Permit records from the City of Seattle permitting system for building permits that create or demolish attached accessory dwelling units (AADU). Records begin in 1990 and are through the current year quarter.The permits in this layer are those that have either been completed or are currently issued. This data does not contain records for those permits that were issued but were not completed so are therefore not comparable to statistics that report permit issuance.Each record represents the number of units added or demolished for each housing type in the project. Therefore a single building permit may appear multiple times if there are a mix of unit types in the project.Housing unit types reflect the unit types regulated by the building codes and change through time. There has been no attempt to standardize these types and therefore reflect the unit types that existed at the time the permit was issued. Unit types will be visible only for the time period they were in the code. For example, small efficiency dwelling units were not created in the code until 2016 and so will not appear as a type until that year.There may be multiple permits at any given address.