Bouten Construction Co. has been selected to add 22 classrooms, a gym and offices in a detached addition at Kennewick's Amistad Elementary School.

Bouten, which has offices in Richland and Spokane, provided the lowest base bid of $12.5 million to the Kennewick School District project.

The Kennewick School Board approved two alternate projects, bringing the total construction cost to $12.8 million. Construction is expected to begin in May and be completed by August 2019.

A $51.1 million state grant aimed at reducing class sizes in kindergarten through third grade will pay for the detached classroom addition. The second phase of construction, which would replace the current Amistad Elementary building, built in 1992, is dependent on voter approval of a 2019 bond.

Bouten’s most recent projects with the district include new buildings for Sunset View, Cascade and Lincoln elementary schools.