The 964-unit Parkside garden apartment complex in upper Bethesda is under contract to be purchased for $36 million by a Washington redeveloper team that plans a conversion to condominium ownership.

Parkside Associates, which includes Gary and Scott Nordheimer, Myer Feldman and Toronto-based Cadillac Fairview, has informed tenants in the tree-lined community that they have a total of 150 days to meet the purchase price, under county regulations.

The seller is Parkside Properties, a partnership involving the Alvin Aubinoe and George and Jeffrey Huguely families. The late Alvin Aubinoe developed the highly regarded rental properties on 68 acres near Rock Creek Park over a 12-year period beginning in 1953.

Meanwhile, a substantial number of members of the seven-year-old Parkside Tenants Association already have formed Parkside Development Corp. in an effort to meet the contract price. Matthew F. McGuire, an attorney who is secretary of the tenant development group, said the objective is to provide tenants with a wider choices of outright condo ownership, plus some rentals and a limited equity co-op section in the project that includes 102 colonial-styled brick buildings in five sections.

McGuire added that the development group, whose members already have paid a $50 fee, have scheduled a meeting Thursday evening at the nearby Holy Cross parish hall on Strathmore Lane. "Obviously most tenants would prefer to continue in a rental status," he added. "I was surprised by the recent notice of the sales contract. But this group is well-organized and has its act together in order to seek financing."

Speaking for the purchasing group, Gary Nordheimer pointed out that abiding by the legal requirements on condo conversions in Montgomery County would take nearly six months until the planned purchase could be made and then another 180 days would be needed to complete the selling process. "It is unlikely that any tenant would be forced to move until March 1981," Nordheimer said.

He said the purchasing group also plans to give tenants a year's lease at current rents, and elderly persons living on fixed incomes will be offered three-year leases. It was estimated that more than 15 percent of all Parkside tenants are retired or semiretired persons, and that there also is a large number of young, one- and two-person households.

Nordheimer added that extensive renovation is planned and that tenants would be offered their own units in "as is condition" 15 percent below what the public sale prices will be later.

Units in the fully leased buildings range from one to three bedrooms and would be priced by the condo developer from $42,000 to $105,000, depending on location and age of the structure. Selling prices are expected to range from $62 to $65 a square foot of living area. Rents now range from $320 to $495 a month, including utilities.

Located at 10520 Montrose Ave., the Parkside in entered from Rockville Pike. The site is near a new Metro station that will be in service within a few years and also opposite the high-rise Grosvenor Park complex that recently was totally converted to condo ownership.

The Nordheimer-Feldman group recently purchased the Georgetown Inn and obtained a contract for the purchase and conversion of Prospect House apartment in Arlington. The group also converted several other District buildings, including 4000 Tunlaw (Road NW), to condo ownership.

Cadillac Fairview, a large Canadian realty investment firm, has a sales contract, along with Daon Corp., to purchase of the large Columbia Plaza rental complex in Foggy Bottom for $51 million.