Alterations/additions to a Manufactured Home may or may not require permits from Department of Labor and Industries. You may contact them for requirements:
Department of Labor and Industries
312 SE Stonemill Drive
Vancouver, WA 98684-3508
Phone: 360-896-2372 or 800-647-0982
If you are attaching anything to the outside of the manufactured home, it would require permitting from both Klickitat County Building Department and L&I.