Completion Certificates Checklist

Before your building control officer will sign off a UK extension, you need to collect a short stack of certificates from every trade who carried out notifiable work. Only once those are in hand can you request the building control final inspection and receive the completion certificate, the single document a future buyer's solicitor will ask for. This checklist tracks which certificates apply to your project and which are still outstanding.

The certificates almost every extension needs

Three documents apply to nearly every extension, whatever its scope:

• The completion certificate itself, issued by building control after the final inspection passes. You do not chase a trade for this one, the BCO produces it.
• The Electrical Installation Certificate under Part P, from your registered electrician, confirming the wiring meets BS 7671.
• The as-built SAP energy assessment, from an accredited SAP assessor, confirming Part L compliance for the new thermal envelope.

The certificates that depend on your scope

The rest are conditional. Tick the boxes above that match what you actually built, and the checklist adds the relevant items:

• A Gas Safe Building Regulations Compliance Certificate if any gas work was done, such as a new boiler, gas hob, or gas fire.
• A FENSA or CERTASS certificate if new windows or external doors were installed.
• A drainage test certificate or CCTV survey if new below-ground drainage was laid.
• A Boiler Benchmark commissioning checklist if a boiler was fitted or replaced, which most manufacturers also require to keep the warranty valid.
• A party wall final award or acknowledgement if a party wall agreement was in place.

Miss a mandatory one and the BCO will not book the final inspection. Miss a conditional one that genuinely applies and the certificate can stall for weeks after the inspection is otherwise ready.

How the timing works

Once the final inspection passes, Regulation 17 gives the council up to 8 weeks to issue the completion certificate. The trade certificates that feed into it run on their own clocks: a Gas Safe engineer has 25 working days to notify, and a scheme electrician self-certifies within 30 days. Those windows are why the paperwork often outlasts the building work, and why chasing early matters.

A worked example

Take a single-storey rear extension with a new consumer unit, a replaced gas hob, new bi-fold doors, and a new soakaway for surface water. Five certificates apply. The electrician issues the Electrical Installation Certificate on the day of final testing. The gas engineer registers the hob and the Gas Safe compliance certificate follows by post. The door installer hands over the FENSA certificate at handover, and the groundworker's drainage test record joins the pack once the drains-before-cover stage is witnessed. The SAP assessor produces the as-built assessment after works finish.

With those five in hand, you notify building control that the work is complete, book the final inspection, and the BCO issues the completion certificate. The trades that leave site first, the electrician and the gas engineer, are the ones to pin down in writing before they go. On one documented Oxfordshire extension the gas certificate alone took many months to prise loose, long after the family had moved back in, which is exactly the delay this checklist is meant to prevent.