Dec. 31 is the deadline for qualifying senior citizens and permanently and totally disabled Fairfax County residents to apply for free auto decals.

To qualify for the free auto decal, applicants must apply between Jan. 1 and May 1, 1985, and be approved for either personal property tax relief, real estate tax ememption or a rental grant in 1985. Applications for those are to be filed between Jan. 1 and May 1, 1985.

An applicant must have a gross annual household income from all sources of not more than $15,000, excluding the first $4,000 of income of each relative, other than the spouse, living in the household. Exclusion of up to $5,000 of any income received by an owner as permanent disability compensation will be allowed.

An applicant and spouse living in the household must have a combined financial net worth of no more than $65,000, excluding the value of the home and up to one acre of land on which it is located.

Applications for the free auto decals are being accepted and assistance is available at the Fairfax County Office of Assessments on the third floor of the Massey Building and at branch assessment offices at 6121 Franconia Rd., Franconia; 6507 Columbia Pike, Annandale; 1437 Balls Hill Rd., McLean; 2511 Parkers La., Alexandria; 11415 Isaac Newton Square South, Reston, and 6140 Rolling Rd., Springfield.

For information, call 691-2437.