The short answer
A building control completion certificate is the document a council issues once it is satisfied that building work meets the building regulations. For an extension, it is the proof a buyer's solicitor and a mortgage lender will ask for when you sell. Do not make the final payment to a builder until you have it.
On this page
It is one sheet of paper, often a PDF, and it is the most valuable document your extension will produce. Without it, the room exists but nobody official has said it is safe.
What the certificate says
The certificate states that, as far as the building control body could tell from its inspections, the work complies with the building regulations. It names the address, describes the work and gives a date. It covers structure, fire safety, insulation, ventilation, drainage, electrics and the rest of the regulations that applied to the job.
It is not a guarantee of quality. A wonky skirting board or a door that sticks is between you and your builder. Building control checks that the work is safe and lawful, not that it is neat.
Who issues it
In England and Wales there are two kinds of building control body, and each issues its own document.
- The council’s building control team issues a completion certificate. The regulations require the council to give one where, after you tell it the work is finished, it is satisfied the work complies.
- A registered building control approver, which is a private firm, issues a final certificate and lodges a copy with the council.
They carry the same weight with solicitors and lenders. Your builder or designer chooses which body to use at the start, and you are entitled to ask which and why.
Scotland works differently. You need a building warrant before work begins. At the end, you submit a completion certificate yourself, and the council, acting as verifier, accepts it once it has made reasonable checks. Using the room before that acceptance is an offence.
When you get it
The certificate comes after the final inspection, and the final inspection only happens when someone asks for it. Jobs are finished, builders move on, and nobody books the visit. Years later the owner finds there is no certificate.
Put it in the contract. The last stage payment should fall due when the certificate is issued, not when the builder says the job is done. The inspector will also want paperwork before signing off:
- an electrical installation certificate
- a gas safety certificate, if gas was touched
- the structural engineer’s calculations
- commissioning records for heating and ventilation
- for glazing, a certificate from a competent person scheme or evidence that it meets the regulations
Collect these as the work proceeds. Chasing a subcontractor for a certificate six months later is slow.
Why it matters when you sell
A buyer’s solicitor will ask for planning documents and building regulations certificates for any work done to the house. An extension with no completion certificate raises three worries: that the work is unsafe, that the council could take action, and that the lender’s security is weaker than it looks.
Councils can require work that breaches the regulations to be altered or removed, and since the Building Safety Act 2022 changed the law they have had ten years from completion to serve that notice, not one. So a missing certificate is not a formality that expires after a year or two.
Sales do go through without certificates, usually with an indemnity insurance policy paid for by the seller. But the policy only covers the cost of council enforcement. It does not make a dangerous beam safe, and some buyers walk away.
If you have lost it
A lost certificate is easy to fix. The council keeps a record and will send a copy, sometimes for a small fee. If a private approver did the job, the council should still hold the copy that was lodged. Many councils let you search building control records by address online.
If there never was one
That is harder. For work in England and Wales that was never signed off, you can apply to the council for a regularisation certificate. An inspector visits and may ask for parts of the work to be opened up, such as lifting a floorboard to see joists or cutting a hole in plaster to check a beam. If the work passes, or once it has been put right, the council issues the certificate. There is a fee, and only the council can do this, not a private approver.
Think before applying shortly before a sale. Once the council knows about the work, indemnity insurance is usually no longer available. Take your solicitor’s advice on the order.
How it works for a factory-built room
A room made in a factory raises a practical question. By the time it reaches your house, the insulation, the wiring and the structure are closed up behind plasterboard. The inspector cannot see them.
Makers of modular house extensions deal with this in one of three ways. Building control visits the factory during production. Or the maker supplies a file of photographs, test records and calculations for each module. Or the system holds third-party certification that the inspector accepts as evidence. Which applies should be agreed with building control before production starts, and written down.
The work at your house is inspected in the normal way: the foundations before concrete is poured, the drains before they are covered, the opening and its beam, and the finished room. Ask the firm who is booking each visit.
One more point for kitchens. The factory certifies the electrics it installed, and whoever connects the module to your consumer unit certifies that connection. You need both. Our page on modular kitchen extensions has more on what is tested where.
A short checklist
- Ask at the start which building control body is being used.
- Agree how factory work will be evidenced.
- Tie the final payment to the certificate.
- Keep every test certificate as it is produced.
- File the completion certificate with your deeds and keep a scan.
To see what the fees add to a project, use the cost calculator, or get a quote and ask the firm to show you a completion certificate from a past job.