The Fine Arts Commission gave preliminary approval yesterday to plans for a three-block, $160 million development in downtown Washington that could include a new main department store for the Hecht Co. and a 450-room downtown Hilton Hotel.

The project, proposed for the city's partially vacant urban renewal site around the Metro Center subway station at 12th and G Streets, NW, also would include about 700,000 square feet of office space and about 70,000 square feet of retail space in two additional office buildings, inlcuding stores on an underground level that would be served by a new escalator to the subway station.

A giant glass-enclosed pedestrian bridge -- 152 feet long and 30 feet wide -- crossing 12th Street also is being proposed by Skidmore, Owings and Merrill, architects doing the preliminary design for the developers, Oliver T. Carr Co. and Hagans Development Co.

The bridge, wide enough to contain small retail shops and perhaps an aerial restaurant over 12th Street, was praised by several members of the federal Fine Arts Commission. "Well, it's not a dinky little bridge," said commission Chairman J. Carter Brown. The only pedestrian street bridge now in the city is a narrow walkway across Independence Avenue at 13th St. NW connecting two Department of Agriculture buildings.

A large public plaza is proposed around the subway entrance at 12th and G Streets and a small park also would be built between the northern-most office building proposed and the 146-year-old Episcopal Church of the Epiphany at 1317 G St.

If the Carr-Hagans proposal is approved by the Redevelopment Land Agency, which controls the three-acre site, "We will have the biggest and most significant downtown Washington project ever," Lawrence Press, acting chief of planning for the city's Department of Housing and Community Development, said yesterday.

"We hope to go before the RLA board early this summer -- maybe July -- and begin construction by the end of the year or early next year," said Betts Abel, project director for Carr.

If negotiations over the price of the land, financing and city approvals go without a hitch, the project would be completed by late 1984, Abel said.

Although the commission generally approved the preliminary plans, it still will have to approve the detailed design of the project. The D.C. Zoning Commission and the RLA Board both will have to hold public hearings before approving it.

Hecht's has been seeking for some time to move to a new downtown site from its historic turn-of-the-century building at 7th and F Streets, NW. Sources here said last spring the company was losing money at the rate of about $1 million a year.The proposed new location would put it almost across the street from Woodward & Lothrop and two blocks from Garfinckel's main store at 14th and F Streets.

Hecht's would occupy three underground floors and five above-ground floors in the main block-long building of the Metro Center development, and face G Street between 11th and 12th Streets NW. Office space would be leased on the seven top floors of the 12-story building.

The hotel site, now under negotiation with the Hilton Hotel chain, would be located on the northernmost part of the T-shaped site, at 12th and H Streets. It would back on the Metro subway entrance and be one block from the city's new $100 million downtown convention center, which already is under construction and scheduled to be finished in 1983.

The two other buildings in the project both would be office buildings with one or two stories of retail space in the building beside the church and three floors of retail shops in the building diagonally opposite Woodies.

Underground parking for 700 cars would be built beneath the hotel and office buildings.

All of the small stores and buildings still standing on the three-block RLA site would be razed except for the front part of the old firehouse on 12th Street, which would be moved to face the public subway station plaza.

The RLA has been attempting to find a developer for the Metro Center site since it condemned and began purchasing the land in 1970.

The Carr-Hagans team was formed to comply with unwritten District housing department policy that requires minority equity of at least 25 percent in such projects.