Mosaic currently identifies the local authority based on the postcode, not the street number. This can cause issues when a postcode spans more than one local authority area. We've raised this as a bug and are investigating a solution.
Incorrect Local Authority area allocated to postcodes on boundary
In this article, we explore why addresses may be assigned to the wrong local authority, ward code, or postcode boundary area.
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Written by Yusef Abulaynain