Despite financing challenges, the average resale prices of homes in the Washington area continue to rise -- particularly for lower- and moderate-priced town houses and condominiums.

Prices rose 4 percent in Montgomery County during the first three months of this year, reported Alfred W. Jarchow, a Rockville appraiser who documents market trends. Town houses, condos and detached houses priced under $80,000 accounted for much of the increase, while prices tended to stay flat for larger homes on the market for $150,000 or more. p

Moderate-priced units are in heavy demand and sell rapidly in a market like this year's, Jarchow noted, while the larger, more expensive properties tend to sit for months before selling. When smaller homes come with assumable financing or secondary loans from sellers, prices stay high. More than half of all Montgomery sales in March carried such arrangements.

When a house has to be sold using a new, conventional loan from a savings and loan association or bank at prevailing rates of 15 percent or higher, prices tend to give way, according to Jarchow's analysis. Only a third of the county's home sales in March used straight conventional financing and about 15 percent used new FHA or VA loans.

Jarchows's data parallels trends in the District, where appraisal officials say that sales of lower-priced units, particularly condominiums, have been hot and price increases sharp.

The city's senior residential assesor, George B. Altoft, says prices for expensive houses have leveled off in many neighborhoods because of the difficulty of financing.

But sales prices of well-located condominium units -- particularly those with assumable loans and seller "take-back" financing -- have been increasing by 2 and 3 percent a month in some cases. The higher priced the condominium, the less likely it is to benefit from the current market squeeze. Close-in, converted units originally purchased by tenants are showing the highest price increases, Altoft says.

The Montgomery County market study by Jarchow on prices and sales during 1979 and 1980 also found that:

There are 38 percent more unsold units available for sale currently than a year ago. At the end of March, 2,900 units, including town houses and condominium apartments, were on the market.

The more money the purchaser is willing to spend, the more there is to chose from. There were about 40 percent fewer homes for sale costing under $60,000 than there were a year ago. In the $60,000 to $80,000 range, there are 15 percent more units unsold. In the $80,000 to $100,000 bracket, there are 50 percent more units available than in the same month in 1979.

In the $100,000 to $150,000 range, thre are 70 percent more houses than a year ago, and above $150,000, there are 80 percent more on the market.

The sales price pattern tends to follow this supply curve. Generally speaking, prices of the houses in the lower ranges are increasing the fastest, while those in the upper ranges rise more slowly.

"A few years back these high-cost homes lead the market," Jarchow said, "But in the last year their average price gain dropped to less than 4 percent. This is one segment of the market where a decline in new building construction will help toward readjustment of supply and demand." CAPTION: Picture, CONDOS -- JBG Associates and Holladay Enterprises report that two-thirds of the 88 units have been sold in their new Dupont West building at 2141 P. St. NW. Prices range from $83,000 to $134,000. Burr, Morris and Pardoe is the agent. By Lucian Perkins -- The Washington Post