When Montgomery County assessors preparing next year's tax bills looked a couple of months ago at the record of recent home sales in a Germantown subdivision, they saw trouble.

In this neighborhood where the houses are nearly identical, sales prices have traditionally clustered in a small range, providing a solid guide to assessors trying to figure where to peg current property values for tax purposes.

But last year, prices wobbled from $72,000 in early 1981 to $77,500 by September to $67,500 by December to $90,000 last month, in the midst of the worst housing market in memory.

Assessors in Montgomery believe those prices reflect not the value of the different houses, but the attractiveness of the financing that the owners were able to offer. That poses a problem that the assessors--and their counterparts around the region and the country--are still struggling to solve.

As high interest rates have driven almost all buyers away from conventional mortgages, so-called creative financing has distorted the traditional relationship between sales prices and home values to the point that officials must find some way to assign just and equal values to widely differing financing packages or run the risk of overassessing homes.

How they solve that problem will affect the taxes paid by every area homeowner.

Arlington County is the sole local jurisdiction that has devised a somewhat scientific method for determining the impact of the seller financing.

Washington and Fairfax County officials said they had lowered assessments this year to account for the impact of creative financing, but they could not explain the amount of decrease nor how it had been calculated. Officials in Alexandria, Prince George's and Montgomery County said they are still studying the problem.

Until this year, assessors usually pegged their property tax assessments to sales prices because the vast majority of buyers paid for houses the old-fashioned way--a buyer went to a bank or savings and loan for a 30-year fixed-rate mortgage and gave the seller a lump sum payment.

But creative financing has done away with the common denominator by making sellers the new mortgage lenders. Instead of receiving a lump sum payment, sellers are getting only part of the sales price in a lump sum and the rest over a period of time. Each seller and buyer are negotiating their own terms for these loans--their own interest rates, amounts to be repaid and the time limit for repayment.

According to several assessors and real estate agents, that means sales prices no longer indicate the value of homes or condominiums but reflect the attractiveness of the financial packages offered by sellers, who supplied part or all of the financing in 60 to 70 percent of the 32,311 home and condominium sales that occurred in the Washington area last year.

At the same time there were dramatically fewer sales, thus compounding the problem of finding a "normal range."

Sellers are offering first or second mortgages at 12 to 14 percent while banks and savings and loans are asking 16 and 17, according to several assessors and real estate agents. As a result, area sales prices have remained relatively stable or fallen only slightly despite the struggling housing market.

Robert King, the District's acting associate director of real property taxes said, "They sellers are keeping the prices up by taking back mortages at lower interest rates. "There is a lot of water in those sales. We realize there is a buffer in those sales because of the financing," added senior D.C. assessor George Altoff.

James R. Vinson, Arlington's director of real estate assessors, said that after conducting a survey to measure the impact of seller financing, his office reduced all property values by 2.5 percent for the assessments that were mailed out in February.

"Instead of just ignoring it and hoping it would go away," Vinson said, his office talked to about 250 people who bought homes in the county last year and asked them to detail their financing arrangements. The assessors then reduced each sale to its actual cash value and compared it to the amount of actual cash the seller would have received with conventional financing.

Arlington assessors found that the cash value to the seller of the creative financing was on the average 2.5 percent less than the conventional financing.

The difference was slight, said Vinson, because "the vast majority had balloon notes due in five years." That is, the buyer would make interest payments for five years only, and then the entire principal amount of the mortgage would come due.

If the below-market financing had been spread out for a longer time, the difference would have been more pronounced, he said, because the longer the payments go into the future the more inflation reduces their value.

District and Fairfax officials said they had also lowered values but the decreases were based on their judgments and not on detailed analysis. They could give no specifics on how much values were reduced or whether reductions were consistent for all homeowners.

"It's not a science; it's not exact," said King. "It's our judgment."

Samuel Reynolds, an independent appraiser and chairman of the District's board that hears taxpayer appeals said if city assessors do not correctly reduce values because of the financing "we will make the adjustments." With creative financing, he said, "Those the sales prices are not the true value."

Samuel Patteson, assessment supervisor in Fairfax said his office was assessing residential property at only 91 percent of its value instead of the required 100 percent to allow for the seller financing and because sales prices often include personal property such as stoves, washers and dryers.

But Patteson said he could not say what part of the 9 percent cut was attributable to the seller financing nor how the amount of decrease had been figured. "It's opinion. It's not math," he said.

John Austin, Alexandria's deputy assessor said, "I guess we are backing out of the problem now." Officials there are just beginning to explore the question, he said.

Gene Burner, director of the Maryland Department of Assessment and Taxation said, "It's kind of an emerging thing and we're trying to take a look at it . . . We're looking to see if there is a problem."

He said his officers, who are located in each of the state's 23 counties and Baltimore City, are not sure that creative financing has replaced conventional fixed-rate financing in a large number of sales.