When Xerox Corp. announced last month that it would build 20 high-rise buildings in a Loudoun County project, G. Richard Pfitzner, the county board chairman in rival Prince William County, got that old feeling: "We're always the bridesmaid, but never the bride."

For years Prince William County officials have been saying their archetypal bedroom community is on the verge of blossoming -- big malls, big office buildings, big companies, big time.

Mostly it has been a tease.

When Marriott Corp. announced in the mid-1970s it would build a huge amusement park off I-66 near Manassas, Prince William appeared set for a tourist attraction to rival any in the Washington area. The proposal was defeated in court, and the enormous tract remains undeveloped.

When a major developer announced in 1978 it would build a shopping mall in Dale City that would rival Tysons Corner, the county and Virginia quickly built a multimillion-dollar highway interchange on I-95 to accommodate the traffic. The mall has yet to materialize.

When the state planned the high-tech research Center for Innovative Technology (CIT) for the Washington suburbs, Prince William officials courted officials in Richmond with everything they had. The project instead went to the Loudoun-Fairfax county line.

That, says Dyan Lingle, Prince William's chief industry hunter, "was a really big disappointment. It would have been the beginning of a new image for us."

"It's important to be patient," says County Executive Robert Noe, his comment illustrating Prince William's attempt to deflect widespread frustration over the failure of commercial development to keep pace with rapid population expansion in the county of 171,000.

Noe also is fond of saying that development in the county "has exploded. You just haven't heard the boom yet."

In a sense, he is right.

The amount of business construction in Prince William in 1985 is predicted to be 10 times what it was in 1978 in dollar terms. Neighboring Fairfax County, on the other hand, built more commercial and industrial space last year alone than currently exists in Prince William and Loudoun counties combined.

The problem in Prince William has been the type of commercial development the county is getting.

There is Potomac Mills, the Fair Oaks-sized shopping mall just west of I-95, set to open partly this summer. But the mall, built by Washington-based Western Development Corp., is going to contain factory-direct discount stores.

"It's a fine development," says Lingle. "But we don't exactly want to be known as the place to come to the outlet mall."

A number of low-rise office parks are approved for the I-66 area north of Manassas, including a 50-acre development called Parkridge and the 133-acre Battlefield Business Park, which overlooks Manassas National Battlefield Park. Still, those projects are tiny compared to what is going on in western Fairfax and eastern Loudoun.

Lingle says plaintively: "Can't I just have an elevator?"

Prince William started out, as one official puts it, as "the typical U.S. 1 jurisdiction," with housing and small businesses strung along Rte. 1. The housing was relatively low cost, built in the 1950s and 1960s for military families at Quantico and Washington.

"Most of our development has been the parasite stuff," agrees Pfitzner, referring to the shopping-center-strip development along the county's main thoroughfares.

The 1970s brought more expensive housing, with projects like Lake Ridge near Dale City. And to the dismay of county officials and business leaders, Prince William's immediate future appears to be as a bedroom community.

"We're so underwater with our residential, we're just drowning in it," says Lingle. Some items:

* The number of homes under construction is at an all-time high, four times what it was a decade ago, swamping the growing commercial development.

* There are 14,000 dwelling units on the drawing boards or under constuction -- a whopping 26 percent addition to the county's current 54,000 units.

* The county has the smallest proportion -- 10 percent -- of its real estate classified as either office or industrial of any jurisdiction in the Washington area. (This in spite of the presence just outside Manassas of the International Business Machines Corp. center, the county's largest employer and taxpayer.)

* About 45,000 of the county's 171,000 residents leave Prince William each morning to go to work elsewhere.

Some, including Noe, say the increasing construction of commercial projects is a signal that the county has turned the corner decisively, if not in the dramatic style the CIT would have provided.

"The change began two years ago," says Martin Crahan, director of development administration for the county. "We've entered the second phase of our development . . . . We're still in the residential growth mode . . . but nonresidential is starting to become a greater share of the total."

"The industrial growth is in its infancy," says prominent commercial real estate broker John Norman. "But it's genuinely under way. In the past, we've talked a lot about it, and I don't think anybody really believed us. But now I think it's really happening."

Among other evidence, Norman cites his own business. "In the last two years, nearly everything I've done has been commercial and industrial," he says. "There has been a major shift in the market I handle . . . . People call me now that I couldn't have gotten through to at all two or three years ago."

Indeed, Lingle says, if a small businessman asked her today where he could lease a typical small warehouse for light manufacturing or assembly, "I'd have to tell him, 'No, I don't have anything like that.'

"Now, if he wanted to build 2 million square feet of space like that, well" -- she pauses and smiles -- "I'd love to talk to him."

County officials say a mix of commercial and residential space is vital if the county is to have the tax base it will need to provide services it needs, such as schools and roads. Prince William has grown by 18 percent since 1980 and is expected to grow by more than 30 percent by the year 2000. Residential real estate taxes alone cannot finance that sort of growth, the officials say.

There are a score of office and industrial park projects in the works, enough to double the amount of such space in the county in the next five years.

The nonresidential development is taking hold along the two interstate corridors, I-95 in the east and I-66 in the west. The interstates, along with a railroad line through Manassas and relatively cheap land prices, make the county ideal for distribution warehouses and light industrial firms, along with retail outlets chasing the growing population, the officials say.

County planners and officials, however, are eager for the enormous, high-profile developments that Fairfax seems to get routinely.

Prince William does have one such jewel, the giant IBM center, where submarine systems are assembled and tested and computer technology is developed.

IBM, which moved to Prince William in 1968, employs at least 4,400 people, and is expected to employ 10,000 by the end of the century.

The company has yet to create the spinoff industries that the county had expected. "We are so much a one-horse town, a one-company town," says board chairman Pfitzner. "If we had another, really classy corporate citizen at each end of the county, it would really enhance our situation."

County executive Noe claims not to be envious of Fairfax and Loudoun as Pfitzner and Lingle are. "I don't share that sentiment. I think there is enough for everybody," he says. "Some very nice things will happen here in time."