Buying a House With an Extension: What Paperwork to Check

Some extensions need planning permission. Others are allowed under permitted development rights, without a full application.

Ask for the planning decision, or a Lawful Development Certificate if one was obtained. Understanding Planning Permission Documents explains what each document shows.

Almost every extension and loft conversion needs building regulations approval, whether or not it needed planning permission. The completion certificate is the key document.

If it is missing, see No Building Regulations Sign-Off for Work on Your Home? for the seller's options.

• Structural calculations for any steel beams or removed walls.
• Party wall agreements, if the work affected a shared wall or was close to a neighbour's foundations.
• A build-over agreement from the water company, if the extension was built over a public sewer.
• Electrical and gas certificates for any new circuits or heating in the extension.

Ask whether the builder gave a guarantee, and whether it can pass to a new owner. Some builders also offer insurance-backed guarantees that survive the company closing.

Understanding a Warranty for Home Improvement Work explains the difference.

Knowing which builder did the work, and whether they are still trading, helps if a problem appears later. On Propvio, a job confirmed by the builder shows exactly who was responsible.