The largest real estate development in the Metro subway system's history, a $100 million office building and hotel project to be constructed in downtown Bethesda, was approved yesterday.

The development, to go atop the future Bethesda subway station, will provide the sprawling and populous community with a new focus for its downtown as well as one of the most striking concentrations of commercial and office space in the area's close-in suburbs.

Construction is expected to begin early next year on the 3.5-acre site on the west side of Wisconsin Avenue just south of Old Georgetown Road and to be completed in stages by 1985. Trains are scheduled to begin running through Bethesda on the Red Line from downtown Washington by late 1983, possibly as far as the Shady Grove terminal near Gaithersburg. b

Included in the project will be a large landscaped plaza, a 17-story office building capped by a roof with a stairstep design, a hotel featuring a large interior atrium, a glass-topped shopping arcade, an underground Metrobus transfer station and three levels of basement parking containing 1,300 spaces for tenants and visitors of the office building and hotel.

Metro board members voiced enthusiasm verging on elation as they viewed a model and an artist's renderings of the project.

"That's a gorgeous spot [for development]," said Rose C. Kramer, a Montgomery County Metro board member. "And just think of all that tax revenue for the county!"

"I'm looking at this with envy," added Marie Travesky, a Fairfax County member. "I wonder where we can put one like it."

Preliminary plans for the project -- tentatively called Bethesda Metro Center -- were approved last spring by the Montgomery County government, clearing the way for Metro to choose a private developer under a 50-year lease that could be extended to 99 years.

The transit authority announced yesterday that it has chosen a firm headed by Allan Rozansky and Alan Kay, major Washington area real estate investors, from among three development groups that had submitted proposals.

The lease the firm has agreed to negotiate with Metro includes both air rights over the subway and underground garage space. The lease will guarantee the transit authority annual rent of at least $1.6 million beginning in 1985, with smaller amounts prior to that time. R & K Metro Associates, the Rozansky-Kay firm that will develop the project, also will pay real estate taxes to the county.

The new 17-story office building will be three stories taller than any existing office buildings in downtown Bethesda. However, an office building and condominium building planned by the George Hyman Construction Co., to be located adjacent to the new development, will rise to 18 stories. Although separate, the Hyman building will appear architecturally to be part of the Metro project, as will another planned office building on adjoining Montgomery Lane.

Once the site of a motion picture theater and numerous small businesses, the Metro site is now surrounded by a graffiti-covered green Metro construction fence. In recent years, numerous high-rise office buildings have sprung up to the east and south of the future Metro station, including the area's pioneer air rights building over the track of the Baltimore & Ohio Railroad branch line to Georgetown.

At the outset of planning for Metro, its supporters and officials predicted that the system's construction would spur clusters of development around its downtown and suburban stations, adding to their liveliness, enhancing the local tax base and -- not incidentally -- providing the transit system with a reservoir of fare-paying customers.

Metro itself has previously leased air rights to developers above Faragut North, McPherson Square and Van Ness stations in Washington. Only one has been completed -- the office building and shopping arcade called the Connecticut Connection at Farragut North.

Henry W. Cord, head of property management for Metro, said the new Bethesda project will be larger than all three of the earlier projects combined. He said Metro has invited developers to submit air rights proposals at yet another Montgomery County station on the Red Line, at White Flint in North Bethesda.

The downtown Bethesda development will resemble large-scale air rights developments atop transit stations in such cities as Jersey City, Toronto, Stockholm and Paris.

The Ronzansky-Kay proposal was accepted for Metro by a panel of officials who reviewed financial, architectural, environment and other factors. Their decision was not subject to review by the Metro board, which was told of the selection by general manager Richard S. Page. The politically appointed board, which includes several elected officials, intentionally stripped itself of power to review such selections to avoid accusations of political pressure by developers.

Under the Bethesda design as approved by the county, Old Georgetown Road will be converted to one-way northbound traffic off Wisconsin Avenue, with southbound traffic routed onto a relocated Woodmont Avenue on the west side of the station project.

Metrobuses will enter and leave the station from the Woodmont Avenue side and will stop at indoor loading platforms one level below the plaza. Passengers may take escalators up to the street surface or down to the trains. The construction timetable calls for the bus terminal to be in service by the time trains start running in 1983, with above-ground construction to follow.

At yesterday's meeting, the Metro board also received a report from a committee of local government officials recommending retention of the Farecard system. The board took no action on the report, but scheduled a special meeting for next Wednesday to consider the recommendation.

Two new board members were sworn in yesterday -- Prince George's County Council member David Hartlove, who replaced Francis B. Francois, and James E. Clark, acting D.C. transportation director, who is an interim replacement for Douglas N. Schneider Jr. Francois and Schneider resigned their local government posts.