Quincy-based JACA Architects has been selected by Corona, California-based Signature Healthcare Services to design a new psychiatric hospital in Westborough.

The project includes the full rehabilitation of an existing 88,000-square-foot building formerly used as a private school and as the headquarters of a gas company, as well as a small new addition which will house the facility’s gymnasium.

The project had an initial estimate at $15 million, and construction is expected to begin on the building within the first quarter of 2016.

Braintree-based Campanelli is the construction manager for the project.

Bill Epp, associate principal at JACA, is overseeing the project in conjunction with Susie Festel, associate interior designer, who is leading planning and design, and Project Manager Paul Girello, who is involved with the documentation and construction of the project.

Editor’s note: An earlier version of this article’s headline mistakenly stated that the hospital is in Marlborough; it is in Westborough. We regret the error.