The Fairfax County board of supervisors, spurred by Saturday's house fire in McLean that claimed seven lives, searched yesterday for ways to stiffen legal requirements for smoke detectors in county residences.

By an 8-to-0 vote, the board ordered a review of its police powers to determine whether county officials have the authority to require installation of the devices in all rental units and in single-family dwellings that undergo remodeling.

In a separate, unanimous vote, the supervisors also agreed to ask insurance companies to offer financial incentives to policyholders in the county who install the safety devices in their homes.

"Fire detectors, like seat belts, can save your life if you use them," said Supervisor Marie B. Travesky (R-Springfield), who introduced the insurance resolution. "We have to keep trying to get the message across."

It is doubtful that yesterday's decision to review the county's constitutionally granted police powers will have much immediate impact on the county's fire safety efforts.

In a 1978 opinion, Virginia Attorney General J. Marshall Coleman told officials in neighboring Alexandria that they lacked legal power to set mandatory smoke detector requirements. Subsequent efforts by that city's ligislators to get a new law passed have been rebuffed repeatedly by the General Assembly.

"I wish them luck [in Fairfax County], but they'll probably have as much success as we did," said Alexandria Fire Chief Charles Rule yesterday. Rule said local jurisdictions' hands re tied by a "Mickey Mouse" state building code that requires smoke detectors be installed in structures built after 1974.

Montgomery County, by contrast, requires that the devices be installed in all residences regardless of the age of the building. A similar measure will take effect in the District of Columbia in 1981.

While some insurance companies offer a discount of 2 percent to home owners who have installed smoke detectors, Allstate, one of the largest insurance carriers serving Fairfax County, does not.

"We would encourage everyone to have them in their homes, but we do not offer any discounts at present," said Allstate spokesman Buzz Harvey, who noted that smoke detectors may save lives but offer no guarantee of reducing property damage from fire.

Harvey said many battery-operated home smoke detectors are useless in any event because consumers fail to replace the batteries regularly. "I have a smoke detector at home and I have no idea whether the thing would go off or not," he said.

According to insurance industry figures, prevailing discount rates, which offer savings of less than $5 a year on a policy for a $60,000 home, do little to encourage the use of the devices.

In one recent year, only seven percent of those holding State Farm Insurance home owner policies qualified for the smoke detector discount, according to company spokesman Jerry Parsons. "It's really more of a goodwill type thing than anything else," he said.

The victims of Saturday's early morning fire at 2014 Great Falls St. died of smoke inhalation, according to autopsy results released yesterday.