The U.S. government is planning to build a second White House on a tract of land in Prince George's County.

The county also will get its own Blair House and Lafayette Park, which will be built across from the new White House. But the new versions of the historic federal buildings will not have chandeliers, or even rooms.

They will be Hollywood-style facades, built in Beltsville for use by the U.S. Secret Service to train new agents in the business of protecting the President and his guests.

"Most of our agents work there at the White House or Blair House at one time or another on temporary or permanent assignment," said Secret Service agent Jim Boyle. "Each of those environments have their idiosyncracies and that's what we want our people to be familiar with."

The most serious attempt on a president in the White House area occurred in 1950, when two Puerto Rican nationalists attempted to shoot President Harry Truman while he was inside Blair House.

The project, which was approved 5-0 last week by the county planning board, will cost the federal government $1.6 million, according to planning documents. It is part of a $10 million, 10-year project to expand the Secret Service's present training facilities in Beltsville. In addition to the building facades, the project includes a simulated shopping mall and a replica of a city street.

The White House facade alone will cost $381,000 and the fake Blair House will cost $56,000. The balance of the money will be used to construct Lafayette Park and the roads around the facades, the documents show.

The Secret Service paid for the initial architectural study for the expansion from its overall 1981 budget, but full funding for the project has not yet been approved by Congress, said Bob Snow, assistant director for public affairs at the U.S. Secret Service. The project is being paid for by the U.S. Treasury Department--the parent agency of the Secret Service--and the General Services Administration.

Tex Gunnels, the staff adviser to the House Appropriations subcommittee that oversees the Secret Service, justified the cost of the project this way. "We can't afford to have a president killed," he says. "My God, the whole country goes into shock."

The idea for the Hollywood-style White House set was conceived about 10 years ago, by a Secret Service agent in the department's training division. Several spokesmen said they did not know which agent had come up with the idea.

Facades have been used for about 20 years to train police recruits, said FBI spokesman Lane Bonner, who noted that fledgling FBI agents practice shooting firearms next to replicas of a barber shop, laundry and a hotel at their training center in Quantico, Va.

Since 1971, Secret Service recruits at the Beltsville center have learned their job by watching films of officials and dignitaries entering and exiting the White House and Blair House and practicing security measures at a mock shopping center in Beltsville, according to spokesman Boyle. They also receive on-the-job training at the actual Pennsylvania Avenue landmarks by other Secret Service agents.

But for the past several years, according to Boyle, it has been difficult to provide sufficient training on Pennsylvania Avenue because the number of recruits has increased dramatically, from several hundred in 1969 to 3,000 this year.

"It's not feasible anymore to run them all through there," Boyle said.

Boyle said the mock White House and Blair House will help agents do their jobs better. "It's best to know the lay of the land, know the environment, know the things that could happen. If you saw what happened March 30 the attempted assassination of President Reagan , what you saw was practical training from an environment such as that the facades ."

Boyle said that the attempted assassination of Reagan did not inspire the plan for the mock White House.

He would not describe how recruits will use the facades for training, saying such details could jeopardize the security of the president and his guests.

"We're trying to stay away from the area you're interested in because we don't want to detail training procedures because we could jeopardize the security aspect of it," he said.

The facades may not be built for several years, according to planning documents, and they will not be visible from the street because they will be surrounded by woods. The Beltsville center is on 490-acres owned by the federal government near the Baltimore-Washington Parkway at Powder Mill Road. It currently includes a shooting range, a dog training area, a mock street and a training building.

Currently, about 225 agents, employees and trainees work at the Beltsville plant each day. After the expansion, the number of people using it will increase by about 10 percent, the planning document states.