Fulton County taxpayers must spend tens of millions of dollars during the next two decades building space for more jail inmates, but thanks to speedier court processes and diversion programs, the outlook isn’t as bad as previously thought.

Consultants from HOK architecture-engineering firm, hired by the county to update jail population projections, said Wednesday that it will likely cost Fulton about $100 million to build space for an anticipated 3,565 inmates by 2026.

That’s better than the report the firm gave three years ago, which said the county would incarcerate 5,000 people by that year and building the additional space would cost more than $200 million.

Gary Retel, HOK’s vice president over justice issues, credited efforts to move offenders through the court system faster and move nonviolent defendants into alternative sentencing programs, such a drug and DUI courts.

“Throughout the nation, inmate population is down,” Retel told the Fulton County Commission. “It’s not just in Fulton County, but it is down more than in other counties, and that’s probably as a result of these programs.”

But he conceded in an interview afterward that the outlook could abruptly change, and said that’s why his firm is proposing that the commission bring it back annually for updates. Local jail populations are down 2.4 percent nationwide, according to a report by the National Institute of Corrections examining 2009-10 figures. Fulton’s population decreased 4 percent in that period.

The cause of the trend isn’t known, Retel said, so there’s a risk that it could reverse itself.

The Rice Street jail has been a drain on taxpayers since it opened 23 years ago as part of an agreement to settle a lawsuit. Another lawsuit filed in 2004 over crowded, filthy and dangerous conditions has the county under a federal consent decree that caps the inmate population at 2,500 and sets minimum staffing levels.

The order also required costly renovations including new cell-door locks, a new heating and air-conditioning system and plumbing repairs to stop sewage from leaking into living areas.

For the past year the jail has been comfortably below the mandated cap, and commission Chairman John Eaves has said he wants out from under the federal order by the end of the year. Senior U.S. District Judge Marvin Shoob, who is overseeing the county’s compliance with the order, has said he might recommend, but not require, that Fulton build a new jail first.

The county is leaning toward adding onto the jail in increments rather than replacing it altogether, according to Chief Jailer Mark Adger. Adding 250 to 700 beds in prefabricated buildings is estimated to cost between $15.6 million and $53.8 million, depending on whether Fulton wants to use precast concrete or less-costly steel.

Steven Bright, senior counsel for the Southern Center for Human Rights, which filed the 2004 lawsuit, said the county should keep investing in programs that keep the homeless, the mentally ill and non-violent drug addicts out of the jail.

“There will be a need for less space,” Bright said, “if the county officials will take the helm and limit the number of people going to the Fulton County jail to the people that really need to be there.”