The average sales price of a single family home in the 20 District neighborhoods west of Rock Creek Park in the first half of 1979 exceeded $183,000, according to new, unreleased D.C. tax assessment data.

The city-wide average price of a detached single-family home sold in the same period hit $90,000, up 29 per cent from the first six months of 1978.

The new figures, termed "incredible, truly astounding" by senior D.C. residential assessor George B. Altoft, confirm that the city's rapid price spiral has not been slowed by 1979's tight mortgage money and high interest rates.

The number of homes sold in the city during the first six months dropped by 24 per cent from 1978's sale pace but the decreases generally came in neighborhoods which had experienced heavy speculative activity on lower-cost units in recent years, rather than in higher-priced neighborhoods.

The largest declines in sales volume came in the far reaches of Capitol Hill and in Columbia Heights, where sales plummented this spring to less than half of 1978's rate.

The prime Northwest neighborhoods, meanwhile, kept up last year's record selling pace or exceed it.

Tax assessment office and realty industry analysts were uncertain what had caused the market to slow down in the southeastern and northeastern fringes of Capitol Hill, but theorized that the city's anti-speculation tax, investor nervousness over a national economic downturn and tougher mortgage terms in the marketplace probably contributed. Sales activity in one neighborhood on the fringe of Capitol Hill dropped from 1978's rate of 246 sales to 99 in the comparable second quarter of 1979.

Sales in the traditional close-in portions of Capitol Hill, meanwhile, have remained strong, and the average sales price in the second quarter went over $135,000, about 18 percent higher than 1978's comparable period.

The $183,000 average price west of the park was recorded on the 275 sales of existing and new homes -- excluding condominiums and cooperatives -- that occurred from January through June. If the Kalorama neighborhood straddling Connecticut Avenue and the park were added into the figures -- with its $258,000 per unit for the first half of 1979 -- the average would have been even higher.

Individual neighborhoods among the 20 west of the park racked up huge mid-year to mid-year percentage gains. Cleveland Park, for instance, showed a 49 per cent jump from mid-1978's $141,000 average (on 27 home sales) to $209,000 this year (on 22 home sales). The Palisades' average went from $106,000 in the first half of 1978 to $140,000 in the first half of 1979, a 32 per cent rise.

The most expensive neighborhood in the city continued to be Massachusetts Avenue Heights ($265,000). Following are Kalorama and Observatory Circle ($244,000), Forest Hills ($244,000), Spring Valley ($232,000), Georgetown ($226,000), Kent ($210,000), Cleveland Park ($209,000), Berkley ($200,000), Wesley Heights ($196,000), central-downtown ($193,000), Woodley ($191,000), and Foggy Bottom ($184,000).

The new D.C. figures also documented an emerging pattern of large dollar and percentage gains in Northeast, Southeast and Northwest neighborhoods undergoing extensive rehabilitation.

Barry Farms in Southeast registered a 75 per cent jump in average sales prices, from $29,000 in the first half of 1978 to $51,000 for the first half of 1979, and to $60,000 during the second quarter of this year.

LeDroit Park, with an upswing in remodeling activity under way in its historic district, saw average sale prices jump by 47 per cent, from $36,000 the first half of 1978 to $52,000 this year.

The central downtown area, including Logan Circle, recorded a 68 per cent increase during the period, from a $115,000 average price per dwelling to $193,000.

Three neighborhoods joined the District's formerly exclusive $100,000-and-up average price category last quarter: Mt. Pleasant ($105,000), Shepherd Park ($123,000), and 16th Street Heights ($102,000).

Thirty-one of D.C.'s 56 residential assessment areas now carry $100,000 and up average price tags. Nine are over $200,000.