The Loudoun County Board of Supervisors, taking decisive progrowth votes in the midst of an election year, approved two major projects yesterday that officials said would accelerate the tide of development sweeping the county's eastern suburbs.

On a 6-to-1 vote, the supervisors approved Potomac Lakes, which, with a planned 3,496 houses, is the third-largest residential project ever approved in Loudoun.

The developer, Kettler & Scott Inc., also plans to build more than 2.5 million square feet of commercial and office space on the 1,342-acre property along Rte. 7 near the Fairfax-Loudoun county border.

The supervisors then voted 7 to 0 to approve Glennwood, a development by another Northern Virginia partnership that could bring nearly 4 million square feet of office and commercial space to a 395-acre site near Dulles International Airport.

Loudoun officials said the votes were important steps in the transformation of eastern Loudoun around Dulles into one of the region's most prosperous development corridors.

The votes also come when many political observers are noting a sourness among many residents countywide at the frenetic pace of Loudoun's growth.

Before the Potomac Lakes vote, several supervisors lavished praise on Kettler & Scott for agreeing to pay for more than $25 million in road construction and other public improvements in and around the project.

"Since I've been on the board, this is this the best development application I've seen," said Supervisor Steve W. Stockman (I-Broad Run), whose district includes Potomac Lakes, and who is running for reelection Nov. 3 as a Republican against independent Gregory Rex Marquis.

"This application goes a long way toward solving critical transportation problems" in eastern Loudoun, Stockman said.

Among other improvements, Kettler and Scott plans to widen heavily traveled Rte. 7 from four lanes to six on a two-mile stretch from Potomac Lakes to Fairfax County.

Supervisor Thomas S. Dodson (D-Mercer), the board's lone dissenter, said he agreed that Kettler & Scott's road construction pledges were substantial, but cautioned other board members that they had not paid enough attention to the demands the development would place on other vital government services, particularly schools.

Board Chairman Betty W. Tatum (D-Guilford) was absent yesterday.

In approving Potomac Lakes, the board rebuffed a group of civic activists who made an 11th-hour plea for turning the property into a national park because of their belief that it could contain Civil War and Indian artifacts. Robert C. Kettler, Kettler & Scott's president, said yesterday his firm would have an archeologist survey the property before building begins, and that he would sell his property "for a fair-market" price to the Interior Department in the event that it is interested.

County officials say such interest is unlikely.

The Potomac Lakes property was considered remarkable in local development circles because of the interest it drew from land speculators.

In October 1984, the tract was sold for $18.1 million.

In the 26 months that followed, it was sold twice more, most recently to Kettler & Scott for $51 milion.

Yesterday's approvals were noteworthy party because they included "aye" votes from some of the same supervisors who have consistently criticized some other projects as bringing excessive growth. One of these officials, Supervisor Ann B. Kavanagh (D-Dulles), whose district includes the Glennwood property, said the quality of the projects and the county's success in extracting promises for roads and facilities from the developers made the difference.