Nassau County is soliciting bids to renovate the new Family and Matrimonial Court building in Mineola — the second phase of a long-delayed, $140 million project scheduled for completion in 2020.

Nassau has issued a request for proposals for an architectural and engineering firm to outfit the interior of the old Department of Social Services building. The vendor will install electrical, plumbing, heating and air-conditioning equipment, the RFP states.

The county says the work will cost an estimated $75 million.

The new building will free up space at Nassau’s cramped Family Court in Westbury, two miles from the county’s court complex in Mineola.

All the Family Court rooms will be relocated to the new Mineola building, although the county’s Juvenile Detention Center will remain in Westbury.

“It’s very tight trying to make things work in that building,” county Public Works Commissioner Shila Shah-Gavnoudias said of the Westbury court.

The RFP comes as work continues on the first phase of construction to overhaul the exterior of the 255,000-square-foot building, located across from State Supreme Court. County lawmakers allocated $72 million for Phase 1, but Shah-Gavnoudias said the work will cost about $65 million.

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The county is borrowing to finance both phases of the project.

The county expects to select a vendor by March. Construction would begin in mid-2018 and be complete by mid-2020.

Lance Clarke, a past president of the Nassau County Bar Association, said the existing Family Court is so small that attorneys frequently hold conferences with clients in stairwells and while leaning on windowsills. “The building is obsolete and antiquated,” Clarke said.

The new Family Court would have 23 courtrooms, including 13 Matrimonial Center courtrooms, a Domestic Violence courtroom, judges’ chambers and holding pens for adult and juvenile detainees.

County Executive Edward Mangano announced plans in 2013 to move the court to the vacant Social Services building, arguing that all the courts should be “be in one area.”

The Family Court project has faced delays due to asbestos abatement and redesigns, said Shah-Gavnoudias.