The Prince William Board of County Supervisors has set the stage for large-scale development in Gainesville -- the area designated by county plans as a major employment center -- by approving a 102-acre office and industrial park at the intersection of Route 29 and Interstate Route 66.

Developer Eugene I. Siegel, president of the Bethesda-based land management firm LandServices Inc., said he expects that the 1 million-square-foot mix of light industrial, research and development, office and retail space will generate 3,000 jobs and about $1.8 million in net tax revenue for the county. Plans also call for a 250-room hotel and two restaurants.

But the site first must be hooked up to public water. John Bailey, engineering director for the county's water authority, said he expects the extension of a water line north to Gainesville to begin later this year.

Meanwhile, Siegel said he plans to start constructing 60,000 square feet of research and development space using private wells, but he cannot exceed 85,000 square feet of development before public water is available. No tenants have been secured for the project, he said.

The county board unanimnously approved the project, called Gainesville Business Center, at Tuesday's meeting. The supervisor for the area, G. Anthony Guiffre, called the project "a precedent-setting rezoning for economic development in the area."

The development, however, is expected nearly to double current traffic loads on Route 29 at Gainesville. Accordingly, Siegel has agreed to widen roads and improve traffic lights at intersections around the site.

The Gainesville Business Center is Siegel's first venture as a developer. He is known in Prince George's County for his consulting work on the 235-acre Inglewood industrial park project and the Konterra mixed-use community near Laurel.

Just like Fairfax and Prince William counties, the Virginia Department of Highways and Transportation is resorting to the private sector for help in digging its plan reviewers out from the under the growing backlog of development plans.

The highway department is negotiating a one-year contract with a Fairfax-based engineering firm, Rinker, Detwiler & Associates, to review traffic issues on rezoning applications and site and subdivision plans in the Northern Virginia district, according to highway department spokeswoman Lynda South.

The appointment of Rinker, Detwiler should help quell mounting criticism from the private sector over lengthening delays in plan review at the highway department. Simple plans that once took 30 to 45 days to get the stamp of approval have been taking up to 100 days, said John Elkin, partner in the engineering firm of Bengtson, DeBel, Elkin & Titus.

Overwhelmed by the plans generated by record levels of development in Northern Virginia, the local highway office this past summer began sending plans all over the state to less-pressured offices for plan review.

The highway department also has gained four new positions for the Northern Virginia district office, doubling the size of its plan-review staff, South said.

Jack Rinker, president of the engineering firm, said he will allocate four staff members to the job, effective Nov. 1, and will bill the state on an hourly basis. Neither Rinker nor highway officials could estimate how much the contract might be worth.

In June, Fairfax County hired private engineers from Bechtel Inc. to tackle its plan backlog. Prince William County followed suit in August, also hiring Rinker, Detwiler. The state's action marks the first time that the highway department has sought outside help for its plan review work.

The Department of Housing and Urban Development announced this week that it has awarded $23.7 million to 108 communities nationwide in the second round of an experimental program to help low-income single parents with young children. The communities are in 33 states, including Maryland and Virginia, and in Puerto Rico.

The purpose of the program, called Project Self-Sufficiency, is to help single parents become economically independent. The HUD awards will help 4,938 families pay rent, while local governments and private groups provide job training, basic education, personal and career counseling, child care and transportation.

In Virginia, the housing certificates will go to families in Alexandria, Fairfax and Prince William counties, Hampton, Norfolk, Roanoke and Suffolk. Maryland families in Cecil, Harford and St. Mary's counties also will get the certificates.

HUD Secretary Samuel R. Pierce Jr. praised local communities taking part in the program for "providing these families with the freedom and security to participate in . . . local support services" while they seek education and employment.

Parents receiving the housing certificates will be chosen by a task force made up of representatives of local business communities, educational institutions, public-housing agencies and public and private agencies that aid low-income citizens.

HUD awarded $23 million to 77 communities in 32 states, the District of Columbia and Guam in the first phase of the Project Self-Sufficiency demonstration in September 1984.

