On a three-acre site just southeast of Garfinckel's Spring Valley store, the W.C. and A.N. Miller constructon firm is building 35 large houses. Located on undeveloped ground that the Miller firm bought in 1927, the project will included 11 detached houses and 24 attached houses. Edward R. Spano designed the traditional, masonry dwellings, which will have three of four bedrooms and two or three baths. A Miller spokesman said that 11 of the houses have been sold, at prices ranging from $229,000 to $249,500. Buyers are expected to begin moving in August.

Montgomery County police report that there have been several thefts recently from houses open on weekends for inspection by prospective buyers and are recommending that owners take some precautions. One Layhill Village resident, Doris Davis, told police that she and husband Kenneth left their house, which was being shown by a real estate agent, on a recent Sunday and came back to find jewelry worth at least $500 missing. Police say small, valuable items should not be left around an open house or even in drawers. Seven other open houses were hit and police later made two arrests and recovered some of the missing valuablees.

The house-price spiral continues. The Rockville government reported that the average sale price of houses in the second half of 1978 was $70,137, an increase of 17 percent over the average price for the same period in 1977 and a 9 percent increase over the average for the first half of 1978. During the second half of last year single-family houses sold for an average of $73,7432. New house sales averaged more than $110,000, topping $100,000 for the first time. That represented an increase of 36 percent over average new home prices in the latter half of 1977.

The Montgomery County Planning Board has recommended approval of a special exeception for developer Roger Eisinger, who wants to extend the "air rights" office complex over the B & O tracks at Wisconsin Avenue in mid-Bethesda and build a reaquetball and squash facility between Montgomery Avenue and the tracks east of Pearl Street. A spokeswoman for the Chevy Chase Village committee, representing neighboring residences, supported the spplication.

Alan Landau and Elliott Burka are the developers of a nine-story condominium at 3110 Wisconsin Ave. NW that features two-story apartments with one and two bedrooms. The building is being constructed by Furman Builders. The 30 units will have fireplaces, balconies and self-contained heat pump systems for heating and cooling. Prices begin in the low $80,000s. Occupancy is scheduled for this summer.