The median sales prices of houses in nearly a third of the District's neighborhoods have doubled in the past five years, a new city survey shows.

It also shows that while the average sales price of houses in the city jumped 19 percent in the past 12 months, the total number of units purchased there dropped 16 percent.

Compiled by George B. Altoft, senior residential assessor for the District government, the statistics indicate that median sales prices for new and resale houses -- excluding condominiums -- have risen significantly in many parts of the city during the past 12 months, despite tight mortgage money and usury limits.

Altoft also said that the "single most consistent, fastest-gaining residential neighborhood throughout the 1975-1979 period" was Mt. Pleasant, even though its 166 percent rise put it in second place behind the central downtown area. Downtown has far fewer house sales a year -- only 25 last year -- and most are town houses being converted into multi-unit income properties.

Mt. Pleasant, on the other hand, is one of the larger residential markets in the city, with an average of 150 to 160 a year, and this provides a sizeable statistical base, Altoft said. Houses in the area, bounded by 16th Street NW, Rock Creek Park, 18th Street and Florida Avenue, have been undergoing extensive renovation in recent years.

The fact that median prices in Mt. Pleasant have gone up by more than 30 percent each year makes it the hottest residential market of the city during the past five years, the statistics indicate.

Observatory Circle's 140 percent rise has also been spectacular, Altoft said, but the number of house sales there is small, with about 30 recorded each year.

Data on condominium sales in the city is still being computed, but is expected to show a large increase in transactions and a moderate increase in sales prices, Altoft said.

The assessment statistics are taken from the property tax recordations of all residential real estate sales in the city. There were 5,310 homes sold in the District last year, down from 6,319 in 1978.

The area with the fastest-rising prices last year was the central downtown assessment neighborhood bounded by Massachusetts Avenue NW on the north and Pennsylvania Avenue BW on the south. The median sales price there jumped 69 percent, from $118,500 in 1978 to $200,000.

Also showing large gains were LeDroit Park, up by 39 percent, from $38,500 to $53,500; Forest Hills, 39 percent, from $172,100 to $230,850; the Observatory Circle area, 38 percent, from $139,500 to $194,000, and Mt. Pleasant, 33 percent, from $78,750 to $105,050.

The 15 neighborhoods with the greatest increases in median sales prices from 1975 through 1979 were central-downtown, up 212 1/2 percent; Mt. Pleasant, 166 percent; Observatory Circle, 140 percent; Kalorama, 139 1/2 percent, and Cleveland Park, 133 percent.

Also, Foggy Bottom, 125 percent; 16th Street Heights, 125 percent; Forest Hills, 121 percent; Berleith, 113 1/2 percent; Palisades, 113 percent, and Crestwood 111 percent.

Also, Capitol Hill, up 108 percent; Glover Park, 102 percent; Barry Farms, 101 percent, and Anacostia, 100 percent.

The presence of the Southeast neighborhoods of Barry Farms and Anacostia and the Northwest neighborhoods of LeDroit Park (94 1/2 percent), Petworth (89 1/2 percent) and Eckington (89 percent), suggests that that double-digit annual appreciation is by no means confined to the affluent third of the city west of 16th Street NW.

In fact, Altoft's five-year study discloses that in percentage terms, some of the most fashionable, high-cost neighborhoods were relatively lackluster in their year-by-year appreciation rates.

Georgetown, for instance, had a five-year median sales price increase of 88 percent, and ranked 31st out of the city's 56 assessment areas in rate of appreciation. The median sales price of $210,000 there last year, however, was the seventh highest in the city.

Another high-priced area, Chevy Chase, ranked 28th in rate of appreciation. The median sales price in that secton in 1979 was $134,200.