Do I need planning permission for an extension?

Extensions are where permitted development is most detailed and most misquoted, because the conditions differ by extension type, by house type, by whether the land is designated, and by what has already been built. Anyone who gives you a single number for how far you can extend has not asked enough questions.

Safety

Step by step

1. Start with the disqualifiers, not the dimensions.Flat or maisonette, listed, conservation area or other designated land, Article 4 direction, rights removed by condition, or the house already extended. Any one of these changes the answer before a tape measure comes out.
2. Identify the type of extension precisely.Single-storey rear, single-storey side, two-storey rear, wrap-around, or an addition in front of the principal elevation. The conditions are different for each and a wrap-around is usually treated as failing the side-extension rules.
3. Check the depth, height and eaves conditions for that type.Limits exist on projection from the original rear wall, overall height, eaves height, and eaves height where the extension is close to a boundary. Take the current figures from the Planning Portal for England and Wales, or the equivalent national guidance elsewhere.
4. Check the coverage and position rules.There are limits on how much of the garden may be covered, and extensions forward of the principal elevation are generally not permitted development. On a corner plot the side elevation facing a road may also count as a principal elevation.
5. Check the prior approval route for larger rear extensions in England.Larger single-storey rear extensions can be permitted development but only after a prior approval procedure in which the council notifies adjoining neighbours and considers their objections about impact. It has to be completed before work starts — it is a process, not a formality.
6. Check what is under the extension.Building over or near a public sewer usually requires a build-over agreement with the water company, entirely separate from planning and building control. Find the drainage before designing the foundations.
7. Assume building regulations apply in full.Foundations, structure and any beam over a new opening, damp proofing, drainage, insulation, ventilation, glazing and electrical work. Apply to building control and agree who is making the application before work begins.
8. Serve party wall notices where they apply.In England and Wales, excavating for foundations near a neighbour's building, or building on the boundary line, brings the Party Wall etc. Act into play. Notices are served before work starts, not when the digger arrives.
9. Get a lawful development certificate if you are not applying for permission.Extensions sit close to their limits, neighbours notice them, and this is the document that ends the argument for good.

Tips

• Draw the original house on the plan as a separate outline. Every allowance is measured from it, and having it drawn stops the commonest measuring mistake.
• Trees near the extension change the foundation design, and a tree preservation order or a conservation area can restrict what you may do to the tree itself.
• Neighbour consultation under prior approval is about impact on amenity, not about whether neighbours like the idea. Talking to them first is still the cheapest thing you can do.

Common mistakes

• Starting a larger rear extension before prior approval is determined — The permitted development right is conditional on completing the procedure first. Starting early can put the whole extension outside permitted development.
• Ignoring the drain running under the proposed floor — A build-over agreement with the water company is a separate consent, and discovering the sewer after the foundations are dug is expensive and occasionally fatal to the design.

Questions people ask