Plumbers certified under chapter 18.106 RCW and electricians certified under chapter 19.28 RCW are required for units constructed in Washington. For the purposes of construction at the manufacturing facility, the manufacturer is not required to be a licensed electrical contractor under chapter 19.28 RCW or a registered contractor as required by chapter 18.27 RCW. Manufacturers may hire registered plumbing contractors or licensed electrical contractors to meet this requirement.

Work performed outside the manufacturer's facility must be performed by a registered contractor under chapter 18.27 RCW, electrical contractor and electricians under chapter 19.28 RCW, and certified plumbers under chapter 18.106 RCW.