Rent for 80 single-family homes and apartments on park property owned by the Montgomery County Planning Board will climb significantly this fall to end de facto salary supplements to county employes living in the houses, planning and park officials said this week.

"This is an attempt to keep rents on park homes current with the open market," said Donald K. Cochran, director of parks. "The planning board has no authority to provide salary subsidies in the form of rent discounts," he said.

Park employes living in the houses have been allowed to pay rents as much as 50 percent below the 1980 appraised value because they did repairs and worked extra hours patrolling the park adjacent to their homes, officials said.

Although all below-market rents had routinely been reported to the Internal Revenue Service as a taxable benefit, a February 1985 Supreme Court decision complicated the issue, said planning board Chairman Norman L. Christeller. That decision made public agencies subject to the Fair Labor Standards Act and made it illegal for any public agency to give rent subsidies, days off or other perks in lieu of wages, Christeller said.

"So we'll have to pay in cash for services rendered on the properties after hours, rather than reducing the rents," he said. The planning board added about $100,000 to its current budget to cover the expected expense of compliance with the Supreme Court ruling, Christeller said.

Under the new rules, all houses will get an across-the-board 7 percent rent discount because of "the nuisance factor in living in these places . . . the high traffic and noise," said Charlie McGovern, property manager for the planning board. Class A houses -- those considered vital to the park or those with historic value -- will receive an additional 8 percent rent reduction.

"If they live on a golf course, in Cabin John or near Brookside Gardens' greenhouse, they have to open the park and chase people out at dusk, and now we're going to pay them for this work," McGovern said.

McGovern and Cochran said the county will be providing more "landlord services" than in the past and that tenants will be eligible for a flat discount rate on any interior or exterior painting they do.

But some tenants are still not sure they will be able to make ends meet under the new rent system.

When park historian Mark Walston comes home at night, he often finds picnickers in his backyard, music blaring and no place to park his car. Walston, 30, lives in a 1794 miller's cottage -- one of about 130 properties owned by the county's planning board. His house sits in Valley Mill Park in Colesville, one of three county parks where beer drinking is allowed with a permit. A public pool and a county-run summer day camp with their attendant hordes are about 70 feet from Walston's front door.

Walston and his wife were paying $280 a month for the two-bedroom, one-bath house, but in October they will pay $600 -- a 112 percent increase that Walston said he had no idea was coming when he moved into the historic property last summer.

"It was in pretty bad shape . . . . I did a lot of replastering, sanding, repainting." Walston also said he had a $2,000 heating bill for the winter because the oil-fired hot water system is old and the insulation inadequate.

"A lot of people are concerned they'll have to leave . . . . One man's rent increase was more than his take-home salary, so he'll have to come up with extra money just to stay," Walston said.

Walston, who like all park-home tenants has a 30-day lease, said that while he was sure the planning board "had the interest of its employes at heart," the 15 percent deductions "don't go far enough. . . . These houses are not in the middle of some subdivision." He suggested an additional 5 percent reduction for those that "maintain the houses in excellent condition."

Fran Tolford, a county housing activist and member of the Suburban Maryland Fair Housing Commission, said her group is concerned that some of the more moderate- and low-income families may be forced to move.

Park officials said that about 25 tenants are non-county employes, but that none of them have come to them with specific complaints as yet.

"We want to work with anyone for whom these rent increases present a hardship," Cochran said. "That's why the board is going to phase the implementation in between October 1985 and April of 1986."

McGovern said each housing site is a special case and will be considered as such when it comes time to pay tenants for work done there.

Margaret Elliot, the assessor hired by the county to do the reappraisals, said her rent calculations took location and age into consideration. She said that despite the complaints she has no plans to lower any of her proposed rent scales. "They are in line with what is available elsewhere."

The 130 park properties are far-flung and varied. They include a cluster of eight homes on the fringes of Little Bennett Park on Route 355; an apartment in Candy Cane Village Park in Chevy Chase; three radio transmission towers; the Seneca Store and several properties housing county-run programs for retarded, elderly or de-institutionalized residents.

Altogether, there are 97 single-family homes and 14 apartment units. All but 82 of the properties were reappraised within the last two years and were exempt this time around, McGovern said.

"We just needed to revise the rents and clean up our practices so we don't get into any trouble . . . . When we did reappraisals in 1980, we got the same kind of responses. We are concerned, but everyone's rents are going up."