Sixty-five single-family houses were sold through the multiple listing service of the Washington Board of Realtors during June at an average price of $106,771. Fifteen condominium and cooperative apartments were sold during the month, at an average price of $68,066.

The Northern Virginia Board of Realtors said 2,338 residential sales were made during June through its multiple listing service for a total volume of $186 million. Sales in June 1978 totaled 2,079 for a total of $148 million. Total volume this year is $100 million ahead of the same period in 1978, the board said.

Half of the 40 semi-detached houses being built at Michigan Park North, at 14th and Gallatin streets NE, have been sold - at prices ranging from $89,000 to $103,000, according to the firm handling sales for Horning Brothers Inc., the developer.

Anthony C. Koones Associates has bought a 6.2-acre site in Rockville's Town Center urban renewal area, where it plans to build 100 rental units in a mid-rise apartment building and 65 town houses for low to moderate-income purchasers. Three-fourths of the rental units will be designed for elderly and handicapped persons. The firm paid $511,000 for the site.

Simon Eichberg has joined the Braedon Companies, which headed by his son, James, as a commercial sales agent. The senior Eichberg has been in commercial real estate selling and leasing for three years. He was previously an executive of De Young Shoes here.

The Charles E. Smith Companies has moved its office from 17 and L streets NW to Crystal City, where it has been developing a large complex of commercial buildings for the past 15 years.

Porter and Associates, a Salisbury, Md., real estate firm, has affiliated with the Better Homes and Gardens Real Estate Service, a franchise based in Des Moines, Iowa.