In many cases, yes: an extension that involves new structural work usually requires a commencement notice or, for smaller works meeting certain criteria, a shorter seven-day notice, lodged through the Building Control Management System before construction starts. Whether a notice is required, and which type, depends on the nature, size and value of the works, so this should be confirmed with your local authority Building Control Section.
A commencement notice is a statutory notice lodged with the local authority under the Building Control Regulations, informing them that building work covered by the Building Regulations is about to start. For house extensions, a commencement notice or the shorter seven-day notice procedure (available for certain smaller-scale works meeting defined criteria) is generally required before construction begins, allowing the local authority to record the works and, where it chooses, carry out inspections during the build.
Whether a full commencement notice, a seven-day notice, or in limited cases no notice at all is appropriate depends on factors such as the floor area of the works, their estimated value, and whether they are new dwellings or extensions to existing buildings, since the regulations set out specific thresholds and categories. Because these criteria are technical and updated from time to time, they should be checked against the current Building Control Regulations or confirmed with a competent professional or the local authority before assuming a particular notice type applies.
Notices are lodged electronically through the Building Control Management System (BCMS), typically by the homeowner's appointed competent professional, along with required documentation such as drawings, and for many works, details of the assigned certifier and builder. Carrying out notifiable works without lodging the appropriate notice can expose an owner to enforcement action and complications when selling or mortgaging the property.