After a five-month deadlock over where to build a $50 million Loudoun County government complex, the Board of Supervisors backed away from the proposal yesterday.

Board members cited the flagging local and national economy and pressures on the county budget in voting unanimously against accepting any of the three developer proposals to build a new complex under a lease-purchase arrangement.

Board members said they would consider reopening the process in December, possibly leading to a March 1991 voter referendum on about $35 million in bond financing for consolidated county offices.

Also yesterday, the board voted 5 to 3 to rezone 1,300 acres near the town of Round Hill for 1,100 houses. The application was approved over the objections of the county staff and some of the 600 residents of the western Loudoun community who said it calls for excessive density.

County Board Chairman Betty W. Tatum (D-Guilford District) said that "quite a few citizens" have told her recently that this is not a good time to commit a large amount of county money to the government project.

Other board members said the public is apparently against the complex, with some supervisors predicting that a November bond referendum would fail.

The Loudoun government rents space at several locations. The central administration building in Leesburg, which opened in 1976, is targeted for eventual use by the county court system.

Consolidation of about one-third of the government in one new complex in Leesburg, Ashburn or on the Xerox Corp. property east of Leesburg was proposed to save rent and promote efficiency.

During spring public hearings on the county budget, many citizens decried the rapid growth of the county government and its spending in recent years. Some said vacant government positions should be eliminated in an effort to hold down real estate taxes.

The county spends $1.5 million a year to rent government office space. That figure is projected to increase dramatically over the next two decades, despite the current glut in the office space market. A recent report found a 42 percent vacancy rate for commercial space in the county.