Rockville appraiser Alfred Jarchow, who makes a continuing study of the Montgomery County housing market, says that while average house prices were up in April over the previous month, active listings were down by one-third from a year earlier. At an annual rate, prices were up 12 percent for the year, he said. Jarchow commented: "It's going to be interesting to see what happens to prices in coming months. We could have a repeat of the 1973 experience if tightening mortgage credit terms should put a damper on sales in Montgomery County."

he recently announced change of leadership in the Yeonas Co., which has been owned since 1969 by Olin-American Inc., brought an end to a family dynasty in local residential construction. Founded 30 years ago by the George Yeonas, the firm was lead until 1969 by a team headed by George's sons, Stephen, Constantine (Gus), James and Paul. When the firm was sold to Olin, Steve formed his own investment firm. Gus stayed on and became president, while James and Paul continued to hold top jobs.

They are leaving at the end of of the month to pursue private real estate development interests. A former officer of the firm, John W. Guinee Jr., has been named president. Gus Yeonas says, "We'll be around if they need our aid or advice. Now we can pursue our own deals without corporate restrictions. But we still have a strong stockholding interest in Olin and thus the Yeonas Co. We want to see it continue to do well."

Washintonian Towers, at 26 stories one of the area's tallest apartments, is scheduled to be converted to condominium ownership by the new owner, the Ward Corp. of Gaithersburg. This growing housing construction and lumber products firm is currently completing a new headquarters building on Route 270, slightly south and east of the Washingtonian. President Dick Ward said that the landmark apartment structure, built on the Washingtonia golf course 10 years ago by Eig Enterprises Inc., has 205 units, 47 of which are being rented on a "day-to-day basis so that should ease any displacement problem."

Rents have ranged from $300 to $600 a month. The sales price of the building was not disclosed but one real estate professional observed that a price of $25,000 per unit or a total of $5 million "would be in the ballpark."

James C. Wilfong, the oldhomes enthusiast who lives in a new rambler near Prince Frederick, Md., recently came upon an unusual sign in the white-fenced Littlecote area of Baltimore County, Md. It reads: "Private Grounds. Where tramps must not, surely ladie and gentlemen will not trespass."