10-acre tracts, say they don't need parks and recreation centers and object to paying for them, cramped subdivision dwellers in eastern Loudoun are demanding them.

Complicating the supervisors' dilemma is the county land use tax, which permits owners of working farms to defer tax payments until their property is sold. The tax cost the county $2.8 million in revenue in the last fiscal year.

But if the tax break is abolished or made more restrictive in an effort to raise additional revenue, more land owners would probably succumb to the pressures to sell, leading to more urbaning of the county - and ultimately to stronger demands for more county services.

In recent days a steady line of people has been coming to Keyes' Leesburg office asking for readjustments on their assessments. One of the petitioners has been Charlotte James, who owns the Hillsboro post office building.

James' two-story building in Hillsboro is 14x25 feet and has no bathroom.

The assessor told James, she told, that the property was appraised so high because of its "commercial potential," a suggestion that she finds more amusing than realistic.

The only thing that has ever been sold in the building - besides stamps and postal money orders - has been penny candy, she said. The previous owners quit selling candy. James said, "because it went up to 2 cents and, besides, all the kids in the neighborhood had grown up." CAPTION: Picture 1, Charlotte James outside Hillsboro post office building, assessed at $20,090. By James A. Parcell - The Washington Post; Picture 2, Alma Atwell lives in a Victorian frame house now appraised at $88,730, twice as much as last years's valuation. By James A. Parcell - THe Washington Post