How Long Do You Get to Sign Off a House Extension?

This question usually comes from one of two places: you've got a planning approval sitting in a drawer and life got in the way, or the extension is built and you've just realised nobody from the council ever came to look at it. The rules are different for each, so here's both.

There are two clocks, not one

A house extension is governed by two separate systems that people blur together:

1. Planning permission is about whether you're allowed to build it at all. The council decides.
2. Building regulations are about whether it's built safely and properly. Building control (the council or a private approved inspector) checks it.

Each has its own time limit, and signing off one does nothing for the other.

Clock one: planning permission

A standard planning permission has to be begun within three years of the date on the decision notice. That applies in England and Wales. "Begun" has a legal meaning: a material operation such as digging foundation trenches or laying a drain, done in accordance with the approved plans. Clearing the garden doesn't count.

Once the work has genuinely started, the permission doesn't lapse. That's why some people dig a trench to "keep it alive". It works, but be careful: if the work then sits unfinished for years the council can serve a completion notice setting a deadline, and partially-built work is a planning problem of its own.

If you've missed the three years, you reapply. Policies may have changed, so it isn't guaranteed to go through on the same drawings. If your project is permitted development rather than full permission, there's no three-year clock, but the rules in force on the day you build are the ones that apply.

Clock two: building regulations

A building regulations full plans application or building notice lapses if the work hasn't started within three years of it being deposited with building control. Lapsed means you apply again, under whatever version of the regulations is current, and insulation and ventilation standards have tightened repeatedly.

Once the work has started, there's no fixed statutory deadline to finish. What there is instead is a duty to notify building control at the key stages before they're covered up. Typically:

• excavation for foundations,
• foundations poured,
• damp-proof course,
• drains before backfilling,
• structural steel and floors,
• insulation,
• and completion.

At completion you ask for the completion certificate. That's the document that says the extension is signed off. It isn't issued automatically. If you never ask, you never get it.

What "signing off" actually means

Sign-off is the completion certificate from building control. It is not the same as your builder saying it's finished, and it is not the planning decision notice. Three reasons it matters:

• Selling. Buyers' solicitors ask for it as standard. An extension without one leads to delays, price chips, or indemnity insurance that doesn't actually fix anything.
• Insurance. Insurers can decline a claim on work that was never approved.
• Enforcement. In England, the window in which a council can take action over non-compliant building work was extended in October 2023 from one year to ten years after completion. The old "wait a year and it's fine" idea no longer holds.

If it was never signed off

You can apply for a regularisation certificate. Building control inspects the finished work, which may mean opening up parts of it, and if it meets the regulations that applied when it was built, they certify it retrospectively. It costs more than doing it at the time and there's no guarantee the work passes, but it's the honest route and the one that unblocks a sale.

The short version

Stage	Time limit
Start work after planning permission	Within 3 years of the decision
Start work after building regs application	Within 3 years of deposit
Finish work once started	No fixed deadline, but notify each stage and completion
Council action on non-compliant work (England)	Up to 10 years after completion, since October 2023

At Diogel, the building regulations drawings are part of our fixed design fee, and we tell you exactly when to call building control and what to ask for, so the certificate ends up in your hands rather than in a to-do list. If you're sitting on an unfinished or un-signed-off extension and aren't sure where you stand, a free call is a good place to find out.