Whether an extension needs a formally assigned certifier depends on its scale and whether it falls within the scope of the Building Control (Amendment) Regulations 2014; many straightforward domestic extensions may not require one, but the extension must still comply fully with the Building Regulations. Homeowners undertaking a one-off house extension may also be able to consider an opt-out declaration in limited circumstances, and this should be checked with a competent professional.
The statutory requirement to appoint an assigned certifier under the Building Control (Amendment) Regulations 2014 was primarily designed around new dwellings and other works falling within its defined scope, and its application to smaller domestic extensions is not automatic in every case. Many extensions to existing homes are carried out without a formally assigned certifier being appointed under this specific regime, but this does not remove the underlying obligation that all building work, including extensions, must comply with the Building Regulations, and professional design and inspection input is still strongly advisable.
Where an extension does fall within the scope requiring an assigned certifier, for example due to its scale or nature, the homeowner must appoint one before lodging the commencement notice, and that person takes on responsibility for design review, inspections and the eventual completion certificate. Where it does not strictly fall within scope, it remains good practice to engage a competent professional, such as an architect or engineer, to design the works, oversee construction and provide appropriate certification, since this protects the homeowner and supports any future sale of the property.
There is also a specific opt-out declaration mechanism available in some circumstances for one-off houses (including certain extensions to them) where the owner is not building for sale, allowing the owner to take on more direct responsibility rather than appointing an assigned certifier, though this comes with its own implications and is not always advisable or straightforward. This option, and whether the specific extension falls within scope of the assigned certifier requirement at all, should be confirmed with a competent professional or the local authority Building Control Section before proceeding.