Statistics gathered by the Montgomery County Planning Board show that the county's population growth has slowed. County households averaged 2.93 persons each in 1977, compared with 3.3 in 1970. Most of the new growth now is in the Rockville, Gaithersburg and Germantown areas, while population in older sections, such as Bethesda and Silver Spring, has been stable.

Although the report warned that statistical averages cannot be considered typical, the average single-family household includes a husband and wife, both working in the county, with one or two children. The average family income ranges from $27,500 to $30,000 a year and the head of the average household is 45 to 50 years old.

People who visit the office of Franklin S. Eaton in the new quarters of Environetics D.C. Inc., an office space planning and interior design firm, would see only telephone and desk working spaces - no executive trappings such as those that the firm might design for a big law firm client. Eaton, who heads the local operation, which did more than $53 million in business here in four years, said that the idea of the new office is to show how small spaces can be effectively utilized. A sofa that revolves to face three different wall displays is used in a display room where clients are courted.

HUD Secretary Patricia Roberts Harris, the second woman to head that cabinet agency, has filled 50 percent of 146 appointive positions with women, a recent report noted. Among those named to top regional positions were Betty Jane Miller in the Denver area and Emma McFarlin in the San Francisco area, a first for HUD in the field.

Realtor Rhea Radin, a founder and director of the newly opened Women's National Bank, opened the first business account in that institution from a bed in the Washington Hospital Center, where she was recuperating from a broken kneecap. Bank president Emily Womack visited Radin in the hospital to acomplish the account opening. Radin, who broke her kneecap recently in a fall near her Capitol Hill business and home, now is recuperating at her residence.