Do I Need Planning Permission for an Extension?

(Short answer: maybe. Longer answer: welcome to planning.)

It’s one of the most common questions we get as architects, usually asked with a hopeful tone that suggests the answer ought to be “no”.

So let’s clear it up—with facts, not fairy tales.

The reassuring bit: Permitted Development (sometimes)

In England, many house extensions fall under Permitted Development (PD). This means you may not need planning permission, provided you stay within some very specific rules (and yes, there are quite a few).

In broad terms, you might be fine if your extension:

• Is at the rear of the house

• Isn’t too big, too tall, or too close to the boundary

• Doesn’t involve balconies, raised platforms, or anything that screams “architect got carried away”

• Doesn’t cover more than 50% of the original garden

• Uses materials that look broadly like the existing house

  
  

For example (deep breath):

• A single-storey rear extension can project up to 3m (terraced/semi) or 4m (detached)

• Under the larger home extension rules, that might stretch to 6m or 8m, but only after neighbour consultation

• Maximum height limits still apply (and are enforced with enthusiasm)

If you’re already glazing over, that’s normal.

The less reassuring bit: When you do need planning permission

You will almost certainly need planning permission if:

• The house is listed

• It’s in a Conservation Area, AONB, or similar alphabet-soup designation

• The extension is two storeys

• You’re extending at the front or side in a noticeable way

• The property has had PD rights removed (this is more common than people think)

Also worth noting: just because your neighbour “did something similar in 2003” does not mean you can. Planning has a long memory, but selective amnesia.

Building Regulations: the thing people forget

Even if planning permission isn’t required, Building Regulations approval still is.This covers structure, insulation, drainage, fire safety, ventilation, and generally making sure the extension doesn’t fall down or fill with mould.

Planning decides whether you can build it.Building Regs decide whether it works as a building. You usually need both. Planning just gets all the attention.

The sensible safety net: Lawful Development Certificate

If your extension is Permitted Development, you can apply for a Lawful Development Certificate. It’s optional—but highly recommended.

Think of it as:

• Planning insurance

• Something your solicitor will love when you sell

• Proof that you didn’t just “crack on and hope for the best”

  
  

So… do you need planning permission?

The honest answer (and the one professionals give):“It depends.”

It depends on:

• The house

• The site

• The extension

• The planning history

• And occasionally, the moon and tides (or so it feels)

If you’d like certainty before spending money—or before explaining things to an increasingly sceptical neighbour—getting proper advice early can save time, stress, and retrospective applications (which nobody enjoys).