For the first time in more than five years, Salt Lake County commissioners are talking about raising taxes.

The county, which administers a budget of more than $250 million, started building its 1991 budget Friday. After four years of holding the line on property taxes, county commissioners appear not to be able to hold the line any longer."I think there will probably be some minor tax increase," said Commission Chairman Mike Stewart, the only Republican who will be left on the commission in January. Voters last Tuesday chose Democrats to replace Bart Barker and Tom Shimizu.

Stewart said a small tax increase now would keep the county from having to impose a large increase later. The county projects its budgets five years in advance, and officials predict big problems will hit in 1992 unless adjustments are made now. Commissioners could cut expenditures, but some feel they have been doing so for too long.

Commissioners won't actually set tax rates until June. However, decisions made during the November budget hearings will have a direct impact on that decision. After examining requests from the county's departments and elected officials, Barker said the commission would have to cut about $24 million from the wish lists to keep from hiking taxes.

County officials say several factors point to a tax increase. One is a $12 million bond voters approved in 1989 to pay for construction of a new jail. Until now, commissioners have used county surpluses to keep from raising taxes to pay the bond. Commissioners don't think the county can keep doing that.

"That's one can I can almost guarantee a tax increase," Barker said. "We've been trying to forestall that."

Another problem is that the natural growth of the local economy appears to be slowing. The county gained only 1.26 percent this year in money from property appreciation and new construction. County budgeters normally count on 4 percent growth.