More than $1 billion worth of real estate changed hands in Fairfax County over the summer, the second consecutive quarter in which sales have reached that level, according to figures compiled by Rufus S. Lusk & Son Inc.

Some 7,713 properties were sold during the months of July, August and September for prices totaling $1.05 billion.

Of these, 92 percent -- or 7,118 -- were houses or condominiums, whose prices totaled $829 million, 78 percent of the dollar volume, according to the survey done by Lusk.

With interest rates declining, it appears the county's real estate industry is headed for a boom year.

Through the first three quarters of this year, 20,622 sales totaling $2.914 billion have taken place, according to Lusk. These numbers are equal to 86 percent and 92 percent, respectively, of the totals for all of 1983.

Lusk also reported that 8,506 mortgages were written during the third quarter -- 7,181 first trusts, 1,242 seconds and 83 thirds -- for a total of $961 million.

Lusk is an information and publishing service that tracks real estate transactions in Washington-area jurisdictions.

The firm's third-quarter report on Fairfax also noted that 10 builders accounted for 39 percent of the new-home sales market.

Ryan Homes was the volume leader with 215 sales, which gave it a 9.6 percent share of the county total of 2,244. Ryan's average price was $96,800, and its dollar volume was just under $21 million.

The Ryland Group was second, with 115 sales averaging $101,000, followed by Pulte with 112 and a $90,000 average.

NVHomes was fourth, selling 80 at an average price of $176,000. NVHomes also had the most expensive subdivision, Fairfax Station, where the firm sold sight homes at an average of $206,551.

The countywide average price for all sales -- commercial and land, as well as residential -- was $137,445.

The average price of a single-family residence was $121,490, while the average for a condominium was $88,318.

Lusk reported that five houses sold for more than $500,000 during the quarter, four of them in McLean.

The largest, however, was in the Mount Vernon on the Potomac subdivision south of Alexandria, where Mr. and Mrs. M. S. Levin sold a house for $680,000 to Mr. and Mrs. Hugo A. Davalos.

The largest transaction in the county was the sale of the Olde Salem Garden Apartments in Falls Church for $21.4 million.

Next largest was the sale of the Greywing Square Apartments in Reston for $13.04 million.

Assumptions of existing mortgages and seller take-backs continued to play a large role in the market.

Lusk noted more than 1,480 assumptions totaling $128 million, and some 350 take-backs worth $37. 6 million.

Perpetual American was the largest lender in terms of dollar volume, with 238 loans totaling $24.8 million.

Dominion Federal had the largest number, 261, totaling $21.9 million.