Real estate sales in Montgomery County rose nearly 20 percent last year to a total of more than $2.7 billion, according to data compiled by a Washington information service.

Some 20,289 pieces of property changed hands in the county last year, including 14,213 single-family dwellings, the information service reported.

The average price of a house in Montgomery was $115,248 last year, while the average for a condominium was $84,361, according to the figures, which were compiled by Rufus S. Lusk & Son Inc.

The Lusk data showed that the county's most expensive single-family homes, on the average, were in Potomac. Some 747 sales there averaged $188,352, the figures showed.

Following Potomac were Chevy Chase, at $180,113, and Bethesda, at $166,014.

The most expensive condominiums were in Takoma Park, where the average selling price was $118,021.

The lowest-priced houses were in Germantown, where the average was $82,199, and the lowest-priced condos were in Damascus, where the average was $51,417.

(The high- and low-price leaders were in areas with substantial numbers of sales. Thus, readers examining the accompanying chart may note some differences between it and this article. For instance, the average condo price in Garrett Park was $178,500, but there was only one unit sold in Garrett Park last year.)

The most expensive residential property to change hands in the county last year was in Potomac, where 10608 Willowbrook Dr. went for $1.35 million, Lusk said. However, the company noted that of the Notes held by sellers were the largest single source of mortgage money. 10 highest-priced sales last year, only three were in Potomac, while seven were in Bethesda.

The second- and third-most-expensive properties were in Bethesda: 5400 Falmouth Rd. at $1.21 million, and 5327 Goldsboro Rd. at $1.2 million, according to Lusk.

The two most active subdivisions were in Germantown. Churchill Townhomes recorded 310 sales ranging from $51,584 to $224,553 and averaging $98,143. Gunners Lake Village showed 290 sales ranging from $43,043 to $88,560 and averaging $64,107.

Some 9,411 new homes (houses or newly built or converted condominiums) were sold in the county last year by 245 builders and developers, Lusk said, meaning that new homes accounted for just over half of all residential sales.

Ryan Homes was the leading builder with 972 sales, for a 10.3 percent market share. Ryan sold homes in 30 developments, with sales totaling $80.1 million and averaging $82,373.

In second place was Pulte Homes with 883 sales and a 9.4 percent market share. Pulte had $83.2 million in sales in 15 developments, with an average price of $94,239.

The Lusk data also revealed that in Montgomery, as in the District, notes held by sellers were the largest single source of mortgage money. Sellers wrote 1,108 first trusts last year for a total of $138.7 million -- accounting for 6.5 percent of all such loans.

The figures showed that seller financing ranged across the price spectrum, and was by no means confined to lower-priced properties. Though most of these loans (962) were for less than $150,000, there were 41 seller takebacks of more than $500,000 each. These upper-bracket takebacks totaled $53.9 million, Lusk reported.

All loans taken out to finance sales last year -- some 18,410 first, second and third mortgages -- totaled $2.15 billion for the county, Lusk said. And the Lusk report noted that, although the number of loans declined slightly from the 1983 total of 19,201, the dollar volume was up 5.6 percent from the $2.05 billion of the earlier year.

The figures also showed 1,887 Federal Housing Administration loans averaging $71,162 and 798 Veterans Administration loans averaging $89,875.

Among institutional lenders, Guaranty Mortgage Corp., last year's leader, again topped the list. The firm wrote 1,095 first trusts totaling $92.3 million, Lusk said. The information service also pointed out that Guaranty has achieved its position while concentrating on what were moderate-sized loans for Montgomery. Among Guaranty's more than 1,000 loans, only 30 were for more than $150,000, and none were for more than $500,000.

Guaranty captured 7 percent of the market for first mortgages by institutional lenders.

Following Guaranty were ICM Mortgage, First Washington Mortgage and Perpetual American Federal Savings Bank, all with about 4 percent of the institutional market.

Among commercial transactions, Kettler Bros.' sale of the Walkers Choice and Hamlet Apartments -- a total of 1,314 units -- to Montgomery Village HWK Associates for $34.6 million was the leader.

In second place, and the most expensive office building sale, was 1451 Rockville Pike, sold by Hines Industrial to Metropolitan Life for $17.8 million, all cash. Hines Industrial developed the building, which sits on 2 1/2 acres, leased it to International Business Machines Corp., and then sold it. Hines Industrial then pulled up stakes and went back to Texas and refuses to talk about its experience, leaving many here to wonder if the project was not as successful as it looked.