Space is so tight at the Patrick Henry Branch Library in Vienna that weekend patrons often must sit on the floor.

But if expanding the library means shutting the library doors for 18 months and filing children's books with adult books, as library officials plan, then many Vienna residents say they are against the expansion.

"The Vienna Town Council is extremely upset about this," council member Rodger Seeman told county library officials last week at a question-and-answer session that drew about 100 angry area residents. "If the renovation means closing the library, then we're against the renovation."

His speech drew hearty applause.

The Marshall Road Elementary School Parent Teacher Association sponsored the meeting primarily to find out why the county plans to relocate permanently about 6,000 nonfiction books from the childrens' collection to the adult stacks as part of the $2.8 million renovation. But as the discussion wore on, it was clear that Patrick Henry patrons were also upset about losing the use of the library during construction, which is expected to begin in 1992 and take more than a year.

The Patrick Henry renovation is one of about 11 library improvement projects that the county plans to undertake in the next few years with $39.1 million in bond money that county voters approved by referendum last year. According to county officials, the projects, including the construction of four new libraries, are necessary to meet the needs of the rapidly developing county.

At Patrick Henry, a community library that circulates as many books as some of the county's larger regional libraries, library officials want to add about 6,100 square feet to the existing 11,000-square-foot building; do away with most interior walls to allow greater flexibility for rearranging library resources; and connect the information desks for children and adults, which are currently separated.

Children will still have an area to themselves, but non-fiction books for children beyond third grade would be stacked on shelves with adult books, partly to make the library "as self-service as possible," said Barbara Webb, associate director of library operations.

"We know how our users use our libraries. We know 70 percent or more of the people do not ask for help," Webb said. "The benefit of interfiling is you have all the books on one subject in one place." Many older children use the adult book section anyway, she said, and adults learning English might use the children's nonfiction books.

Sixteen of the county's 22 libraries have integrated collections.

Nevertheless, many parents expressed skepticism that interfiling would work for Patrick Henry patrons.

"You've developed ideas about libraries and you impose it on the communities. Why don't you go into the communities and ask them what they want?" said Martha Pruett, a Town Council member.

Patrick Henry patrons also objected that the library would be shut down for the duration of the expansion project. The changes would affect about 90 percent of the building. Renting temporary quarters would be too costly during this tight fiscal year, according to county officials, but patrons could use Tysons-Pimmit or other branch libraries.

The residents disliked the idea of going elsewhere. "There are some people who will never learn to use the library because of that 18 months. There are others who will get turned off if they can't walk there," said Pruett.

Library officials were a little bewildered about how to handle the clamor. But after a week, the Fairfax Public Library Board of Trustees decided they will appoint a committee of Vienna residents to vocalize the community's concerns to the library board.

""There's certainly lots of room and lots of time to incorporate any changes if changes need to be made," said Edwin S. Clay III, director of libraries. He will address the Vienna Town Council at its regular meeting on Monday.