Inova Health System officials said yesterday they have asked Virginia state health planners for permission to spend $205 million to enlarge two Fairfax County hospitals that now are so busy that ambulances are rerouted away from them about a third of the time.

The nonprofit organization disclosed that it wants to add 177 beds to Inova Fairfax Hospital--a massive undertaking that would include a 156-bed heart institute and a 1,400-car underground parking garage.

If Inova wins state approval for the expansion, Fairfax Hospital's bed count would increase to 833, second only to the District's 907-bed Washington Hospital Center in the mid-Atlantic region.

In western Fairfax, Inova wants to build a 71-bed, three-story patient building at Inova Fair Oaks Hospital. The new patient rooms, which would mostly replace existing beds, would raise the hospital's licensed capacity from 151 to 160 beds and ease a regional shortage of critical-care beds.

Inova, which operates five hospitals and dominates inpatient care in Northern Virginia, already has begun smaller improvement projects at its other properties.

"This is really a systemwide effort to make sure we're meeting the needs of the community and ensuring appropriate access not just today but in the long term," said David W. Wright, Inova's vice president for marketing and business development.

Wright said Inova would use its ample cash reserves, sell bonds on Wall Street and launch a fund-raising campaign to finance the construction projects--$131 million at Fairfax Hospital and $74 million at Fair Oaks Hospital.

The application for the necessary state certificate of public need is part of a rush by many Northern Virginia hospitals for permission to renovate or expand.

Some facilities, such as Columbia Reston Hospital Center and Arlington Hospital, plan to spend millions of dollars on renovations without adding bed capacity. But tiny Loudoun Hospital Center, which has 80 beds and is full most of the time, hopes to apply in June for up to 50 more.

Yet the surge of interest in hospital expansion in Northern Virginia does not represent the situation across the Washington region, which for years has been flush with extra hospital beds--a surplus that has put several D.C. hospitals in financial jeopardy.

But in the western reaches of Northern Virginia, including parts of Fairfax and Prince William counties and all of Loudoun and Fauquier counties, the population is booming, fueled by explosive job growth in technology firms and construction of thousands of housing units.

Fairfax's population jumped by 111,000 from 1990 to 1998, pushing the latest census estimate to 929,000. In Loudoun, one of the nation's fastest-growing communities, the population grew from 86,000 in 1990 to 144,000 in 1998--a 67 percent increase.

In recent months, some emergency workers have complained that Fairfax hospitals have rerouted ambulances with increasing frequency because they have no room for critical-care patients. Inova spokeswoman Lisa Wolfington said that most of the rerouting is caused by such conditions and that during those periods, Fairfax Hospital accepts only children, victims of trauma and unstable patients.

Everyone else is transported to the next available facility, which means longer trips for emergency technicians and more time before their units are available for other calls.

State and regional health planners will study the Inova applications, and the Virginia health commissioner will consider their recommendations and make a final decision by June.

Dean Montgomery, Northern Virginia's chief regional health planner, said that despite the recent shortages, the northern region of the state still has too many unused hospital beds--except at Loudoun Hospital Center and Inova Fairfax Hospital.

"For two or three years, we have been saying there . . . needs to be an expansion [at Inova] fairly soon," he said. Yet "substantial excess capacity" remains elsewhere, he said. "Most other hospitals can get along just fine with the capacity they now have."

Wright said the key element of the Fairfax Hospital expansion is the creation of the heart institute, which would be built where a parking garage now sits. The move would free up at least 113 beds in the hospital to cope with the demand for critical-care services.

The new building would become the home for all cardiac services at the hospital and would raise the number of cardiac catheterization labs from four to six. There would be dozens of additional cardiac intensive-care beds, more short-stay beds for people undergoing outpatient procedures and many other service expansions.

The expansion would serve the dual purpose of creating the heart institute and freeing up areas that can be devoted to critical-care beds, Wright said.

At the same time, the Fairfax Hospital emergency department would be enlarged from 28 treatment rooms to 53, Wright said. The new parking structure built beneath the heart institute would increase total parking capacity by 625 cars, he said.