Not every extension in Cornwall needs a full planning application. Permitted development (PD) rights allow some alterations to houses if they stay within set limits and conditions. The rights are narrower than many people expect, and they are often reduced or removed on the homes and landscapes that are common here. This guide covers the limits homeowners most often ask about, and why you should still check before you build. It is general guidance only, not legal advice. Always confirm figures against the current Planning Portal guidance, Cornwall Council advice and the General Permitted Development Order.
This is different from our guide to planning permission in Cornwall, which covers how to apply when an application is needed.
What permitted development covers — and what it does not
Householder PD rights apply to houses, not generally to flats or maisonettes, and they are restricted for some homes created through a change of use. A proposal has to meet every relevant limit and condition, including earlier extensions. The "original house" means the house as first built, or as it stood on 1 July 1948 if older. An extension added by a previous owner still counts.
Even when planning permission is not needed, Building Regulations usually still apply. Structure, insulation, fire safety, drainage, ventilation and electrics are assessed separately from planning. PD also does not remove listed building consent, party wall procedures or highway consents where those regimes apply.
Common limits for rear and side extensions
The figures most homeowners ask about are set out on the Planning Portal and in the government's householder technical guidance. As a current general summary for houses in England (always check the latest official text before relying on a measurement):
A single-storey rear extension must not extend beyond the rear wall of the original house by more than four metres if detached, or three metres for any other house, and must not be more than four metres high.
Where the house is not on designated land or a Site of Special Scientific Interest, those rear depths can increase to eight metres (detached) or six metres (other houses) through prior approval — the larger home extension or neighbour consultation scheme. You cannot simply start work at those larger depths.
The extension must not be higher than the highest part of the existing roof, and its eaves must not be higher than the existing eaves.
If any part of the extension is within two metres of a boundary, eaves height must not exceed three metres.
A side extension must be single storey, no more than four metres high and no wider than half the width of the original house. On designated land, side extensions are not permitted development.
Extensions of more than one storey must not extend more than three metres beyond the original rear wall and must be at least seven metres from the rear boundary. Two-storey side extensions, and two-storey rear extensions on designated land, need planning permission. See our two-storey extension guide for more detail.
Only half the land around the original house can be covered by extensions and other buildings.
Exterior materials should be of a similar appearance to those on the existing house.
These limits apply to the total enlargement. If you already have a rear addition, a new one joined to it is measured as part of the whole.
Wrapping around a rear and side wall
Homeowners often ask about an extension that turns the corner, filling the space beside and behind the house. Official technical guidance treats this as needing to satisfy both the rear-wall limits and the side-wall limits: rear depth and height, plus the side rules on height, single storey and width. Many wrap-around schemes cannot meet both at once, so they need a planning application. If you are planning a larger home extension in Cornwall, have the design checked against current PD rules first. Our home extension overview covers the wider design choices.
Where rights are reduced or removed in Cornwall
Cornwall has many conservation areas, listed buildings and land within the Cornwall National Landscape (formerly the AONB). These are treated as designated land (Article 2(3) land) in the PD rules. On designated land:
Side extensions are not permitted development.
Rear extensions of more than one storey are not permitted development.
The larger 6 metre / 8 metre rear extension scheme does not apply.
Cladding the exterior is not permitted development.
Listed buildings are a separate control. Works that affect their character need listed building consent whether or not they would otherwise be PD. Our guide to building in an AONB or conservation area explains how these designations overlap.
Cornwall Council can also remove or restrict PD rights with an Article 4 direction, or a condition on an earlier permission may have removed them. That is fairly common in sensitive historic areas and on some newer estates. Flats and maisonettes do not benefit from these householder extension rights at all.
Lawful Development Certificates and why checks matter
If you believe a proposal is permitted development, you can apply to Cornwall Council for a Lawful Development Certificate (LDC). An LDC is not planning permission. It is a formal decision that the work would be lawful, which is useful before you start on site and later when the house is sold.
Limits change when the Order is amended, and small differences in how a wall, eaves line or original rear wall is read can decide whether a scheme is PD. Always check the current Planning Portal pages and the General Permitted Development Order, and ask Cornwall Council or a planning professional if you are unsure.
Permitted development FAQs
If I stay within the size limits, can I definitely start work?
Not automatically. You still need to meet every other PD condition, confirm that rights have not been removed, and remember that Building Regulations, listed building consent and other regimes may still apply. An LDC is the formal way to confirm the planning position.
Do Building Regulations apply if I do not need planning permission?
Yes, in most cases an extension will still need Building Regulations approval even when it is permitted development.
Can I use permitted development in a conservation area or the National Landscape?
Some tightly limited rear extensions may still be possible, but many usual rights are reduced or removed on designated land, including side extensions, two-storey rear extensions, cladding and the larger rear-extension scheme. Check before you design.
What if my house is a flat or has already been extended?
Householder PD rights generally do not apply to flats and maisonettes. Existing extensions count towards the totals, so a house that has already been extended may have little remaining allowance.
Talk to us about your extension
If you are planning an extension and want to understand the building implications alongside the planning checks, we can help you think the project through. Call Cornish Building Company on 01208 816991 or contact us to discuss your plans.
