The average sales price of a house in the District rose to $101,556 in the third quarter of this year, a 30 percent increase from a year earlier when the average price was $78,091, city assessment records show. And the current high interest rates don't appear to be slowing the rate of increase.

While the total number of house sales in the city dropped by 7 percent in the third quarter, the average sales price per house in 24 of the District's 56 neighborhoods increased by 25 percent or more. In a dozen neighborhoods the average sales price was more than $200,000. A year ago, three neighborhoods were in that category. These figures -- derived from the city's property transfer records for all single-family units, except condominiums -- are the most accurate guage of neighborhood-by-neighborhood price movements in the city. The figures don't show how much a particular house in a neighborhood may have risen in market value this year, but only how the number of sales and the average price of houses sold in each neighborhood compare with similar periods in the preceding year.

The statistics also do not include the months of October and November, when the combined impacts of the Federal Reserve's toughened credit policies and local usury ceiling problems depressed real estate activity in the District.

Average prices in the city have been rising by more than 20 percent annually for more than two years, leading analysts to expect a slowdown in this year's tight-credit environment. District prices for the first nine months of the year, however, rose 28.5 percent over 1978's levels.

Unit sales of single-family houses were down by 17.7 percent in the same period, but the major part of that decline came in two large areas on the fringes of Capital Hill, where 1978 sales had a speculative burst and have since returned to lower levels. About the same number of houses were sold in most neighborhoods as were sold the year before. In five neighborhoods -- Garfield, Hawthorne, Fort Dupont Park, Anacostia and Randle Heights -- average sales prices declined. The largest drop, 18 percent, was recorded both in Hawthorne, where three sales averaged $151,527, and in Fort Dupont Park, where 22 houses were sold for an average of $40,195.

Condominium sales, which officials began tracking formally last year, jumped by 18 percent in the first nine months of 1979, to 2,181 units. Average prices through the third quarter were $69,000 per unit, up by 10 percent from 1978's $63,000. However, George B. Altoft, senior residential assessor for the city, noted that measuring condominium values by average sales prices may understate the real appreciation under way.

Current condo sales, he said, are largely of one-bedroom and efficiency units that have just been converted from rental status and are selling for more moderate prices -- in the $40,000s and $50,000s -- than were 1978's condominium offerings.

As a result, the average condominium price increase of 10 percent appears significantly lower than for detached houses -- "and that is not necessarily the case," Altoft said.

The average sales price for condominiums by project reported thus far in 1979 range from $21,600 at Altamount Manor in Northeast to $432,500 for the Karorama Square town house in Northwest.

Neighborhoods with big increases in average house prices this year have been:

Central-city and downtown (south of Dupont Circle and Massachusetts Avenue NW), up 76.3 percent to $226,857.

Observatory Circle in Northwest, up 64.2 percent to $229,000.

Barry Farms in Southeast, up 58 percent to $45,212.

Foggy Bottom, Northwest, up 55.5 percent to $179,000.

Wakefield, Northwest, up 43.1 percent to $161,150.

16th Street Heights, Northwest, up 40 percent to $96,273.

Forest Hills, Northwest, up 37.9 percent to $238,616.

LeDroit Park, Northwest, up 36.3 percent to $53,540.

The neighborhoods with average house sales prices this year in excess of $200,000 are all in Northwest. They are: Massachusetts Avenue Heights, $290,016; Kalorama, $261,771, Spring Valley, $248,720; Georgetown, $243,492; Forest Hills, $238,616, and Wesley Heights, $237,888.

Also, Observatory Circle, $228,999; Central-Downtown, $226,857; Woodley, $216,950; Kent, $212,897; Berkley, $206,865, and Cleveland Park, $202,122.