A local company is looking to build a sizable addition onto its manufacturing facility and double its current staff of just over 100 employees. Century Box, run by the Kakan family of Methuen, has been a city mainstay since 1978. This coming fall, its owners hope to begin construction of a 55,000-square-foot addition to the current plant on Chase Street; a zoning variance for the project has already been approved, and the Conservation Commission is set to review it as well. Methuen Community Development director William Buckley says the permitting process should move along quickly. If approved, construction will begin in October, with the project expected to take between a year and 18 months.