Fairfax County Executive J. Hamilton Lambert yesterday called for a $6.8 million increase in the county budget for the current year, and he said that some spending will have to be deferred until next year because tax collections have been running behind forecasts.

The proposed budget changes for fiscal 1988, which began July 1, are needed to adjust spending for some existing programs and, in a few instances, to provide money for programs the board has ordered since this year's $1.7 billion budget was approved in April.

In his midyear budget report to the county Board of Supervisors, Lambert said that some spending deferrals will be needed because personal property tax receipts and sales tax receipts are lagging behind projections by more than $6 million and are forcing the county government to juggle priorities.

Among the proposed increases to the budget are:$2.6 million for the purchase of 584 electronic voting machines. After the November elections, politicians and voters complained that the county's voting machines were antiquated, leading to breakdowns that inconvenienced voters and slowed the election tallies. The new machines are scheduled to be purchased in time for the March 8 Super Tuesday presidential primary. $1.1 million to implement the first phase of a career development plan for the county fire department. When the budget was passed, union officials representing the fire department complained bitterly that county police officers got a better pay package -- including raises -- than did firefighters. $40,000 to fund a new School Age Child Care Center at Layton Hall Elementary School, 3705 Old Lee Hwy.

Lambert said the proposed reductions and deferrals include $3.7 million for a new Equipment, Management and Transportation Agency garage in Centreville; $2.3 million for the renovation and expansion of the Franconia Government Center; $1.2 million for the expansion of the Clifton Fire Station, and $1.2 million intended for a new pedestrian trail near Annandale Road.

The county board authorized a public hearing on the proposed changes for Jan. 11, the first scheduled meeting of the newly elected board under the chairmanship of Democrat Audrey Moore. The Annandale District supervisor will become chairman Jan. 1. Yesterday's meeting was the last for John F. Herrity, who lost to Moore last month.

In other matters yesterday, the board received a report from a panel that said that the county has 1,150 too few beds for people who need mental health, substance abuse and child support services.