The Defense Department is planning to spend almost $1 billion for a major renovation of the 44-year-old Pentagon building and construction of a four-building annex in its south parking lot, according to Pentagon officials.

Under preliminary plans, it will cost $425 million to construct the new annex and up to $500 million for what officials describe as "a top-to-bottom rebuilding" of the Pentagon. Calculated to provide 3 million square feet of new office space and to accommodate 18,500 additional employes, they will be the largest office construction projects ever undertaken in this area.

When it was constructed in 1942, the Pentagon cost $83 million and consolidated the War Department from 17 locations into one. Department of Defense (DOD) rentals of nongovernment-owned office space have since ballooned to 73 buildings in the Washington area -- a number that officials hope to reduce to 45.

The Pentagon project is so large that the General Services Administration (GSA), which takes care of almost all government office construction, does not have enough money to finance it, according to defense officials. Instead, the Reagan administration is asking Congress to use military construction funds spread over the next seven years.

The Defense Department tried to get the Pentagon construction program started this year, but Congress balked at taking a first step.

In the fiscal 1987 military construction bill now on Capitol Hill the Defense Department sought $22.5 million to design the annex and $2.5 million to begin site preparation at the south parking lot area. Officials also wanted congressional appropriations committees to reprogram some $10 million in fiscal 1986 Air Force money to begin design work in the next few months.

However, Rep. W.G. (Bill) Hefner (D-N.C.), chairman of the House Appropriations subcommittee on military construction, put a hold on both the reprogramming and the first portion of the money.

"We don't like the way they handled it," one congressional aide said recently. The Pentagon and GSA, he said, have been working on the project for more than a year but did not tell the appropriations panel about it until this spring, and then only after questions were raised in a public hearing.

In addition, Virginia lawmakers criticized the effect it would have on traffic in the Pentagon area.

The new, comprehensive development plan for the Pentagon area (which includes the Pentagon, the FB2 building or "Navy Annex" and the projected buildings on the south parking lot) would expand on preliminary GSA site studies dating to 1979.

The Reagan administration's sharp increase in defense spending has brought a 10 percent to 15 percent growth in the number of DOD employes in the Washington area. As a result, the cost of rented office space doubled in five years, from $41 million in 1980 to $87 million in 1985. Over half the Washington area's 70,800 DOD employes are now in rented buildings and their numbers are growing. At the same time, the amount of less-costly, government-owned space occupied by the Defense Department has actually dropped slightly, from 8.6 million square feet to 8.5 million square feet.

Aside from the need for additional space, the Pentagon, which provides 6 million square feet of government-owned defense office space, is rapidly deteriorating. Portions are becoming unusable, sources said.

Water leaks, antiquated electrical and plumbing systems and a lack of electrical capacity to handle modern heating, air-conditioning and ventilation systems have led to plans for what one official last month told Congress was a "phased, top-to-bottom renovation of the Pentagon which is projected to commence in 1989."

The "early planning" for the renovation, a Pentagon spokesman said yesterday, calls for a five- to six-year program "costing $400 million to $500 million."

Supplementing that program is the plan to construct the Pentagon annex and to improve the Navy annex to the west of the Pentagon.

Plans for the four buildings in the south parking lot annex began in May 1985, when GSA advertised for architect-engineering services to develop a master development plan for the Pentagon, according to a GSA letter to Congress last year.

In private negotiations with House and Senate Public Works Committee members, who fully support the program, agreement was reached that the Defense Department should seek military construction funds rather than more limited GSA money for the project.

The Washington firm of Keyes Condon Florance completed a draft of preliminary concepts for the master plan last March and settled on two alternative site plans in June. Final approval of the plan by the Defense Department and GSA is not expected until December.

Two closed meetings with the National Capital Planning Commission resulted in the dropping of several approaches, one of which contained a mix of buildings from four to 14 stories tall. A proposal to add another ring to the Pentagon itself was also dropped.

Two concepts are being studied, both of which include four buildings on what is now the south parking area. In both plans the buildings would have five stories above ground -- the same number as the Pentagon -- and two levels underground for offices, a retail concourse and visitor arrival center.

All parking for the new buildings, 6,000 spaces in all, would be beneath the project. A new parking structure would be built in the north parking area, and parking for visitors would be placed in structures south of Shirley Highway. Shuttles would bring visitors from the new parking area to the new annex.

Under one concept, the full 3 million square feet of new office space and full complement of new employes would be at the south parking site.

Under the second concept, the annex buildings would be somewhat narrower, according to material given Congress, and house only 15,500 employes. Under this plan, the Navy Annex building would be replaced with a new structure, which would accommodate the other 3,000 added employes.

That structure is located on a ridge west of the Pentagon and now houses 6,000 employes. It would be changed from a traditional military building above ground to an underground, or "below-grade" structure surrounding a large atrium.

Pentagon officials told Congress recently they have designed a number of new approaches to ease obvious transportation problems that will develop from the large increase of workers and small increase in individual parking spaces.

Perhaps the most controversial will be a plan to make drivers pay for parking their cars in the Pentagon area.

"A program to charge for parking will be gradually implemented," according to the draft "Transportation Program for the Expansion of the Pentagon" presented to Congress last month. "The cost for parking will eventually reflect market costs for parking in the area which would further provide further incentives for high-occupancy vehicles."

The number of parking permits allocated to different types of parking will be altered to cut down on one- and two-occupant vehicles, while the number for van pools and car pools with four or more passengers will be more than doubled.

Overall, the new program says that "the number of parking spaces per employe will be reduced from approximately one in three which now exists to one in four."

To meet criticism that the increase in employes will create traffic snarls, Pentagon officials said they plan to introduce staggered hours, with more workers starting their day between 6 a.m. and 7 a.m.

There is also the hope, envisioned in the plan, that the number of employes using public transit or walking will increase from 30 percent to 37 percent.

Officials want the first of four buildings in the complex to be ready for initial occupancy in 1989, so that it can be used temporarily to relocate employes whose Pentagon office space is being renovated, Congress was told. According to DOD officials, it would cost $25 million a year for five or six years to rent space for those Pentagon employes if the new annex building was not available.