Planning permission vs Building Regulations: what’s the difference?
Reviewed October 2026 · Not legal, planning or financial advice
Planning permission and the Building Regulations are separate systems. An extension can be exempt from planning and still fail the Building Regulations — and a defect there is the owner’s problem when the house is sold.
Planning: whether, where and how big
Planning concerns the use of land and the effect of development on neighbours and the area: size, height, position, appearance, overlooking. Either the extension is exempted development or it needs permission.
Building Regulations: how it is built
The Building Regulations set minimum standards for structure, fire safety, ventilation, drainage, energy performance (Part L), accessibility and more. They apply to every extension, exempt or not. Technical Guidance Documents explain how to comply.
BCAR and extensions over 40m²
Under the Building Control (Amendment) Regulations, an extension to a house with a floor area over 40m² is generally subject to the full statutory process: a commencement notice with compliance documentation, a design certifier, an assigned certifier who inspects during construction, and a certificate of compliance on completion. This adds professional cost — often several thousand euro — and is worth factoring into whether to stay at or under 40m².
Why it matters when you sell
A buyer’s solicitor will ask for evidence that the extension is either exempt or has permission, and that it complies with the Building Regulations. An architect’s or engineer’s opinion on compliance is the usual evidence. Missing paperwork can delay or derail a sale.
Regulations and their interpretation change. This guide explains the position as we understand it in October 2026. Confirm what applies to your house with an architect, planning consultant or your local authority before relying on it.