EVEN Without A firm headcount, it's safe to assume that a goodly number of Fairfax County residents don't want a property tax increase if it can be avoided. On Tuesday, people in this particular camp have an opportunity at the polls both to underscore that view and to express approval for construction of a sorely needed new county courthouse. A referendum is scheduled on a proposed $18.7-million bond issue to finance the project, and county officials emphasize that approval of the bond sale can prevent a tax increase that would be needed for pay-as-you-go financing.

There would be no need to raise the property tax, they explain, if this bond issue is approved and the total county sales don't exceed $46.6 million in bonds annually. Sales of more than that amount would require additional revenues to cover increased payments to bond purchasers. Officials state that there are no plans to go over the $46.6-million figure.

Conditions in the present county courthouse are, in a word, terrible. Judges, lawyers, other court officials and jurors all can point out that the building is not only dilapidated, but overcrowded to an extent that jeopardizes fairness in the administration of justice. For one thing, some of the courtrooms are so small that lawyers and their clients can't find a quiet corner to talk privately and judges can't hold bench conferences without being overheard by the juries. In the hallways, jurors find it difficult not to mingle with witnesses, members of the public or defendants. Participants in juvenile case must share the crowded facilities. Since the Juvenile and Domestic Realations Court is housed there, too. Under the construction plan, this court would stay there and the circuit and general district courts would move to the new facility.

Saying "no" to the project isn't likely to kill it, anyway - for under state law, the circuit court chief judge could order the county to build the court chief judge could order the county to build the courthouse if the bond proposal were defeated. It happens that he supports the project. Thus, the most sensible way to pay for it is through a bond sale. That is why the Fairfax County Federation of Citizens Associations, the League of Women Voters and other responsible citizens groups support a vote "Yes" - and why they are urging votes who agree to be sure to cast their ballots on Tuesdaotes who agree to be sure to cast their ballots on Tuesday. We wish them success.