How Big Can a House Extension Be Without Planning Permission?

If you're planning to extend your home, one of the first questions you'll ask is: how big can a house extension be without planning permission?

In many cases across England, you can build a significant extension under Permitted Development (PD) rights - but there are strict rules around size, height, location, and design. Getting this wrong can lead to costly delays, enforcement action, or even having to remove the extension entirely.

This guide breaks down everything you need to know, from exact size limits to common mistakes, so you can confidently plan your project and understand when you'll need full planning permission.

 

 

What Is Permitted Development and How Does It Apply to House Extensions?

Permitted Development rights allow homeowners to carry out certain building works without submitting a full planning application. These rights are granted nationally but can be restricted locally.

For house extensions, this means you may be able to:

  • Extend your home to the rear or side
  • Add single-storey or (in some cases) double-storey extensions
  • Build within set size and height limits

However, PD rights only apply if:

  • The property is a house (not a flat or maisonette)
  • The building has not had PD rights removed
  • The extension meets all specific criteria

Key takeaway: Permitted Development is not a free-for-all, it's a tightly controlled set of rules.

 

 

How Far Can You Extend Your House Without Planning Permission in the UK?

One of the most searched questions is: “how far can you extend without planning permission?”

Here are the current limits for rear extensions under Permitted Development in England:

Single-Storey Rear Extensions

  • Detached houses: up to 4 metres from the original rear wall
  • Semi-detached & terraced houses: up to 3 metres

Larger Home Extension Scheme (Prior Approval Required)

You may be able to extend further under the neighbour consultation scheme:

  • Detached houses: up to 8 metres
  • Semi-detached & terraced houses: up to 6 metres

This requires prior approval from the local council, but not full planning permission.

 

 

What Is the Maximum Size of a House Extension Without Planning Permission?

Beyond depth, there are overall size limits that often catch people out.

Total Land Coverage Rule

Extensions and outbuildings must not cover more than 50% of the land around the original house

Height Restrictions

  • Maximum height: 4 metres for a single-storey extension
  • Eaves height must not exceed 3 metres if within 2 metres of a boundary

Width Limits for Side Extensions

  • Must be single storey only
  • Cannot be wider than half the width of the original house

Important: These rules apply to the “original house” as it stood on 1 July (or when first built if later).

 

 

Can You Build a Double-Storey Extension Without Planning Permission?

Yes - but with tighter restrictions.

Double-Storey Extension Rules Under Permitted Development

  • Cannot extend more than 3 metres from the rear wall
  • Must be at least 7 metres from the rear boundary
  • Roof pitch must match the existing house
  • No balconies, verandas, or raised platforms

Because of these limitations, many homeowners find that double-storey extensions often require planning permission, especially in built-up areas.

 

 

How Close to the Boundary Can You Build an Extension Without Planning Permission?

Boundary rules are critical - and often misunderstood.

Key Boundary Rules:

  • If within 2 metres of a boundary, the maximum height is 3 metres
  • Eaves must not exceed 3 metres regardless
  • Materials should be similar in appearance to the existing house

This is particularly important for:

  • Side return extensions
  • Narrow plots
  • Urban or terraced properties

 

 

hat Are the Height Restrictions for Extensions Without Planning Permission?

Height limits apply to both the structure and roof design.

Standard Height Rules:

  • Maximum overall height: 4 metres (single-storey)
  • Eaves height: Max 2.5–3 metres near boundaries. Two-storey extensions must not exceed the height of the existing house

Roof design also matters:

  • Flat roofs are allowed but must stay within height limits
  • Pitched roofs must match the existing property

 

 

What Types of Extensions Can Be Built Without Planning Permission?

Permitted Development covers a wide range of extension types, provided they meet the rules.

Common Extension Types:

  • Rear kitchen extensions
  • Side return extensions
  • Wraparound extensions (if within limits)
  • Single-storey open-plan extensions
  • Some double-storey rear extensions

However, design complexity can affect compliance. For example:

  • Large wraparound extensions often exceed width or coverage limits
  • Open-plan layouts may require structural changes impacting compliance

 

 

When Do You Need Planning Permission for a House Extension?

Even if you're close to the limits, you'll need planning permission if:

  • The extension exceeds size limits
  • You live in a conservation area or protected zone
  • Your home is a listed building
  • Permitted Development rights have been removed
  • The design significantly alters the appearance of the property

Common Scenarios That Require Planning Permission:

  • Large wraparound extensions
  • Extensions with balconies or raised platforms
  • Extensions that significantly overlook neighbours
  • Modern designs that differ from the existing house

 

 

Do You Need Building Regulations Approval for an Extension?

Yes - Permitted Development only removes the need for planning permission. You will still need Building Regulations approval to ensure:

  • Structural safety
  • Insulation and energy efficiency
  • Fire safety compliance
  • Drainage and ventilation

Skipping this step is not an option and can cause serious legal and resale issues.

 

 

How Do You Prove Your Extension Is Within Permitted Development?

Many homeowners apply for a Lawful Development Certificate (LDC).

Why This Matters:

  • Provides official confirmation your extension is legal
  • Required by solicitors when selling your home
  • Protects you from future disputes

While not mandatory, it is strongly recommended.

 

 

What Are the Most Common Mistakes When Building Without Planning Permission?

Understanding the rules is one thing - applying them correctly is another.

Common Mistakes:

  • Measuring from the wrong “original house”
  • Ignoring the 50% land coverage rule
  • Overlooking height restrictions near boundaries
  • Assuming previous extensions don't count
  • Not checking local PD restrictions

These mistakes can lead to:

  • Enforcement notices
  • Project delays
  • Additional costs

 

 

Get Expert Advice on Your House Extension in Cornwall

Every property is different, and even small details can affect whether your extension needs planning permission. Let our team evaluate your property and provide expert recommendations on size, layout, and compliance.

Contact Us  Our Home Extensions