The Loudoun County Board of Supervisors this week unanimously approved rezoning for a 600,000-square-foot retail shopping center, the first regional mall to win zoning approval in the county.

Located on the west side of Rte. 28, about two miles south of Rte. 7, the center would lie on 72 acres and have 400,000 square feet of office space in addition to the 600,000 square feet of retail-store space.

Rosser H. Payne Jr., a former director of planning for Fairfax County and now chief planning consultant for the mall developer, Nattak Development Co., said Monday that the facility will have three major retail stores, but he could not say which stores those would be.

Payne said that there will be about one year of paperwork and then construction would begin, taking three to 3 1/2 years.

After the board's vote, Payne told the supervisors that "after 28 months of work on the application , I thank you from the bottom of my heart. Unanimous is wonderful."

As an inducement to win the county's approval, Nattak offered to build approximately $1.5 million worth of off-site road improvements, including two additional lanes on Rte. 28 between Rte. 625 and Rte. 846. Another inducement, also accepted by the county, was that Nattak would contribute 10 cents per square foot of commercial building space, which the county would use for fire and rescue services in eastern Loudoun.

Although Nattak, owned by Alan Kay of the Bethesda building firm Rozansky and Kay, is the first to receive the county's assent for a regional mall, the company may have some competition early next year when a rezoning application from Lerner Corp. goes before the county. Lerner Corp. wants to build the 600,000-square-foot Windmill shopping mall at Rte. 7 and Rte. 28.

The supervisors do not expect, even if another mall (such as Lerner) is approved, that two shopping centers will be built here.

"They will vote on each application according to its merit," said county administrator Philip A. Bolen. Board chairman James F. Brownell said, "Our thinking is that the marketplace will determine which one goes in."

The county's comprehensive plan and other planning documents do not recommend one site over the other for a shopping mall, according to Loudoun zoning administrator Michael Congleton.

In other business, Mercer supervisor Thomas S. Dodson, who was expected to begin duties as board chairman next month, told the board that for "basically personal and business reasons" he would not accept the position. Instead, Dodson said, he will nominate Leesburg supervisor Frank Raflo for the chairmanship at the board's first meeting in January.

Last January, in order to break a partisan deadlock in a 4-to-4 vote on the chairmanship, the supervisors agreed to elect a Republican chairman for this year, and a Democrat for 1985.

This week was board chairman James F. Brownell's last meeting in the chair, and the supervisors presented him with a plaque in honor of his work over the last 12 months. Presenting the plaque, county administrator Philip A. Bolen said Brownell had told him that "on Dec. 31, he's going to be celebrating, but not New Year's." Quipped Brownell about the plaque, "My only comment is, believe me, I earned it."

In other matters this week, the board:

* Approved a rezoning for the construction of 32 houses and 127 town houses on 40 acres near Sugarland Run. The tract's southern boundary is adjacent to the Northern Virginia Community College property.

* Heard from Bolen that the position of assistant county administrator has been filled. James Keene, currently budget and management officer for the city of Rockville, Md., and a resident of Lovettsville, will begin work with the county next month, Bolen said.