If you're like Senate Majority Leader Howard Baker, you may be convinced by now that Pennsylvania Avenue will forever be torn up and heading toward official designation as the Free World capital's first start-and-no-finish construction project.

But rest assured that the masters of dig-it-up and pave-it-over at the Pennsylvania Avenue Development Corporation don't see it that way at all. As bulldozer-meister Thomas J. Regan, also known as PADC's executive director, says confidently, "We're right on schedule and we're right on budget."

"Right on schedule," as it turns out, means that road reconstruction will continue until 1985, and building construction along the avenue will go on into the next decade.

For drivers it means the most of the south side of Pennsylvania Avenue has been rebuilt during the last year. It also means that the reconstruction of the north side section between 10th and 13th streets NW will be finished in another month, Regan assures.

But Baker and the other 35,500 Washington motorists who travel each day on the ceremonial avenue of presidents will have to wait until October before the Sixth-to-10th-street portion is completed. Meanwhile, construction crews will be starting to dig up the Third-to-Sixth-street section, a project Regan says will take another 15 months or so and be completed just in time for the 1985 presidential inauguration.

Baker, who frequently travels the avenue on his way to White House meetings, recently told the Senate that he suspects the roadway project might be endless.

"I think it's a case of bureaucracy out of control," Baker said. "I think there is a work crew assigned to Pennsylvania Avenue without a stop order. First they tear up one side, fix it and tear up the other. Then they tear it all up again."

Regan promptly dispatched an aide to assure Baker that such was not the case. For those not entitled to a private briefing, here's a Pennsylvania Avenue update:

"Right on budget" means that the road-paving, sidewalk-building, park-constructing, tree-planting and assorted other planning for private development that PADC has undertaken along America's Main Street will cost the taxpayers $78 million by the time it's supposed to be completed in 1986, Regan says.

It's all part of the federal government-private venture effort, first suggested more than two decades ago by President Kennedy, to turn the ragtag hodgepodge of development along Pennsylvania Avenue's north side into a showcase of public parks, walkways, private office buildings and prime commercial development. PADC was created by Congress in 1972 to oversee development of the avenue.

While some longtime Washington residents may be skeptical of the promises, the boyish-looking Regan voices confidence from where he now sits high above the avenue in a 13th Street office. It is filled with maps and pictures and charts and architect's sketches of buildings yet to appear.

"The public improvements will be done," he says flatly.

He pointed to a map of the avenue and noted that at 15th Street, Pershing Square, with its ice-skating rink and large stone tableaus commemorating Gen. John J. (Black Jack) Pershing's World War I exploits, is completed. Ditto for the adjacent Western Plaza, just across 14th Street, where one of L'Enfant's original designs for the layout of Washington is embedded in a raised stone and concrete plateau.

More parks will follow, Regan said, with the John Marshall Park, immediately to the west of the U.S. Courthouse, to be completed by May. But plans are not settled for Market Square, a combination office and park complex between Seventh and Ninth streets, and Indiana Plaza, a minor park near the intersection of Indiana Avenue and C Street, just north of Pennsylvania Avenue.

In addition, PADC has planted some of the 700 willow oaks it is placing along the avenue so that a canopy-like effect will be created with the foliage. Twenty mature elms and oaks between Sixth and 10th streets were deemed inconsistent with that esthetic touch and were knocked down last summer. A three-tiered lighting system with specially designed street lights and lights for the sidewalks is also planned.

For the moment, however, it is the avenue's reconstruction that has attracted the public's attention.

The existing roadway was deteriorating to the extent that "successive overlays of asphalt couldn't cover the damage," Regan said. "It was falling apart." He said that because the underground Tiber Creek flows beneath much of Pennsylvania Avenue, it creates moisture problems that eventually generate cracking in the roadway surface.

He said the reconstructed Pennsylvania Avenue has a base of 12 inches of crushed gravel, topped by 10 inches of concrete and a four-inch asphalt covering.

"It will last 50 to 100 years," Regan promised.

As for the glass-and-steel private office buildings, stores, hotels and hoped-for 1,200 housing units that PADC is encouraging private developers to construct, Regan assured that they all will materialize, too, by 1990 or 1992.

Some private construction is already finished or within months of completion along Pennsylvania Avenue. The curving glass-fronted office and retail complex built by the Cabot, Cabot & Forbes development firm at 1201 Pennsylvania is now open.

The renovation of the Old Post Office Building across the avenue is nearly finished. The National Endowment for the Arts and the National Endowment for the Humanities plan to move into refurbished offices there by the end of March, while the Evans Development Co. of Baltimore says that 50 stores and restaurants will open on the three lower levels in the building's huge atrium on July 1 in a commercial venture called The Pavilion at the Old Post Office.

Meanwhile, the renovation of the National Theatre is expected to be completed in October, as is the reconstruction of the block of E Street between 13th and 14th streets in front of the theater. Construction of the flagship Marriott Hotel and the adjoining National Place office and retail complex and the reconstruction of the National Press Building all are expected to be finished next year, Regan said.

The renovation of the twin-turreted Apex Building on the triangular-shaped tract at Seventh Street is under way and the headquarters of Sears World Trade Inc. will be housed there.

Work on other projects, such as the long-delayed refurbishing of the Willard Hotel and construction of an office-retail complex at 1001 Pennsylvania and an office-housing-hotel complex at 601 Pennsylvania Ave., is expected to start this fall.

"People are going to come here to have fun, to shop," Regan said. "Five o'clock now and this place is dead. By 1990 or 1992, that's when we hope to finish with the private development . I'm not just whistlin' Dixie. This will happen."