Daytime facilities at the city's biggest homeless shelter were shut down yesterday for the start of an extensive renovation project designed to refurbish the entire three-story building before next winter.

But there was uncertainty over precise financial arrangements for the remodeling, and some advocates for the homeless feared that the project might become stalled.

The shelter, located at 425 Second St. NW in an old Federal City College building, is operated by the Community for Creative Non-Violence. The portion of the facility closed yesterday is the building's 47,000-square-foot basement, used as a daytime shelter. Upper floors, used as nighttime shelter for some 800 persons daily, remain open for the time being.

The federal government several months ago agreed to fund an estimated $5 million renovation. But the activists who operate the decaying, roach-infested shelter -- and are involved in the renovation -- say the work could cost far more, and say they are concerned that no specific financial arrangements have been made. Unless funding becomes available soon, they say, the shelter's remodeling may not be finished by next winter.

"Unfortunately, no decision has been made by the federal government at this point in terms of how the whole job will be handled," said Constance Bloomfield, a New York City-based architect involved in the renovation plans. "We are sort of worried how it's going to be done . . . . We're at a standstill."

"We waited . . . because we didn't want to see this happen during the winter because somebody could die" if the shelter were closed for refurbishing during cold weather months, said CCNV's spokesman Mitch Snyder. "But each day that goes by is getting more and more precious."

Last November, Snyder ended a 51-day hunger strike after the Reagan administration promised to make the shelter a national model. At the time, preliminary estimates placed the renovation at about $5 million, a figure architects for the shelter now say is "substantially" low.

Harvey Vieth, chairman of the Federal Task Force on Food and Shelter, said yesterday that the federal government remains committed to the shelter's repair. In the next two weeks, Vieth said, he and Snyder need to discuss the project's cost.

Meanwhile, Snyder said, he plans to continue emptying the shelter to make way for the planned renovation. On June 1, sleeping space for 300 to 400 men is scheduled to be vacated so repairs can begin. Snyder said he will not permit the homeless to return to the shelter until the work is finished.

"We are doing all we can," Vieth said. "There are certain things involved in rehabbing a structure. Whatever time frame, we are just as dedicated to getting started and getting done as they are."

However, Vieth said he cannot promise when the work will begin and whether it will be finished in time for winter.

"I don't want to mislead anyone," he said, adding that he has only "rough" plans regarding the renovation.

Working with funds provided by the National Endowment for the Arts, Conrad Levenson and Associates in New York City drew up the renovation plans with the assitance from City College School of Architecture students.

Architect Bloomfield said the shelter will be redesigned into five dormitories under one roof: four for men and one for women. Plans call for greatly enhanced food services, privacy and security.

Snyder said the shelter should also include job, disability and social security counseling, an infirmary and a medical laboratory.

"I'd like to see it finished," said Lamont, a 22-year-old unemployed man who slowly carried himself and his white plastic bag full of his possessions into the new day's heat. "I guess it won't ever get fixed up if all these people are laying up in it."