Developer Leggat McCall hopes to convert the former Sullivan Courthouse to 430,000 square feet of office space and 48 affordable apartments. Photo by Steve Adams | Banker & Tradesman staff.

The development firm trying to convert the former Sullivan Courthouse in East Cambridge to a mixed office-residential development is asking for another year to get the project underway.

Plans call for Leggat McCall Properties to perform significant abatement and demolition work to turn the 22-story shell into 430,000 square feet of office space, plus a residential podium with 48 affordable housing units.

Leggat McCall won the right to redevelop the state-owned tower in 2012 and signed a $33 million purchase-and-sale agreement with the state Division of Capital Asset Management and Maintenance in 2013. The firm estimates asbestos abatement and other construction costs will run as high as $50 million.

In a letter sent to the Cambridge Planning Board, Leggat McCall formally requested a one-year extension to the special permit allowing the conversion. The project has received one extension already, due to expire on Sept. 28, to let it negotiate a critical parking spot lease with the city.

“At this time, the proponent is actively involved in ongoing design review with the Community Development Department Staff as well as meeting with representatives of other city departments as it finalizes its building permit application,” the company’s lawyer James Rafferty wrote. “It is anticipated that the proponent will be in a position to apply for a building permit in the coming months.”

The tower would add significant new office space next to Kendall Square, perhaps the region’s most desirable neighborhood for biotech companies.