Extending or converting your home is one of the most effective ways to create additional living space without moving. However, before work begins, it is essential to understand whether you need planning permission, how permitted development operates, and what building regulations approval is required. In Devon & Cornwall, additional controls may apply due to conservation areas, national parks, Areas of Outstanding Natural Beauty and other designated land.
This guide explains the legal framework governing house extensions and conversions in England, helping homeowners make informed decisions and avoid costly mistakes.
Do I Need Planning Permission for Extension?
A common question homeowners ask is: do I need planning permission for bespoke extension works? The answer depends on the scale, position and design of the proposed extension, as well as the status of the existing house.
In many cases, you do not need planning permission because extensions can fall under permitted development. However, planning permission is required where works exceed permitted development rules, where the property is a listed building, or where development rights have been removed.
If you are unsure, it is always advisable to seek advice from your local planning authority or apply for a lawful development certificate to confirm that the works are permitted.
Permitted Development Rights
Permitted development rights allow homeowners to carry out certain types of home extension and alterations without submitting a formal planning application. These development rights are granted by national legislation but are subject to strict development rules.
Permitted development rights apply only to houses, not flats or maisonettes, and relate to the original house as it stood on 1 July 1948 (or when first built if later). Any previous extensions added by a previous owner are counted when assessing what is allowed today.
In designated land such as conservation areas, national parks, world heritage sites, Areas of Outstanding Natural Beauty and sites of special scientific interest, permitted development rights may be restricted.
Single Storey Extension
A single storey extension is one of the most common forms of development undertaken under permitted development. Many homeowners choose this option to expand ground floor living space without the complexity of a multi‑level build.
For a detached house, a single storey rear extension may extend up to four metres beyond the rear wall. For a semi detached house or terraced house, the limit is up to three metres, subject to height restrictions and design conditions.
Single storey extensions must not exceed the maximum height of four metres overall and three metres when located within two metres of a boundary.
Permitted Development Rules
Permitted development rules set out clear limits on size, height, materials and positioning. These rules apply cumulatively, meaning previous works count towards the total allowance.
Key permitted development rules include:
- Extensions must not extend beyond the principal elevation
- Materials should be of a similar appearance to the existing house
- No more than half the width of the original house can be extended
- Development must not cover more than half of the land around the original house
- Extensions must not include raised platforms
- Roof alterations must not exceed the existing roof height or alter the roof pitch facing the highway
Where these permitted development rules apply correctly, an extension may proceed without planning permission.
Single Storey Rear Extensions
Single storey rear extensions remain popular across Devon and Cornwall. Under permitted development, these extensions can project beyond the rear wall while remaining compliant.
A detached house may extend up to six metres beyond the rear wall using the neighbour consultation scheme, while semi detached and terraced houses may extend up to six metres subject to prior approval.
This “prior approval process” allows the local authority to assess the impact on neighbouring amenity before work begins.
Original House
Understanding what constitutes the original house is critical. The original house is the building as it existed in 1948 or when originally constructed.
Any existing extension, side extension, loft extension or storey extension built since that date counts towards development limits. Even works carried out by a previous owner are included in the calculation.
Failure to account for earlier changes is a common reason homeowners unintentionally breach planning control.
Without Planning Permission
Many homeowners aim to carry out work without planning permission, relying instead on permitted development rights. While lawful, this approach requires careful compliance with the applicable development rules.
Although planning permission may not be required, building regulations approval is still mandatory for most structural works, conversions, and changes affecting fire safety, insulation, ventilation and drainage.
Obtaining a lawful development certificate provides formal confirmation that the works are permitted and can be invaluable when selling your property.
Neighbour Consultation Scheme
The neighbour consultation scheme applies to larger single storey rear extensions that exceed the standard permitted limits.
Under this scheme:
- Neighbours are notified by the local authority
- They have 21 days to raise concerns
- The local authority assesses impact on amenity, light and outlook
- No planning permission is required if approved
This scheme applies only to single storey rear extensions and not side extensions or two storey works.
Single Storey
A single storey extension is generally easier to deliver under permitted development than developments involving more than one storey.
Single storey projects typically involve extending the ground floor to create open‑plan living space, kitchens or dining areas while keeping work below the existing roofline.
Height restrictions, boundary distances, and materials remain critical to compliance.
Loft Conversion
A loft conversion can often be carried out under permitted development, provided it remains within volume limits and does not extend above the existing roof.
Roof lights, roof space alterations, and internal reconfiguration are commonly permitted. However, dormers facing the highway, raised roof heights, or excessive volume may require planning permission.
Loft conversions must comply with building regulations, particularly regarding fire safety, floor space, insulation, head height and access.
Maximum Height
The maximum height of an extension is strictly controlled under permitted development rights.
- Single storey extensions: 4 metres maximum
- Side extensions: limited to 3 metres
- Two storey side extensions are not permitted under PD
- Eaves and roof heights must align with the existing roof
Properties with flat roof designs remain subject to height limits relative to the existing roof and surrounding ground level.
Extension Without Planning Permission
An extension without planning permission is lawful only where all permitted development conditions are met.
However, if any element breaches the rules, the extension risks being considered unlawful development, potentially resulting in enforcement action.
A lawful development certificate confirms legality and protects homeowners from future disputes, particularly during property sales or remortgaging.
Larger Rear Extensions
Larger rear extensions exceeding the standard permitted development depth are possible through the prior approval route.
This option allows greater projection beyond the rear wall while still avoiding a full planning application, provided the local authority is satisfied that neighbour impacts are acceptable.
This route cannot be used on listed buildings or properties within restricted designated land.
Apply for Planning Permission
You must apply for planning permission where proposals fall outside permitted development limits or where rights do not apply.
Planning permission is required for:
- Two storey side bespoke extensions
- Extensions on listed buildings (plus listed building consent)
- Works in restricted conservation areas
- Developments affecting designated land
- Multi‑storey extensions
- Extensions impacting the principal elevation
Applications are submitted to the local authority and may attract the community infrastructure levy depending on scale.
Final Considerations
Even where planning permission is not required, building regulations approval remains compulsory. This covers structural safety, drainage, thermal efficiency, ventilation, soil and vent pipe arrangements, external walls, roof tiles, roof lights and fire safety.
For peace of mind, professional advice should always be sought before commencing work, particularly in Devon and Cornwall where conservation areas and environmental designations are common.
Authoritative Reference
This article is based on official UK legislation and guidance:
UK Government – Permitted development rights for householders: technical guidance