What Size Extension Can You Build Without Planning Permission? 

Building an extension is a great way to get more space without the hassle of having to move. If this sounds like the answer to your prayers, it’s worth finding out what size extension you can build without planning permission. If your project qualifies as a permitted development extension, you may be able to save yourself some time, red tape and fees.  

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Permitted Development Rights

You used to need planning permission to do any work on the exterior of your house, such as renovating the façade, changing windows and doors and building extensions. However, the 2015 Town and Country Planning Order changed all that by providing for some home renovations to be done under Permitted Development Rights (PRDs). Within certain limitations, and provided you don’t abuse them, PRDs work as an automatic grant of planning permission, so you don’t need to apply. 

What Size Extension Can You Build Without Planning Permission? 

What size extension you can build without planning permission will depend on the size, location, features and proximity to your boundary. We’ve provided a quick rundown below, but the rules can be complex, so make sure you hire an experienced, reliable construction company or architect that can talk you through everything and help you understand your options. 

All Extensions

The main types of extension are either one- or two-storeys built onto the rear of the house, ‘side returns’ built down the side or ‘wraparounds,’ which extend out from the rear and around the side. 

Exactly what you can build without planning permission varies for each type of extension. In general, though, you don’t need permission as long as:  

  • Your extension is on the back of the house or the side and doesn’t face the road.     
  • The roof is lower than the original roof or eaves. 
  • The roof isn’t higher than 3m at the eaves when the extension comes to within 2m of the boundary. 
  • For loft conversions (which count as extensions), the original roof shape and volume must be maintained.  
  • Your extension doesn’t occupy more than 50% of the total land around your house.
  • It doesn’t have balconies, verandas, raised platforms, a chimney or vent pipe, TV aerial or satellite dish. If you want to include these in the design, you’ll need planning permission. 
  • You are using use materials compatible with the original structure. 

Note that the above don’t apply to any home renovations on a listed building or in a conservation area. If this is your case, you’ll always need planning permission. 

Also, even if you don’t need planning permission, your extension will need to meet with Building Regulations approval so it can be certified as legal and up to modern fire safety and energy efficiency standards. 

If you’re working on walls shared with neighbours, you’ll also need a Party Wall Agreement. If you have sewers on your land, you’ll need a Build Over Permission from the water board. 

Side Extensions

‘Side returns’ can be built without planning permission as long they are single storey, not more than 4m high and no wider than half the original house. 

Single Storey Extensions

Single storey extensions to the rear of the house don’t need planning permission provided they don’t extend more than 8m from a detached house or 6m from other types of home. They must also not exceed 4m in height. 

Multi-Storey Extensions 

You can build a multi-storey extension on the back of your house without planning permission as long as it doesn’t extend over 3m from the rear wall. The pitch of the roof must also match the existing house, as far as practical, and any upper windows at the side must be obscure-glazed to protect neighbours’ privacy.  

How Close to My Boundary Can I Build an Extension? 

A rear extension must not come closer than 7m of your rear boundary. Side returns can be built right up to the boundary as long as the height doesn’t affect neighbours. However, you’ll still need a Party Wall Agreement to build within 3m or dig within 6m of shared boundaries at the side of your property.  

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