Substantial alterations are a part of the Seattle Residential Code (SRC) that is applied when a large percentage of the house is being renovated or a large addition is being constructed; SRC Section R107.9. The requirements of substantial alterations are to update the house to the current code for major life safety items of fire, egress, and seismic. Substantial alterations may be required when an attached accessory dwelling unit (AADU) is created as an addition to an existing house. Traditionally, SDCI has given a break from substantial alterations when an AADU is being created in an existing house to support the housing need in Seattle, though this break has not been given when a new addition to a house is for an AADU.
Recently, SDCI has revised the policy to allow fire wall construction in compliance with Seattle Building Code 706 to separate a new addition from an existing dwelling unit per SRC 107.5. If the project meets the exceptions in SRC R107.5 the existing dwelling unit may not need to comply with upgrades required for substantial alterations when adding a new addition. The purpose of this policy change is to allow homeowners to expand their house without making upgrades to the existing house. The fire wall assists in this by maintaining independence for fire protection and egress between the spaces. We recommend reviewing the code interpretation 706.5 Fire Wall Horizontal Continuity to fully understand fire wall requirements and SRC R107.5 to understand the substantial alteration exception requirements.
For questions about this new policy, please contact Richard Pellinger, firstname.lastname@example.org.