Can I sell a house without planning permission for an extension?

There is no legal bar to putting a house with an unauthorised extension on the market, but in practice the conveyancing process almost always brings the issue to a head. A buyer's solicitor conducting due diligence will ask the seller to demonstrate that any extension either fell within exempted development under the Planning and Development Regulations 2001, or was built with a valid planning permission and, separately, complies with the Building Regulations, typically evidenced through a Certificate of Exemption, an Opinion on Compliance, or the original grant documentation.

If the extension cannot be shown to be authorised, most lenders will refuse to fund the buyer's mortgage on the property until the position is regularised, since an unauthorised structure represents a legal and financial risk they are unwilling to underwrite. This often forces the seller to apply for retention planning permission before the sale can close, or to negotiate a reduced price or a retention condition into the contract, with funds held in escrow pending the outcome of a retention application.

Cash buyers who do not need mortgage finance have more flexibility to proceed regardless, but even then most solicitors will still flag the risk to their client, since an unauthorised structure could affect future resale, could be subject to enforcement action by the council, and typically cannot be insured to the same standard as an authorised one.