SACO – The Saco planning board gave the go-ahead Tuesday for a $100 million project to redevelop a 15-acre complex of former mills on the city’s Saco Island.

The decision clears the way for Saco Island LLC to buy the land and transform the island over the next four years into a mix of condominiums, retail stores, offices and a marina with 62 boat slips.

Saco Island bridges the downtowns of Saco and Biddeford, and it used to be the economic heart of the region.

Work should begin in August, said Saco Island LLC partner Kevin Mattson. The first thing to be built will be a brew pub restaurant, which is scheduled to open in December in one of the old mill buildings.

“We close on Aug. 27,” Mattson said. “Then on Aug. 28, it’s work time.”

The board’s approval culminated a three-year effort to get the OK to redevelop the site. The project is similar in scale to the Maine State Pier development project in Portland, but the effects are expected to be greater because it’s being done in a smaller community.

After Tuesday’s vote, city officials in Saco and Biddeford said they expect the project to boost the local labor market during construction and bring lasting benefits to the downtown districts in both cities.

“It’s huge,” said Saco development director Peter Morelli.

The Saco City Council last fall approved a $35 million tax break for the project in hopes of infusing new life into the former mill complex on the island.

Mattson said workers will stabilize throughout the winter old mill buildings that in some cases have been left exposed to the elements because of missing windows and leaking roofs. The first condominiums should be up for sale by next fall, he said.

As part of the deal, the city agreed to sell a piece of land to the developer for use as a parking lot, with the city using the proceeds to build a new train station for the passenger train that travels between Portland and Boston.

With Tuesday’s vote, Saco Mayor Mark Johnston said the city will try to complete the train station by next spring.