On the crest of the inflationary 1970s, well-located apartments have become more valuable. And conversion to condominium ownership has meant bonanzas for many developers and landlords.

That's why the 1,500-unit Grosvenor Park in North Bethesda, the 800-unit Towers and a building of the Van Ness Complex in Northwest Washington, the Kenwood in Bethesda and hundreds of other prime rental properties here are going condo.

Five years ago, when the conversion market here was depressed, one of the three high-rise Grosvenor buildings was converted to condo ownership by the owner. Now the other two high-rises and an enclave of garden apartments along Rockville Pike are being converted by a Chicago-based redevelopment specialist, American Invsco. The firm has been active in more than a dozen cities.

Predictably, the more than 1,000 tenants who continue to rent at Grosvenor were shook up when they learned in January that American Invsco had purchased their apartments from the Karl T. Corby Corp. for $30.5 million. Corby built the attractive complex in the early 1960s.

The Grosvenor has been fully rented for most of its history, and long-time tenants have been spared crunching rent increases over the years. But Corby recognized market realities and received an offer that must have been too good to turn down.

In recent weeks, American Invsco and its management, sales and maintenance affiliates have mounted a quiet campaign to sell the apartments, generally in "as-is" condition, to tenants - who were offered first choice and a lower price - and to the public.

Prices for the apartments range from $28,000 for the smallest efficeciencies to $260,000 for the largest penthouse apartments. Condominium fees for heating, cooling and maintenance are to be based on square footage.

Tenant reaction to this offer has been mixed.

Some, particularly those who are members of an existing tenant association, recoiled from what they describe as a totally one-sided offer from the redeveloper.

Thus far, more than 500 prospective buyers - about one-fifth of them tenants - have placed deposits on the apartments, American Invsco says. The units are expected to be available late this year.

Grosvenor tenants, whose incomes tend to be above average, have untill Tuesday to take advantage of the developer's incentive offer. It includes a credit for six months of rent (before conversion), 1 percent off prices and a payment of association fees for a year as well as a required reserve payment.

But tenants generally have been critical of the offer, and many have rejected it. Tenant association attorney Petter Messitte described is as "heavy-handed and one-sided." Messitte told 500 tenants at a meeting last month that by accepting the "as-is" contracts, they also would be waiving the right to have their units looked at by a professional home inspector before they buy.

Messitte urged the tenants to try to get better terms by confronting the redeveloper. Invsco said that because the tenant association would not reveal its membership, negotiations would have to be undertaken on an individual basis.

But earlier this week, Messitte said that a representative of the redeveloper had agreed to meet with him as the authorized representative of 275 potential purchases who now live at the complex.

In letters to the Maryland attorney general and the Montgomery County consumer affairs, Messitte also raised questions about sales and conversion practices at the Grosvenor that he said violate state and county statutes regarding contracts.

Tenants have expressed reservations about the condition of the 16- and 18-year-old buildings. An engineering report mentions problems with waterproofing and leaks. Heating the building can be difficult when the temperature falls 20 degrees, the engineers found.

But the redeveloper said contracts have been let to improve the structure.

"It's really typical of a building of its age," said one engineer who inspected it.

Invsco's main man on the scene, Nicholas N. Mason, said that although outside interest in the Grosvenor is strong, the reaction of tenants has "become a challenge."

He also pointed out that prices for units being resold in the south Grosvenor building, which went condominium five years ago, have nearly doubled.

Although the Grosvenor situation is somwhat of the hundreds of other condominium conversions in this area, it is also atypical for several reasons.

First, the redeveloper is a large, out-of-town firm that marketing director George Prochaska calls "tightly knit." Second, Grosvenor tenants tend to have above-average incomes and have been paying relatively low rents.

The Grosvenor complex is also well-located, a prime factor in the value of real estate. A tunnel under Rockville Pike will connect the campus-like complex to a Metro station that is scheduled to open in 1981.

Grosvenor is near the Beleway, White Flint Mall, the new Bethesda Marriott, the Jewish Community Center, the Navy Medical Center and the National Institutes of Health.

The Georgetown Preparatory School's nine-hole golf course adjoins Grosvenor to the north.

A spokesman for Georgetown Prep said that there are no immediate plans to develop any of the 78-acre course for residential housing or commerical development. But other areas around Grosvenor and the new Metro station are still held for development by the Corbys and others. The condominium property report indicates that the Corbys have land for another high-rise building.

If there's a real zinger for tenants of the Grosvenor, it's the scarcity of alternative rental housing. Developers haven't been building rental apartments for non-subsidized tenants in recent years because of high interest rates, rent controls, the increased cost of ground and decreasing returns from the best of rental properties.

Not all of those converted apartment are occupied by owners, however. About one-fourth of all new and converted condo apartments are said to be rented out. Mortgage costs and condominium fees help push up the rents of condo units after they are converted, however.

And finally, the additional tax yield from the individually owned apartments has obviously tempered local officials in their decisions to freeze, impede or discourage condominium conversions for upper-income-rental buildings. CAPTION: Picture, Remaining rental units at Grosvenor Park are going condominium. By Lucian Perkins - The Washington Post