The city of Rockville took one more step toward its touted Town Center development plan yesterday when it announced that a joint venture headed by Winmar Co., a Seattle-based subsidiary of Safeco Corp., had received options to buy two key tracts of city-owned downtown land.

"This represents the whole revitalization of the downtown," Rockville Mayor William E. Hanna Jr. said at a news conference yesterday. "It will provide a living, dynamic center for downtown Rockville." Douglas F. Horne, the city's community development director, said later that if Winmar goes through with the purchases, the "key portions" of the development plan would be in place.

Under a "letter of understanding" dated last Wednesday, the Winmar group has exclusive rights to purchase the two plots for the next six months. During that period, Winmar and its consultants will draw up plans and schedules for building on the parcels, a marketing study of the demand for commercial and residential rental space and an outline of the possiblities for intergrating the moribund Rockville Mall into the revitalization plan.

Winmar's partner in the deal is Nordal Associates, a New York-based financial consulting firm that specializes in commercial real estate development. The two firms have retained Beda Zwicker of Gruen Associates as architect for the project. Zwicker lost narrowly in 1978 when Rockville selected Washington architect Arthur Cotton Moore to draw up its Town Center plan.

The first site, bounded by North Washington St., Middle Lane and Courthouse Square, will almost certainly contain a hotel along with other buildings, Horne said. The second its situated down the block on the north side of Middle Lane. A third city-owned plot at the intersection of Jefferson St. and Rockville Pike was sold last year to Gateway Building Inc., an area developer. Gateway has told the city of more than 200,000 square feet of office space.

Although the details of downtown development remain vague -- especially because Winmar and Nordal still can walk away from the project -- city and company officials said yesterday that plans would follow the outlines laid down by Moore two years ago. The Moore plan, which cost the city $94,000, called for a hotel an arts center on the first plot and a set of office and residential buildings on the second.

Hanna said he expects a public hearing when Winmar and Nordal submit their plans in February, with the city council passing judgment on the proposals in March.

One key question is how Winmar's development plan will mesh with Rockville Mall, known officially as the Commons at Courthouse Square. The Commons project was seen as the center of downtown development when it opened in 1972, but the management never has come close to renting all of its 55 retail spaces. It now contains little more than 10 shops and restaurants wedged between shuttered storefronts.

Saying that the mall's management owed it more than $174,000 in unpaid garage rent, the city last May shut off access to the adjacent parking garage that it owns and revoked the free parking that mall customers had enjoyed.

Despite the clause in yesterday's agreement requesting plans for the integration of the Commons, city officials were quick to disavow any connection between the mall and Winmar's activity.

In the past, developers often have steered clear of Rockville's central district, citing problems of access and competition from nearby Montgomery and White Flint malls. But city officials have remained optimistic, partly because a Metrorail stop is scheduled to open downtown in 1983.