The average sales price for single-family houses in the District topped $100,000 for the first time last year, but average price increases slowed from 26 percent in 1979 to 5.6 percent, the lowest rate of appreciation in more than a decade.

Those are among the findings of the city's real estate assessments office in a new study of the 7,433 house and condominium sales made last year in the District. The statistical analysis documented these kay changes in the market last year:

About half the units sold here were condominiums, compared with less than 15 percent of the sales five years ago. From September 1980 through February 1981, in fact, more condominiums were sold in the city than houses, the first time that has ever occurred.

Median prices in some of the city's most prestigious neighborhoods, where double-digit increases have been the norm for the past four years, were on ice during 1980. A leveling off of prices had been long-expected in the city, but the most dramatic evidence of this was in Northwest neighborhoods such as Spring Valley, where the median sales price rose by only 1 1/2 percent, from $242,500 to $246,750.

Cleveland Park's median went up by 1.7 percent, from $179,800 to $182,500; Chevy Chase rose by 2.8 percent, from $134,200 to $138,000 and Georgetown went up by 7 percent, from $218,000 to $225,000.

Despite the slowdown of price rises in individual neighborhoods, the District continued to be a market where houses are exceptionally expensive. Out of 56 assessment neighborhoods, 28 had median home prices of $125,000 or more, and seven showed medians of more than $200,000.

The city-wide average price for houses sold last year hit $100,420 last year, up from 1979's average of $95,080.

Median prices for condominiums sold in the city remained level at about $71,000 during 1980, but only because of the large number of low-cost tenant conversion projects that year.

A separate analysis of condominium resales by one of the study's authors, city assessor George B. Altoft indicates that condominium prices rose sharply during the year, with an average monthly appreciation of 2.7 percent. A few condo purchasers who bought at tenants' prices last year saw their units gain in value by 10 percent to 13 percent a month, Altoft said.

The condominium boom is turning into a tax revenue-producing machine for the District government. The 4,109 converted and newly constructed condominiums that were added to the city's assessment rolls in 1980 will push the city's fiscal 1982 municipal tax base $128 million above what it would have been without them, the assessors found.

Sales volume for houses continued to drop, falling by 23 percent from 1979. The 5,310 houses sold in 1980 represented a 36 percent decline from 1978. Assessor Charles Horwitz, co-author of the study, said in an interview that the pattern of declines in house sales "can only continue," since an average home price of $100,000 and mortgage rates of 15 percent "are obviously affordable by only a small percentage of the market."

Builders are now virtually abandoning the under-$70,000 market and are concentrating on construction of houses priced at $130,000 and up. In the span of 12 months, the assessors said, the median sales price of newly constructed houses in the District rose by 100 percent, from $65,500 to $130,750.

While the bulk of the new homes sold during 1979 -- primarily town houses and smaller houses in the Northeast and Southwest -- were priced from $50,000 to $80,000, there was a strong shift to luxury-class construction last year. About half of all new houses sold in the city last year went for more than $125,000, and a number of those were Northwest twon houses priced at $200,000 or more.

While median prices declined in some neighborhoods, actual home values did not, the assessors noted. The declines were an indication that a larger proportion of lower-priced houses had come on the market than in previous years, Horwitz noted. Within those neighborhoods, however, homes are likely to still be gaining in resale value, he said.

"Real estate values in the District are still moving up," Horwitz said, "but at a slower rate than they did in any recent year."