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.
When alterations or additions to single-family residences and duplexes are extensive enough, the Seattle Residential Code (SRC) requires updates to important life safety and structural systems. These upgrades include bedroom egress windows, exiting requirements, smoke alarms, carbon monoxide alarms, separation between dwelling units, and seismic strengthening of the building.