Mason Neck, the southermost reach of Fairfax County, is still so immune from the rapid spread of suburbia that a landowner can build an illegal house on the shore of Belmont Bay and the county assessor's office does not find out.

Such immunity suits environmentalists fine. It means that Mason Neck's most celebrated resident, the reclusive and endangered southern bald eagle, can roost contendedly in the soaring loblolly pines. It also means wild rice can flourish in Great Marsh and the bobwhite and wild turkey can find a home in the woods.

But some of the Neck's most vigilant protectors are fearful that the stage is being set for subdivision-style development.

What has them worried is a proposal calling for a sewer pipeline to be built across the Neck.

Fairfax County Supervisor Warren E. Cikins (D-Mount Vernon) and Thomas M. Schwarberg Jr., who heads the regional office of the State Water Control Board, say the pipeline would have only one purpose: to shunt sewage away from a small plant that is both costly to operate and sometimes ineffective. The pipeline, they say, would carry sewage only from Harbor View Estates, on the western edge of the Neck, to the newly upgraded lower Potomac treatment plant on the eastern side.

But Elizabeth S. Hartwell, who has been leader of many past battles to save the Neck (from a "satellite" city gas pipeline and dredging and pesticide spraying, among other thing fears the pipeline will inevitably used by developers who hold large, vacant tracts that, at present, have no access to sewers.

"If that pipeline goes through," Hartwell said, "this could open up the land to rapid and certainly much more dense development than is on the master plan . . . Such development would practically nullify the expenditure of $7.6 million for parkland to protect this unique area."

"I think Liz is correct," said Noman M. Cole Jr - former chairman of the State Water Control Board and resident of Hallowing Point River Estates, one of the few subdivisions that has been built on the Neck. "I don't think you can put a limit on such a pipeline. It could easily get out of hand."

The president of the Hallowing Point Community Association. Dr. Anthony L. Colasanto, said, "There would be development - no doubt about it - and as dense as they could get it. We're (the community association) going to be opposed to the pipeline."

One of the biggest landowners. Charles R. Hooff Jr. (405 acres), said that if he could get sewer for his property, "I would go for five-acre lots." That would amount to low-density development, and even Hartwell said that would not be harmful to the wildlife and ecology of the Neck.

Hooff's son, Charles R. Hooff III, said, "From what I can glean from the county people, they're not ever going to discuss "the possibility of getting sewer on the Neck.

The younger Hooff said, "I don't think there should be massive development. The land should be treated property and the delicately."

While large sections of the Neck are held publicly - a state park of 2,000 acres, two regional parks of 1,000 acres each and a national wildlife refuge of 1,000 acres - about 1,500 acres remain undeveloped.

The land has not remained undeveloped because it is not desirable. Many sites front on or are within view of the pretty, isolated coves and bays on the Potomac River. Other sites are part of the still-rural landscape that distinguishes the Neck from spreading suburbia as close as the town house clusters on nearby Rte. 1. All this choice land is a half hour away from the 14th Street Bridge.

Despite all these attractions, developers cannot build because there is no sewer. They cannot use septic disposal systems because of poor soil and the high water table.

Richard A. Scarbrough, who owns two lots on Belmont Bay, was told by the county in 1975 that he could not build on them because the soil was unsuitable for a septic system. Scarborough went ahead anyway and built a makeshift A-frame that his neighbors in the statelier homes of Belmont Park want the county to condemn.

Because building inspectors hardly ever get to the Neck - there is not much construction to inspect - Scarborough's house has gone unnoticed in the county bureaucracy. The county land books list both of his lots as "unimproved," or vacant.

The future of the controversial sewer line will probably be decided in September, when the Fairfax Board of Supervisors takes up the issue.

Until then, Hartwell and her other conservationists will be sounding the alarm as loudly as they can. They have had a lot of experience. As Hartwell said, "For the past 10 years, there's been a threat to the land about every six months."