St. Mary's County officials are weighing zoning code changes to head off what some residents fear will be a proliferation of the large, barge-type houseboats now moored permanently in at least one commercial marina here.

Charles Kimball, head of Chesapeake Marine Development and the owner of Cedar Cove Marina, objects to the code revision, saying the houseboats he builds are non-polluting, seaworthy vessels that are in fact a partial answer to the county's chronic housing shortage.

If the zoning change is approved as expected by county commissioners later this month, it is likely that the 12 floating homes already tied up at Kimball's 45-slip marina will be allowed to stay.

"It is anticipated that these boats will be grandfathered in. It's pretty hard to outlaw something retroactively," said County Administrator Edward V. Cox. But at least two other marina owners who had expressed interest in renting or selling floating homes similar to those built by Kimball will be stopped, Cox said.

Kimball's wife, July, said she and her husband had hoped to put about 20 of the one- and two-story boat houses at their marina, but said they could live with the law and would not attempt to challenge it.

July Kimball said all 12 of the boats were sold as investment properties and all are currently rented to hunters, fishermen, vacationing families or defense contractors and engineers at the nearby Patuxent Naval Air Test Center.

The boats -- made locally for Mariner Houseboats of Mays, N.J. -- sell for between $50,000 and $80,000 depending on accessories and how large their fiberglass hulls are. The average price is $63,700 -- far lower than a waterfront lot and house would be, Kimball says -- and the boats are either 14 by 32 or 14 by 42 feet.

They can be towed elsewhere, and Chuck Holmes of Anchors Aweigh in Annapolis says that's exactly what some people do: "Follow the sun and take it to Florida in the winter."

"Live aboards" -- as year-round residents of houseboats, cruisers and sailboats are called -- are banned in neighboring Anne Arundel and Calvert counties. In February 1984, Annapolis prohibited houseboats exceeding 20 feet in width, 46 feet in length and 14 feet in height from tying up at the city docks for more than 48 hours. At commercial marinas, no more than 5 percent of the slips can be occupied by floating homes, said city attorney Frederick C. Sussman.

At the Gangplank Marina in Washington at least 80 people -- paying an annual slip fee of $1,800 -- make their home aboard yachts, sailboats and houseboats moored off Maine Avenue, said spokeswoman Pat Byrne.

The question of floating homes invariably boils down to a matter of definition, said Holmes. "If it has shingles, is it a house or a boat?" he asked. Many of the boats being constructed primarily as second homes or investment properties have no pretense of mobility, he says.

"They are too wide to be transported on highways. Many are conventional two-story, two- and three-bedroom homes on a barge. Although they can be moved, they are not really self-propelled," Holmes said.

Besides questions of environmental quality and potential pollution to the already fragile Chesapeake Bay and its tributaries, the Taxpayers League in St. Mary's County also raised questions of tax equity. President Marylynn Whestine argued that floating-home owners should assume some tax burden, especially when assessments on waterfront properties "have been soaring in recent years."

Should the ordinance become law, floating homes -- "whether self-propelled or not -- would be banned," Cox said. Nothing moored at a marina that was "used, designated or occupied as a permanent dwelling unit, place of business or for any private or social club" would be permitted, he said.

This new definition includes structures built on a barge "that are primarily immobile and out of navigation or any structure which functions substantially as a land structure while the same is moored or docked within Maryland," Cox said, quoting from the proposed zoning amendment.

The St. Mary's ordinance also would prohibit floating homes with a habitable space greater than 3,000 square feet as measured by the draft of the boat and its depth.