The real estate boom that has been under way in Fairfax County since the end of the recession continued last year as the county recorded property sales totaling just under $4 billion, according to figures compiled by Rufus S. Lusk & Son Inc.

The dollar volume represented an increase of 26.2 percent over 1983. The number of transactions reached 27,473, a 14.7 percent rise from the previous year, Lusk reported.

More than 25,000 homes were sold in the county last year -- 21,758 houses and 4,916 condominiums -- but the average price slipped somewhat. In 1983, condominiums averaged $84,814, compared with $81,949 last year. Lusk has revised slightly the way it tracks house sales, so the two years could not be compared directly, but it appeared that the 1984 average of $121,871 was below the 1983 level.

Nonetheless, business in what brokers like to call the "upper brackets" was brisk, the Lusk figures showed. The top residential transaction in the county last year was the $4.5 million purchase of Merrywood, the childhood home of Jackie Onassis on Chain Bridge Road in McLean, by Dianne Kay, wife of developer Alan Kay.

The top 10 home sales in the county all exceeded $500,000, and average selling prices in the 10 most expensive subdivisions all exceeded $325,000, Lusk noted. The highest-priced subdivision was River Oaks in McLean, where the average sale ran $469,438.

Nine of the top 10 subdivisions were in McLean. The lone exception, Mt. Vernon on the Potomac, in the Alexandria section of the county, ranked sixth, with an average price of $381,696. Lusk also noted that McLean was the most expensive neighborhood overall, registering an average house price of $215,725.

In the condominium market, however, Great Falls was the leader with an average of $190,873.

The accompanying map shows Lusk's average house and condo prices for the various areas of the county, based on postal zip codes.

Lusk also reported that 24,939 first mortgages taken out at the time of sale were recorded last year, totaling $2.89 billion. Some 4,885 of these involved assumptions -- about 20 percent of the total -- and another 1,241, or 5 percent, were seller take-backs. Assumptions totaled slightly more than $400 million.

Dominion Federal Savings was the top institutional lender in the county in terms of numbers of loans. Dominion wrote 867 mortgages totaling $75.4 million, Lusk reported.

1st Washington Mortgage, which wrote 804 loans, recorded a dollar volume of $86.1 million, Lusk said.

The 10 busiest lenders accounted for 5,880 loans, or 31.3 percent of the market.

Similarly, the 10 most active builders also had just under a third of the market -- 31.5 percent -- with a total of 3,225 sales.

Ryan Homes recorded the largest number of sales, 905, for an 8.8 percent market share. Ryan's sales totaled $85.6 million, for an average price of $94,634.

Following Ryan was the Ryland Group with 439 sales, 4.3 percent of the market, for a total of $45.1 million and an average of $102,660. Just behind Ryland was Pulte with 438 sales, also a 4.3 percent share, for a total of $40 million and an average of $91,391.

NVHomes was fourth on the list with 354 sales and a 3.5 percent share. But in terms of dollar volume, NV ranked second with $55.6 million and an average price of $157,030.

Together, the top 10 builders sold $348.7 million worth of homes at prices averaging $108,124, Lusk said.

Of the 10 most active subdivisions, Reston had the largest number of house sales, 1,387. The most expensive house sold there went for $284,794 and the cheapest for $49,000. Following Reston were Franklin Farm in Herndon, with 593 sales ranging from $218,890 to $72,000, and Burke Centre, with 574 transactions ranging from $180,000 and $60,000.

On the commercial side, 799 properties changed hands. This was 2.9 percent of all sales but, at $922 million, 23.2 percent of the dollar volume. The largest transaction was the sale of an office building at 8120 Old Courthouse Rd. in Tysons Corner to Prudential Insurance Co. for $26.8 million, all cash.