After abruptly closing Pennsylvania Avenue in front of the White House, the Clinton administration is making a modest effort to beautify the two-block area by planting grass in the middle of the street.

The plan, to be announced today by National Park Service officials, stops short of earlier, more ambitious plans to embellish the section between 15th and 17th streets NW with fountains, benches, trees and even an underground tunnel for traffic.

But Park Service officials say the simple, $700,000 plan is only a temporary solution, for the next few years, until a more expensive, permanent design is selected.

"We didn't want this to make much of a statement and presuppose a long-term solution," said James L. McDaniel, the Park Service's liaison to the White House. "The whole idea was to keep it neutral, a little less of an asphalt landing strip, yet not assuming this would be the final plan. That needs to be done in a more thoughtful way."

Under the interim plan, to be put in place by the end of November, the asphalt in the middle of Pennsylvania in front of the White House will be replaced with 40-foot-wide strips of grass to make the street more inviting to pedestrians. The street was closed in May because of fears that terrorists could use a truck bomb to damage the White House.

The grassy area is supposed to make the street blend in better with the North Lawn of the White House and Lafayette Square. Asphalt lanes on either side of the grassy median will allow authorized vehicles to travel through the area, including those taking part in the Inaugural Parade.

It will take at least two years before permanent changes are completed along Pennsylvania Avenue and surrounding streets, officials said. Park Service officials rejected the idea of a national design competition, deciding that a panel of nationally known architects and planners will meet in December to propose several options that will become part of an environmental impact review. A final design could be ready as soon as January 1997.

At President Clinton's insistence, Park Service officials said, the public will be asked for their ideas before the panel's December meeting. Letters went out today to 5,000 planners, architects and others asking for their thoughts. A public forum is scheduled for Oct. 17 and 18 at the White House Visitors Center.

Until a permanent plan is approved, Washington will have to live with the interim plan, which was developed by the Park Service and a panel of 11 local and national architects. Their goal was to keep the street simple, with a grand space in the middle to break up the asphalt and "foyers" at the 15th and 17th street entrances to the avenue, instead of an abrupt barricade.

"My interest was to make it as subtle as possible, a pedestrian area," said Harry G. Robinson III, dean of Howard University's architecture school and member of the architectural panel. "There needed to be a soft connecting seam with the North Lawn and Lafayette Park."

Instead of a raised median, the grassy sections will be level with the street, with bricks around the edges. That way, officials said, a person looking down the street from either end will not see an ugly mass of asphalt.

"We wanted to break it up, create some relief," McDaniel said.

The makeshift concrete planters and barriers blocking off Pennsylvania at 15th and 17th streets NW will be replaced by more attractive square and circular planters containing magnolia trees and pink roses. The concrete planters will be painted a neutral beige.

Guard booths at each end of the street will substitute for the police cruisers now parked there to intercept traffic. At the checkpoints, there will be steel barricades that move up and down when a vehicle goes through.

In front of Blair House, sections of the street will be removed and replaced with grass to set off a driveway there. A series of concrete posts on the sidewalk in front of the White House, criticized by some architects as creating a fortress effect, will remain for now.

The plan offers no ideas on how to deal with the concrete barriers that mar the sidewalk north of Lafayette Square along H Street NW other than to paint them a color other than white. Metrobuses will continue to use Madison Place NW.

Later this year, officials will decide on improvements for the closed part of State Place and Executive Avenue south of the White House. It is possible, McDaniel said, for changes to be made to State Place that would allow two-way traffic again on E Street, which is one-way eastbound now.

The temporary plan could cost more than $1 million before it is completed, officials said. The Park Service is seeking private donations to lower the cost to the government.

Entrances at each end would contain:

* Square and circular concrete planters containing magnolia trees and pink roses.

* Guard booths instead of current police cruisers.

* Steel barricades that can be raised or lowered for traffic.

Blair House:

* Portions of street removed and grass strip added to create driveway.

Grass strips:

* 40 feet wide.

* Level with street.

* Bricks around edges.

East entrance:

* Parking spaces added on north side of street.