Wash. Admin. Code § 51-51-4505
R4505.1 Additions to an existing building. Additions shall comply with this section and other applicable provisions of this code for new construction.
R4505.2 Structure for horizontal additions. Where an addition involves new construction next to and attached to an existing building and includes alterations to the existing building, the addition shall meet all of the requirements of this code for new construction. Alterations to the existing building shall comply with the requirements governing alterations within this code. In wood light-frame additions, connection of the structural components shall be permitted to be provided using wall top plates and addition studs that abut the existing building. Wall top plates shall be lapped and spliced in accordance with Section R602.3.2. Abutting studs shall be fastened in accordance with Table R602.3(1).
EXCEPTION: | The structural components of the addition shall be permitted to be connected to the existing building in accordance with accepted engineering practice. |
R4505.3 Structure for vertical additions. Where an addition involves new construction that adds a story to any part of the existing building or vertically increases the height of any part of the existing building, the new construction and the existing building together shall meet all of the requirements of this code for new construction.
Exception: | Where the new structure and the existing structure together are evaluated in accordance with accepted engineering practice and are shown to be sufficient to support the combined loads from the new structure and existing structure, no structural alterations are required. |
Wash. Admin. Code § 51-51-4505