Prices of houses resold last year in Montgomery county increased 9 percent, according to figures compiled by appraiser-consultant Alfred Jarchow. The rate of appreciation was down from 14 1/2 percent in 1979 and 17 percent in 1978 but exceeded the 7 percent increase in 1977. Jarchow also pointed out that the upward trend in resale prices was below the 12.4 percent rise in the consumer price index in 1980. Since 1971, resale prices have increased annually at an average of 9 1/2 percent.

Passive solar energy features designed by architect Richard Theiss of Springfield have been incorporated into detached houses being built in Wildwood Hills Estates, near Burke Center, by Gaddy Construction. Theiss said that the houses are designed to qualify for recently enacted HUD solar bank mortgage loan subsidies up to $5,000 or energy credits up to $4,000.

A two-day seminar on real estate investing will be given Feb. 21 to 22 at the Key Bridge Marriott by John Haley and Dennis Makielski, who are realty developers and investors based in Clinton. Information on the seminar is available by calling 868-4410.

Jacyn Development Group has opened a group of luxury town houses in Columbia. Furnished models of the homes priced from $95,000 are located on a 15-acre site off Cedar Lane east of Harpers Farm Road. Houses have cedar siding exteriors and wood decks.

Long & Foster Realtors reported that its 35 offices here sold $936 million in property last year, up 5.2 percent over 1979. Most of the volume increase was attributed to the firm's new-home and condominium division. The average unit price of houses sold was $88,534, an increase in 9 percent.

FHA-insured financing for single-family houses has been extended to mobile homes, the Department of Housing and Urban Development announced recently. Former Fha Commissioner Larry Simons recently said that FHA-insured financing "represents the real future of manufactured housing in American," the Manufactured Housing Institute noted.

The redevelopers of the Foxchase of Alexandria apartments have announced that work has been completed on 470 of the 2,100 apartments. Robert Wheeler, project director, said the completed apartments are being rented at the development located on Jordan Street, Alexandria.

Century 21 of Springfield Inc. reported sales volume of $24 million in 1980. The firm now has two offices in Springfield.

Apex Realty, which recently opened an office in Springfield, now is half owned by JayBee Builders Inc. along with Virginia Lamp, who founded Apex. Apex is based on Rte. 1 in the Woodlawn area.

H. Harland Crowell Jr., who operated his own real estate office in Northern Virginia before becoming a homebuilder several years ago, has gone into business with Jack Lewis at Town & Suburban Properties. The new firm is in McLean. Crowell said he will continue to build contemporary houses in north Arlington.

The area chapter of the Community Association Institute will hold a seminar on warranty claims related to communities Feb. 24 in McLean. Information is available at 569-2147.