The Loudoun County Board of Supervisors yesterday approved by a 5-to-3 vote a developer-backed change to the county master plan that will allow a regional shopping mall to be built along Rte. 7 in eastern Loudoun.

The supervisors, who will formally adopt the county budget May 16, also voted to keep the real estate tax rate at its current level--$1.13 per $100 of assessed value--and to cut the personal property tax from $5 per $100 valuation to $4.75.

The decision to cut the personal property tax came after the supervisors voted to impose a new 2 percent tax on motels and travel campgrounds and discovered they could borrow $300,000 from the state to cover the costs of school improvements.

The board unanimously approved a $41.8 million operating and capital expenditure school budget and increased the pay of the supervisors who will take office in January to $9,000 a year from the current $6,000. The board chairman will be paid an additional $1,000 a year.

The controversial proposal to allow development of a shopping mall along Rte. 7 near its intersection with Rte. 28 was opposed by the county planning commission and numerous other groups. They claimed that a mall there would attract other commercial development and lead to traffic congestion on Loudoun's major commuter highway.

Developers have eyed sites on the highway for some time and the issue of development has been debated frequently in the county. In recent months, builders have lobbied hard to get the board to amend the county master plan, which specified the intersection of Rte. 28 and Rte. 625 as the site for a mall.

Board members who supported the plan amendment yesterday said they believed the only way the county could get road improvements in the area would be if the developers competing for a shopping mall offered to build them.

Supervisor Frank Raflo (D-Leesburg), who supported the amendment, said it would not actually allow a shopping center on Rte. 7, but supervisor James F. Brownell (R-Blue Ridge) called that a "syntactical sleight of hand." The board said it will not allow the developers to build a mall with direct access on Rte. 7 or Rte. 28, but will insist the mall be situated on either major road with access road links.

Supervisors John Milton (D-Catoctin) and Travis Sample voted against the amendment along with Brownell. Board chairman Thomas S. Dodson (D-Mercer), Carl F. Henrickson (D-Broad Run), Betty Tatum (D-Guilford) and Supervisor Andrew R. Bird III (R-Sterling) voted along with Raflo for the amendment.