The following items are on the Sept. 26 agenda of the Fairfax County Board of Supervisors. The meeting will be held in the board room on the A-level of the Massey Building, 4100 Chain Bridge Rd., Fairfax: 10 a.m.

* Approval of the minutes.

* Presentations by the Economic Development Authority and the Environmental Quality Advisory Council.

* Board members' items. 11 a.m.

* Board actions on 1984 fiscal plan adjustments.

* County executive's items. 11:30 a.m.

* County attorney's items. 2 p.m.

Public hearings on:

* Proposed amendment to clarify that it is illegal to dump trash on any county road, highway or property or private property without owner's consent.

* Restricting parking for residents of Residential Permit Parking Districts only during certain hours of the day and establishing a specific Residential Permit Parking District near the Huntington Metrorail station.

* County proposals to give up title to a portion of Trammell Road, Mason District, a portion of Parkers Lane, Mount Vernon District, and a portion of Old Keene Mill Road, Springfield District.

* The possibility of permitting 16 to 20 residential units per acre, with an option for offices and small shops, on the east side of Rte. 1, opposite Engleside Street, Mount Vernon District. This area is planned for office use.

* A request to permit a marina at 10729 and 10719 Old Colchester Rd., a residential zone in the Mount Vernon District. 2:30 p.m.

Public hearings on requests:

* To build a house at 2006 Cutwater Ct., in an area subject to flooding, Centreville District.

* To build a two-story addition at BMW of Fairfax, 8427 Lee Highway, Providence District.

* To permit a radio, television and microwave facility 2610 Reston Ave., in a residential district, Centreville District. 3 p.m.

*Public hearings on rezoning:

* About five acres at Braddock Road and I-66 from residential to high-rise office use. Springfield District.

* About two acres of Gambrill Road to higher density residential use. Springfield District.

* The residential density upward on about four acres at North Chambliss Street and Lincolnia Road. Mason District.

* About 53 acres on Rte. 50 near West Ox Road from low-density residential to community residential and office use. Providence District. 3:30 p.m.

Public hearings on:

* Increasing the residential density of about three acres at Heron Drive and Eames Avenue. Springfield District.

* Rezoning about 11 acres on Guinea Road, opposite Chatsworth Court and Pomeroy Drive, from community office and retail to residential use. Annandale District.

* Increasing the residential density of 38 acres on Braddock and Ox roads. Springfield District.

* A final plan for 232 town-house units on 84 acres on Silverbrook Road near Hooes Road. Mount Vernon District.

* Increasing residential density of one acre at Baccarat Drive and Waller Road. Annandale District.

Board decision on the exchange of county-owned property for a privately owned parcel, concerning the Chantilly soccer fields. Providence District. 7:30 p.m.

Public hearings on:

A request to permit a mini-warehousing establishment at 9910 Richmond Hwy. on 12 acres of land zoned for industrial and office use. Mount Vernon District.

* A request to permit a waiver of minimum lot width requirements for single-family homes on 70 acres west of Moore Road, east of Centreville Road and south of Braddock Road. Springfield District.

* A request to build an addition at 2837 Bolling Rd., an area subject to flooding.

* A request by developer Roger DeMarco to change a proffer that provided a tennis or multipurpose court and two tot lots, by eliminating the court and one tot lot, and including other recreation areas instead. The property is on the north side of Old Courthouse Road. Providence District.

* Access restrictions, road improvements and preservation of existing property on four acres at Virginia Drive and Rolling Road. Springfield District. School Board Actions

The following were among the actions taken at the Fairfax County School Board at its Sept. 15 meeting:

ENROLLMENT--Received a report that enrollment as of Sept. 6 was 121,576 regular and special students, or 630 more than had been projected and 115 less than last year's opening day figure. A complete enrollment report will be made Sept. 30. In past years, enrollment has increased in the month of September.

JOBS--Authorized 50 new staff positions and 25 contingency positions to meet current and anticipated needs. The new positions will cost $1 million, to be allocated in the school system's mid-year budget review.

VANDALISM--Reported that, for the year ending June 30, vandalism and theft in county schools was down 17 percent from the previous year. Total loss was $361,000, the lowest in six years. Langley, McLean and Mount Vernon high schools were singled out for high equipment losses.

SCHOLARS--Reported that 137 county high school seniors have been named National Merit semifinalists in the 1984 program. This amounts to 42 percent of the Virginia semifinalists, although the county has only 15 percent of the state's students.

SUBSTANCE ABUSE--Received copies of a draft letter to community leaders and organizations from the chairman and the superintendent asking for support in efforts to eliminate drug abuse in the school system.

RESOLUTIONS--Named Nancy L. Gibbs, music teacher at Wolftrap Elementary School, as teacher of the year for the school system. The board also named the Marshall High School computer lab the Ralph W. Frieden Computer Laboratory in honor of the man who was a math teacher at Marshall from 1969 until his death last spring.