Real estate experts and retailers say they expect the announcement last week of plans for William Center, a 600-acre regional shopping mall, office space and residential project just outside Manassas, to help persuade hesitant developers and builders to begin, or to speed up, projects in Prince William County.

The William Center Mall, which is being developed by Hazel/Peterson Cos., Northern Virginia's biggest real estate developer, and Edward J. DeBartolo Corp., the nation's leading shopping mall builder, "is going to lend credibility to the marketplace," said Ernie Hueter, a sales consultant for Coldwell Banker who has been analyzing the retail space market in the Manassas area since 1984. "It shows that the area is not just a small bedroom community with small retailers."

Hueter and other real estate analysts say the 1.2 million-square-foot shopping mall at I-66 and Route 29, which is expected to be finished by 1992, will not put such neighboring shopping centers as Manassas Mall or Fair Oaks mall out of business. If anything, Hueter said, it will spark increased competition and provide shoppers with a better mix of stores.

Alice Jones, general manager of Manassas Mall, on Rte. 234 less than five miles from the William Center site, said her center is undergoing a major renovation to adapt to the rapidly growing population and rising average household income of Prince William.

The 16-year-old mall has 450,000 square feet of shopping space, but the size is expected to double in the next year and then increase to 1.1 million square feet in two years. The mall's occupants include Hecht Co. and Montgomery Ward stores. A Leggatt's department store outlet is scheduled to open this month, and a Sears store is to be opened there this summer. Within the past year, the mall also has attracted Benetton and Britches for Women stores.

"We realize there's rapid growth in the area and that higher income families are moving to the county," said Jones, who has worked at Manassas Mall since 1977. "The shoppers are looking for value and quality, and with this expansion and renovation we're trying to satisfy their needs. And quite honestly, we want to keep them from going into Fairfax."

Gil Brooker, general manager of the 1.4 million-square-foot Fair Oaks Shopping Center at Rte. 50 and I-66, which will be eight miles from William Center Mall, said the Washington area is "under-retailed," and as a result he doesn't see the new mall seriously affecting Fair Oaks. "The competition will act in a synergistic fashion," he said. "It makes each mall have a more regional draw."

Arthur N. Fuccillo, general counsel for Lerner Cos., which is developing the Dulles Town Center mall at Rtes. 7 and 28 in Loudoun County, said the William Center, which will be about 15 miles away, is going to serve a different market than the Dulles mall. "It's not going to have any impact on us," he said.

Since many of the newer malls attract national or regional chains, smaller local retailers may feel much of the competitive pressure, some analysts said. "Local stores are going to have to compete and will have a hard time," said Armond Spikell, vice president of Smithy Braedon's retail services group.

DeBartolo executives, who have developed dozens of malls nationwide, say they don't know which retailers will occupy the five major department store sites at William Center. Fair Oaks has a full complement of big local retailers, such as Woodward & Lothrop, Garfinckel's and Hecht Co., and most national retailers with a Washington area presence have stores within 25 miles of William Center. Such national stores as Macy's and Nordstrom's, which are expanding locally, may opt for the new mall, analysts said.

Most real estate and retail analysts say it doesn't matter which retailers choose William Center. Beth Pierce, a commercial sales agent for Barrueta & Associates, said, "With the growth in Manassas and Prince William, there's a need for another regional mall. It's going to spur healthy competition -- and that's good for the consumer."