A slow market in 1975-76 forced the renting of 72 Shadowood garden condominium apartments in Reston, where the leasing market is strong. Now there's generally a 90-day wait for new home deliveries in the Fairfax County new town. The rented Shadowood dwellings will be sold later this year. . . . Carla Hills, former Secretary of Housing and Urban Development, has leased office space through the Brannock firm in the Commonwealth building at 1625 K St. NW. The eight-person law firm is Latham, Watkins & Hills. . . . Howard Thompson, a director emeritus of Columbia Fedeal Savings and Loan Association, said the other evening that he likes living in Rossmoor. He's also delighted that he settled on a semi-detached dwelling before the prices were raised sharply late last year. Now several dozen similar units are available for immediate occupancy. . . . Later this year, subsidiaries of MCD Enterprises will open new singles and town houses in the Olney area and begin serious marketing of Prospect Plantation houses and big lots near the water and a golf course on Kent Island in Queen Anne County Md. . . . The Los Angeles Times reports that "a large number of unsold homes are beginning to show up in the higher price brackets" in the southland area of California. Also noted was a drop in the median price of resale houses in California to $63,102 from the all-time high of $64,424 last October. . . . Architect Robert Calhoun Smith is back at work again on a part-time basis after emergency surgency in Houston. An owner of a private jet volunteered to fly Smith quickly from Acapulco to Houston for surgery - which was performed by Smith's brother. . . . A downtown leasing specialist reported that the federal government has taken a 50,000-square-foot chunk of space for a price near $7.50 a square foot, higher than GSA has been accustomed to paying.