Prince William County's Mall at Manassas is going into competition with Fair Oaks Mall in Fairfax County.

A New Jersey-based development company, Interstate Properties, has won approval of its plan to almost double the size of its shopping center on Sudley Road outside Manassas.

Edward P. Latimer, the attorney for the project, told Prince William supervisors this past week that his clients want to turn the mall at Manassas into a regional mall rivaling that at Fair Oaks, with 1 million square feet of retail space and six department stores.

The board gave its unanimous consent to the project, but not without some misgivings about the effect the mall will have on traffic in the area. Residents of the neighboring Tudor Hall subdivision protested that road improvements offered by Interstate Properties would do nothing to solve the already dangerous conditions on their local roads.

"We are sitting on a time bomb. Somebody is going to get hurt," said a Tudor Hall resident, Ron Sisk, who complained that traffic already cuts through his subdivision to reach the mall at Sudley Road, a major commuter route.

The Washington landscape is dominated by giant building cranes, a fact documented in hard numbers by the latest F. W. Dodge construction report. Construction contracts this year totaled $596.8 million in the District through the end of August, up 21 percent from $495.2 million in the first eight months of 1984. To no one's surprise, the largest share of the new construction is for nonresidential buildings, chiefly new office and retail buildings.

Real estate brokers believe that cleaning and sprucing up homes enhances their market value when they are sold, but also advise against substantial remodeling projects, according to a survey done for the ERA Real Estate firm.

More than two-thirds of the 400 brokers surveyed said some redecorating -- such as painting the interior -- should be done before selling a home, while nearly half urged that a general cleanup be undertaken inside and outside. Smaller numbers advised that worn carpeting should be replaced or landscaping improved.

"Major home improvements probably only make sense if the homeowner plans to stay there and personally get the benefit of them," said Dain Zinn, an ERA vice president. "Why finish a basement recreation room as a gift for the next resident?"

IN THE BUSINESS . . . Associated Tax Services in Vienna, which provides real estate tax services to mortgage lenders, has become American Realty Tax Services Inc. . . . The National Association of Realtors reported that the seasonally adjusted annual rate of sales of existing single-family homes reached 3.43 million units in August, the highest figure in nearly six years . . . Virginia housing sales remained high in August, up 24 percent over those for August 1984 in a survey of 22 multiple-listing services, according to the Virginia Association of Realtors. A total of 6,273 housing units were sold . . . Brenneman Associates Inc. and Condominium Realty Corp. have merged into Condominium Realty Brenneman Partnership . . . Linpro Co. says it will break ground this fall on the first phase of the Commonwealth Corporate Center along Route 50 in Fairfax City, near the interchange of Route 123 and Interstate Route 66. The project eventually will include four identical five-story office buildings with 500,000 square feet of space . . . Members of the D.C. Builders Association, the Commercial Industrial Institutional Council, the Northern Virginia Builders Association and the Suburban Maryland Building Industry Association are staging their 22nd annual fall convention in style, with a Nov. 2-9 cruise on the Carla Costa to the Caribbean . . . The Virginia Association of Roofing Contractors has elected Thomas G. Petty of Richmond as its new president, with Jerry M. Sare of Arlington as its first vice president . . . The Washington-Lee Shopping Center in Arlington has been acquired by a new partnership called Webb/Sequoia Venture 100 and will be turned into a $63 million corporate office park to be known as Sequoia Plaza. The partnership's principals are Webb/Sequoia Inc., a partnership between Sequoia Building Corp. of Fairfax and the Webb Cos. of Lexington, Ky., and the Investment Group Capital Corp. of Washington, headed by developer Conrad Cafritz . . . Leggat McCall Cos. has purchased Minot DeBlois and Maddison Inc., which says it is the nation's oldest real estate consulting organization. The new group will be known as Colliers Leggat McCall Consultants Inc. with offices here and in Boston. Real estate economist Lewis Bolan will be the managing director of the firm . . . Ryland Group is celebrating its 50,000th home by donating $500 to health-related charities across the country for each home sold in October and a microwave oven to each home buyer.