Units in 13 condominium projects in the District sold for an average of more than $100,000 last year, according to data compiled by the city's assessment office.

Kalorama Square, a cluster of 25 condominium town houses in the embassy area of Northwest, had average sales prices of more than $252,000. Two other projects in Northwest -- Foxhall East at 4200. Massachusetts Ave. and the Plaza on 25th Street across from the Watergate -- averaged more than $170,000 a unit.

Sales prices for some individual units rose as high as 40 percent over the year. Condo apartments in one building north of Georgetown have racked up steady price gains of more than 3 percent a month over the past three years.

The price figures are the first to emerge from the District's on-going study of its rapidly expanding stock of condominiums. The city assessment office has never included condominium units in its detailed quarterly studies of house prices by neighborhood. Senior residential assessor George B. Altoft began compiling separate data on condos last year. Among his preliminary findings:

Despite the fact that they tend to be smaller, condominiums aren't that much less expensive than detached houses, when sales prices are averaged. The average price for 6,300 single-family houses sold in the city last year was $74,000. The average for 1,800 condominium units -- which included some town houses as well as apartments -- was $62,944. Although the study didn't concern itself with price per square foot, a measure commonly used by developers and builders to compare costs, condominiums on the average probably cost more per square foot to buy than other kinds of housing in the city.

The highest-priced condos are concentrated in a small section of Northwest, while the lowest-priced ones are in Northeast and Southeast. In addition to the projects mentioned above, high average sales prices were also recorded in Northwest last year for 2029 Connecticut Ave., $137,000; the Worland at 2828 Wisconsin Ave., $124,354, and the Bancroft, 2231 Bancroft Pl. NW, $120,000.

Also, Cathedral West at 4100 Cathedral Ave., $115,000; Foggy Bottom House at 2405 I St. NW, $114,000 and California House at 2205 California St., $105,000.

In Southwest, Tiber Island, at 400 M St. had average sales prices of $101,400.

At the budget end of the price scale last year were the Wyatt-Dougherty building at 1034 6th St. NE, where the average unit sold for $18,810; Altamount Manor, 2400 Good Hope Rd. SE, $19,454; the Ridgecrest at 510 Ridge Road SE, $21,500, and Seward Square, 414 Seward Square NE, $22,718.

Well-located condominium units, purchased immediately after they are constructed or converted from rental status, can appreciate in value as impressively as any house in the city.

At one development examined by Altoft, the 70-unit Beecher House at 2400 41st St. NW in Glover Park, average sales prices were up by 84 percent over three years. Some units that sold in the low $20,000s there in 1975 went for about $55,000 last year. Altoft calculates that some units in the building are appreciating by as much as 39.6 percent annually.

At one large conversion project, the 278-unit Carlyn apartments at 2500 Q St. NW, units that sold for $31,000 in 1973 were going for $52,000 five years later.

Kenneth R. Harney is editor of the Housing and Development Reporter, published weekly by BNA, Inc. CAPTION: Picture, At Kalorama Square, the average sales price last year topped $250,000., The Washington Post