Salt Lake County commissioners are preparing to call a bond election as early as May to seek voter approval of a property tax increase needed to build and operate a proposed 350-bed minimum security jail.

No one is yet ready to say how big a tax hike the commission will request because the county isn't sure how much construction of the facility for low-risk inmates will cost.Construction costs have been estimated at $8 million to $10 million, but commissioners now say the facility - described as a barracks with a chain-link fence - may cost less. Annual operations costs are estimated at $2.7 million.

One reason final construction estimates aren't set is that county officials and South Salt Lake planners are still negotiating the conditions of a conditional use permit the county must get before the jail can be built. Those conditions could affect cost estimates.

The county's application for the permit is scheduled to be heard by the South Salt Lake Planning Commission during its March 2 meeting.

Commissioners want to build the jail on about 15 acres of a 40-acre parcel of county property at approximately 12th West and 33rd South. The proposed jail site is bounded on three sides by the Jordan River. An 18-home subdivision lies just to the east.

South Salt Lake may require the county to buy out those homeowners as a condition of its approval for the project. County officials are not opposed to a buy out because the additional 10 acres would allow more direct vehicle access to the property and provide space for future jail expansion.

Expansion plans are already in place that would increase the facility's capacity to 500 inmates as more jail beds become necessary. County officials had expected to buy some of the existing homes, and now they feel an up-front purchase of the subdivision may be best.

But subdivision residents are expected to split into two camps should any buy out be proposed. Some residents have indicated a willingness to sell, county officials say. Others oppose the jail project and are expected to ask the city to reject the county's application.

Commissioners are hoping to call an election in May, see an early summer beginning of construction and open the jail by November 1990.

Preliminary plans call for appointment of an executive committee that will conduct a public information campaign to educate voters on the need for the jail during the six weeks preceding the election.

The county wants a jail bond election for two reasons: a voter-approved tax increase is politically more acceptable than a legislatively-approved hike, and a bond issue OK'd by voters is the cheapest way for for the county to borrow money.

Officials say the new jail is needed to ease overcrowding pressures on the 22-year-old downtown jail. The metro jail has an official capacity of 550 inmates. However, in recent weeks jail population has reached as many as 650 prisoners a day.