Do You Need Planning Permission for an Extension? West Sussex Guide 2026
- Green Manor Building LTD
- 18 hours ago
- 6 min read
Before any extension project can move forward, one key question needs to be answered.
Does the work require planning permission, or can it be carried out under Permitted Development?
In many cases, homeowners in West Sussex, Brighton, Hove, and surrounding areas are able to extend without submitting a full planning application. However, this depends on the property, its location, and the scale of what is being proposed. Understanding the correct planning route early helps avoid redesign, delays, and unnecessary costs later in the process.
This guide explains how planning permission for extensions in West Sussex works in 2026, when it is required, and what should be checked before progressing too far.
Planning permission vs Permitted Development
There are two main routes for residential extensions
Permitted Development (PD)
Permitted Development rights allow certain extensions to be built without submitting a full planning application, provided the proposal remains within defined limits and conditions.
Key considerations include:
Size and depth
Single storey rear extensions up to 6 metres deep for detached houses
Single storey rear extensions up to 3 metres deep for terraced and semi detached houses
Two storey extensions up to 3 metres beyond the original rear wall
Height
Maximum height of 4 metres where the extension is within 2 metres of a boundary
Eaves and ridge heights must not exceed those of the existing property
Other factors
Distance from boundaries
Materials and overall appearance
The original size of the property, as PD allowances are measured against the house as it was originally built
Permitted Development is commonly used for single storey rear extensions and some modest side extensions. However, it is not automatic and does not apply in every situation.
Planning permission
A full householder planning application is usually required where:
The proposed extension exceeds Permitted Development limits
The property is located in a restricted area such as a conservation area, listed building, or National Park
The design significantly affects the external appearance of the building
Previous extensions have already used up remaining PD allowances
Article 4 Directions have removed PD rights in the area
In practice, most extensions fall clearly into one route or the other once the property and proposal are properly assessed.
Quick assessment guide
While every property should be reviewed individually, the following can act as an early guide:
Standard single storey rear extensions outside restricted areas are often suitable for Permitted Development
Larger extensions, corner plots, or visible side extensions may require planning permission or prior approval
Properties in conservation areas, listed buildings, or South Downs locations are more likely to require planning permission
Where there is any uncertainty, confirming the route early helps prevent redesign later
This is normally clarified before detailed design work progresses.
When planning permission is more likely to be required
Based on residential projects across West Sussex, planning permission is more commonly required in the following situations.
Conservation areas and listed buildings
If a property is located within a conservation area or is listed, Permitted Development rights may be restricted or removed entirely. West Sussex includes numerous conservation areas across Worthing, Horsham, Chichester, and coastal towns where design, scale, and materials are more closely controlled. Listed buildings almost always require formal consent for external alterations, regardless of size.
South Downs National Park
Large areas of West Sussex, including parts of Brighton, Lewes, and Mid Sussex, fall within the South Downs National Park boundary. Properties within the Park fall under a separate planning authority and are typically subject to greater design scrutiny. Even modest extensions may require planning permission depending on visibility, setting, and design approach. Confirming whether a property lies within this boundary is an important early step.
Article 4 Directions
Some areas of West Sussex are subject to Article 4 Directions, which remove certain Permitted Development rights. These are commonly found in conservation areas and locations of architectural or historic significance. Your local planning authority can confirm whether an Article 4 Direction applies to a specific property.
Properties that have already been extended
Permitted Development allowances are measured against the property as it originally existed. If a home has been extended previously, remaining PD rights may be reduced or removed, even where works were carried out by a former owner. Reviewing the property planning history helps avoid incorrect assumptions.
Larger rear extensions and prior approval
Some larger rear extensions may still be possible under Permitted Development through the prior approval process.
This can allow:
Single storey rear extensions up to 8 metres deep for detached houses
Single storey rear extensions up to 6 metres deep for terraced and semi detached houses
Prior approval is not full planning permission. It requires:
Formal notification to the local authority
Consultation with neighbouring properties
Assessment of design and potential impact
This route can affect both design and project timeline and should be considered early where larger extensions are being explored.
Lawful Development Certificates
Even where an extension appears to fall within Permitted Development, many homeowners choose to apply for a Lawful Development Certificate. An LDC provides written confirmation from the local authority that the proposed works are lawful.
While not mandatory, it is often useful for:
Avoiding future disputes or enforcement issues
Providing clarity during a property sale
Confirming compliance before construction begins
For many projects, this is a sensible risk management step and typically costs significantly less than a full planning application.
Planning permission and Building Regulations
Planning permission and Building Regulations are separate approval processes.
Even where planning permission is not required, almost all extensions still require Building Regulations approval.
Building Regulations cover:
Structural safety
Insulation and energy performance
Drainage and foundations
Ventilation
Fire safety
Electrical and mechanical systems
This process runs independently and should be included within the overall project programme.
How planning affects your timeline
Planning is often one of the longest early stages of an extension project.
Typical timeframes include:
Permitted Development with Lawful Development Certificate: 6 to 8 weeks
Prior approval process: 6 to 8 weeks
Full planning permission: 8 weeks standard determination, which can extend to 13 weeks for more complex proposals
Properties in conservation areas or within the South Downs planning authority may require additional consultation, which can extend timelines further. Understanding this early helps set realistic expectations for when construction can begin.
For a complete breakdown of the entire extension process from design through to completion, including the construction phase, read our House Extension West Sussex: Timeline and How Long It Takes (2026) blog.
Moving forward with clarity
Establishing the correct planning route before committing to detailed drawings or build dates is one of the most valuable early steps in any extension project.
Green Manor Building delivers residential construction projects across West Sussex and works alongside architects, designers, and consultants familiar with local planning requirements.
Clarifying feasibility early helps reduce uncertainty and supports a smoother delivery process once works progress on site.
Clear planning, defined scope, and early confirmation of requirements form the strongest foundation for a successful extension.
Green Manor Building provides construction services across West Sussex, working with homeowners and developers on extension projects from planning through to completion.
Phone - 01273 494059
Frequently Asked Questions
Do I need planning permission for a single storey extension in West Sussex?
Most single storey rear extensions don't require planning permission if they stay within Permitted Development limits (up to 6m deep for detached houses, 3m for semi-detached/terraced). However, this doesn't apply in conservation areas, to listed buildings, or where Article 4 Directions are in place.
How long does planning permission take in West Sussex?
Standard planning applications take 8 weeks to determine, though complex proposals may take up to 13 weeks. Prior approval processes typically take 6-8 weeks. Properties in conservation areas or the South Downs may require additional consultation time.
What is the difference between planning permission and Building Regulations?
Planning permission concerns the external design and impact of your extension. Building Regulations ensure structural safety, insulation, drainage, and compliance with building standards. Almost all extensions require Building Regulations approval, even if planning permission isn't needed.
Do I need planning permission in a conservation area?
Extensions in conservation areas typically have restricted Permitted Development rights. Even modest extensions often require planning permission. Check with Brighton & Hove City Council or your local planning authority to confirm what applies to your property.
What is a Lawful Development Certificate?
An LDC provides written confirmation from your local authority that proposed works are lawful under Permitted Development. While not mandatory, it's recommended for avoiding future disputes, providing clarity during property sales, and confirming compliance before construction begins.
Can I build an extension without planning permission?
Yes, if your extension falls within Permitted Development limits and your property doesn't have restricted rights (such as being in a conservation area or having an Article 4 Direction). However, you'll still need Building Regulations approval even if planning permission isn't required.
Further resources
For general guidance, the following references may be helpful:
These resources provide general information only. Each property should always be assessed individually. For project-specific advice, consult with your local planning authority or an experienced construction professional.
