A coalition of business groups in Northern Virginia is launching a campaign to shave 15 to 20 percent from the price of the average new home by loosening Fairfax County's building regulations.

If more lower-priced homes were available, middle-income households would be able to buy them instead of crowding lower-income families out of the rental market, said William Bestimt, president of the Fairfax County Chamber of Commerce. The chamber, the Northern Virginia Builders Association and the Northern Virginia Board of Realtors plans to sell this message to citizens associations, civic clubs and parent-teacher groups using speeches and educational films.

The coalition has not yet set a timetable for launching its educational program.

Its efforts coincide with the county Board of Supervisors' call for the private sector to get involved in solving the low-income-housing problem. But the initiative is getting a cool response from some supervisors.

"In my experience, builders don't pass the full savings on to the consumer. I'm sympathetic to the extent that we eliminate unnecessary costs, but to the extent that we are chiseling away at inspection standards and lowering overall quality, I don't support it," said Supervisor Thomas Davis III, chairman of the board's housing subcommittee.

Supervisor James M. Scott said low-cost housing for sale is a short-term panacea for the housing crisis. The county worked successfully with Yeonas Co. to reduce housing costs at Lake Braddock, only to find that prices shot up when the second and third buyer purchased them, Scott said.

"It was a windfall for people selling. It didn't keep the lower-priced housing in the market," he asserted.

Bestimt is ready for such skepticism. "People need to see this is not a fast and loose way to feather the builder's nest," he said. "It's a way to make housing more affordable."

The coalition offers the following suggestions for keeping costs down:

* Streamlining the development review process to reduce processing time.

* Encouraging cluster developments that reduce sewer and water line runs.

* Allowing innovative and cheaper construction materials such as PVC pipes.

* Providing higher-density zoning.

These measures could produce savings that would make median-priced homes in Northern Virginia affordable to families with the median income, Bestimt said. Median means one-half is higher and one-half is lower.

Statistics compiled by the Northern Virginia Builders Association show that even in affluent Fairfax County, 9 percent of median-income households could not afford a median-priced house ($108,933) in 1983. In Manassas Park, the median-income family had only 57 percent of the income needed to become home owners in 1983, NVBA said. Median income in Fairfax County was $47,100 in 1983.

By presenting these facts to citizens associations, the coalition hopes to build a groundswell of support for regulatory change, former NVBA president William Berry said. This would create the political environment needed for reform, he said.

"What it requires is politicians to stand up and say there is nowhere for median-income families to live in this county and we are going to do something about," Berry said.