Termites attack 18,000 houses annually in Northern Virginia. Yet homeowners find that many common methods of anti-termite treatment are inadequate.

Considering these unsettling realities, the Fairfax County Board of Supervisors voted last Monday to seek an amendment to the state building code giving localities the power to require one of the most effective controls against termites - chemical treatment of soil.

County executive Leonard Whorton indicated that if the amendment was adopted by the State Housing Board - the body in charge of the housing codes - the county would require pretreatment of soil before new houses are constructed.

In a study he presented last year while serving as acting county executive, deputy county executive J. Hamilton Lambert said traditional termite controls, such as use of treated lumber, have often proved inadequate. The most effective control, Lambert said the studies have shown, is chemical treatment of soil. The chemical used is the highly toxic hydrocarbon chlordane, which has been banned in most of its uses.

Soil treated before construction costs about $45, the Lambert study said. Treatment after construction can run as high as $250, the study said.

At present there are no state regulations requiring that houses - new or old - be protected against termites. Certification that a houses is free of termites is required by the Federal Housing Administration and the Veterans Administration before those agencies grant loans to home buyers. Some private lending institutions alos require certification of some homes.

Lambert's 1976 study said that a telephone survey of pest-control firms indicated that 71,500 inspections are made annually in Northern Virginia, and that about a quarter of these reveal infestation. While the study had no estimate of termite damage, it did say that treatment and control costs Northern Virginians $7.2 million annually.

In discussing the different controls, the study said that termite shields can deteriorate with age and be extremely expensive to replace. The study also said that termites are capable of crossing treated sections of wood to get to untreated sections.