Two defendants have each pleaded no contest to 36 counts of involuntary manslaughter in a fire at an illegally converted California warehouse that occurred during an unauthorised concert.

Under the terms of the plea agreement on Tuesday, Derick Almena could be sentenced to nine years in prison and Max Harris could receive a six-year sentence. A judge will sentence them at a later date.

Authorities say the 48-year-old Almena rented the warehouse and illegally converted it into an entertainment venue and residences before the December 2016 blaze.

The 28-year-old Harris helped Almena collect rent and schedule concerts

Prosecutors say the men turned the cluttered building into a "death trap" with few exits, rickety stairs and dark and dangerous passageways.

Both men are only expected to serve half their sentences. They have been in jail for a year.

Parents of those killed in the warehouse fire say they're disappointed by the prison sentences expected for the pair.

David Gregory's 20-year-old daughter, Michela Gregory, was among the victims.

He told the San Francisco Chronicle that he was shocked and disappointed by the short sentences. He says he would have preferred a trial.

Sami Kopelman, mother of 34-year-old victim Edmond Lapine, says she also thinks the sentences are too lenient.

Kopelman told the East Bay Times that she would have liked to see each defendant get 36 years in prison.