Fairfax County developer James H. Burch and the Prince William County Park Authority have reached an agreement that clears the way for what he said will be the largest boat-storage operation on the upper part of the Potomac River.

Burch said his company will build the Water Hammocks of Occoquan, a $1 million indoor boat-storage facility for 536 boats, in the town of Occoquan, just off Interstate 95.

A smaller warehouse-style boat-storage facility is located in Woodbridge. Most storage near Occoquan is open-air slips and docks.

The warehouse-style building, docks and supporting facilities, including a restaurant, will be built in what is now an abandoned water and sewer pumping building on 6.7 acres in Occoquan.

Under the agreement, Burch has a 20-year renewable lease on the valuable waterfront property. "Prince William is putting up the land, and I'm putting up the $1 million," Burch said.

Although the building could be described as an upgraded warehouse, Burch said it will look like it is made of wood even though it is actually steel. He said it will be the centerpiece of a park-like setting catering to what he called the increasing need of Northern Virginia boat owners to find places to store their boats.

Burch said state boat registration figures show there are more than 4,000 registered boats in Prince William and more than 15,000 in Fairfax, but there are fewer than 3,000 boat slips.

Many owners now park their boats in their yards, on top of boat trailers and in storage facilities near their homes far from the water, Burch said.

At Water Hammocks, boats will be stored in hammock-style units allowing them to hang in the air, stacked, without touching each other, Burch said.

Xerox Corp. is offering to build an overpass worth $8.4 million that would feed traffic onto Route 7 from Potomac Park, a corporate office park and residential community proposed for the corporation's 2,267-acre estate just east of Leesburg.

Loudoun County officials have voiced fears that huge developments planned alongside Route 7 would cause a gridlock on the county's primary commuting corridor. The county planning commission voted unanimously on Wednesday to recommend approval of the Xerox rezoning application to the board of supervisors. The board is expected to act in November on the plan, which calls for building 11 million square feet of corporate office space, including a hotel, conference center and golf course, 1,830 executive-style homes and a 120,000-square-foot neighborhood shopping center.

The Xerox offer came in a package of road improvements filed last week with Loudoun County. The improvements are designed to cope with the estimated 40 percent increase in traffic that the first phase of the Potomac Park project is expected to bring to Route 7.

Construction of the overpass, only the second interchange in the predominantly rural county, would begin no later than June 30, 1990, or when the first 800 homes, 400 hotel rooms and 1.4 million square feet of office space planned for phase one of Potomac Park is built, Xerox's statement said.

To keep traffic moving in the meantime, Xerox offered to build a third lane for almost three miles on westbound Route 7 alongside its property. Xerox also said it will pay $400 into a highway trust fund for every residential unit it builds and 40 cents for each square foot of commercial development, which Loudoun would have to use for future road improvements nearby.

Milt Herd, the county's chief of comprehensive planning, said the money -- a total of $624,000 in the first phase of Xerox's project -- is enough to pay for 1 1/2 miles of another lane on Route 7.

IN THE BUSINESS . . . RE/MAX International Inc., a real estate company, is expanding in the mid-Atlantic area, adding 250 full-time sales associates and 15 new offices in the next year, according to board Chairman David Liniger . . . JBG Cos., a Washington real estate development and management firm, has bought the 200,000-square-foot office building at 1250 Connecticut Ave. NW . . . Cabot, Cabot & Forbes plans early next year to start construction of a 99,000-square-foot office and research-and-development building at the Loudoun Tech Center on Route 7 near the intersection with Route 28 . . . Kaiser Permanente Health Care Program is building a new medical center in Kensington, on a 4.6-acre site at the intersection of Connecticut and Perry avenues . . . The 384-room Hyatt Regency Bethesda opens next Friday at the intersection of Wisconsin Avenue, Old Georgetown Road and East-West Highway. It is part of a $160 million mixed-use development.

PERSONNEL FILE . . . Ann L. Mable has been named assistant vice president of the real estate financing firm of Ivor B. Clark Co. . . . David B. Low is the new executive vice president, treasurer and chief financial officer of the National Corporation for Housing Partnerships . . . Mark J. Riedy, president and chief operating officer of the Federal National Mortgage Association (Fannie Mae), has been elected to a three-year term on the board of Perpetual American Bank . . . Warren S. Oliveri, vice president of Weaver Bros. Inc., has been elected to the board of the Building Owners and Managers Association International . . . Oxford Mortgage & Investment Corp. has promoted Alan F. Greenwald to vice president . . . Joseph M. Imber has been named controller for the HBW Group, a Rockville-based commercial and industrial real estate firm . . . Krista Amason, most recently finance officer of the Fairfax County Redevelopment and Housing Authority, has been named executive director of the Ballston Partnership, a coalition formed this year to promote the redevelopment of the Ballston neighborhood in Arlington . . . Shopping center developers Alexius C. Conroy, Melvin Simon and Herbert Simon have joined to form the Alexius C. Conroy Co., based in White Plains, N.Y., to develop and buy regional and community shopping centers. Conroy was president of Cadillac Fairview Shopping Centers (U.S.) Ltd., while the Simons own and manage more than 60 million square feet of retail space in North America . . . Metropolitan Industries, a builder specialties contractor, has named Ray Dunie as executive vice president.