Ask your solicitor and a suitably qualified building professional to check whether the extension has the required planning permission, building-control compliance and completion or exemption documentation. Compare the extension with the approved plans and inspect its structure, insulation, drainage, roof and signs of damp or movement. Your mortgage lender, valuer and insurer may also need details of the work, and missing paperwork could affect lending, resale or future alterations. If the extension appears unauthorised, ask about the available options and likely costs before committing, and confirm the position with your local authority or Citizens Information.