Do I Need Planning Permission for an Extension? Guide

Why Planning Feels Complicated (But Doesn’t Have to Be)

Thinking about extending your home is exciting — more space, more light, a home that finally works the way you want it to. But there’s always one big question at the start:

Do I need planning permission?

The rules can feel like a maze. Permitted development rights, conservation areas, neighbour consultation — it’s easy to feel confused. Yet with the right guidance, it’s much simpler than it looks.

At James Williamson Architects, we guide Surrey homeowners through this process every day. By clarifying the rules early, you can avoid delays, prevent disputes, and move forward with confidence.

👉 Want to know what applies to your property?  Let’s chat through what applies to your home

Key Factors Before You Begin

Before diving into design ideas, it helps to understand the key rules that shape what you can build.

1. Party Wall Act and Neighbour Consultation Scheme

• Party Wall Act 1996: If you’re working on or near a shared boundary — for example, excavating for new foundations — you’ll need to notify neighbours in writing.

• Neighbour Consultation Scheme: For larger single-storey rear extensions, you may need Prior Approval from your local council. Neighbours are formally consulted before a decision is made (usually within 42 days).

Example: In Guildford, we supported a family with a 6m rear extension under the Neighbour Consultation Scheme. Because we submitted clear drawings early, the neighbours raised no objections and approval came through quickly.

2. Listed Buildings and Conservation Areas

If your property is listed or lies in a conservation area (common in Elmbridge, Guildford, and Mole Valley), expect stricter rules. Even small changes may require permission, and some alterations (like modern glazing) can be restricted. Always check before committing to designs.

3. The “Original House” Rule

Extensions are measured against the size of the “original house” (as it stood on 1 July 1948, or when first built if later). Previous additions count towards the total. If your new extension would cover more than half the land around the original house, or exceed certain limits, you’ll need planning permission.

Types of Planning Permission

Not all projects fall neatly under permitted development. In some cases, you’ll need to apply for planning permission.

• Full Planning Permission: Required for larger or more complex projects. Involves submitting full plans for approval. Provides certainty but takes longer to prepare.

• Outline Planning Permission: Establishes whether a proposal is acceptable in principle before detailed plans are submitted. Useful in early stages, but still needs a full application later.

👉 Unsure which route your project falls under? Schedule a 90-Minute Strategy Session.

When Extensions Fall Under Permitted Development

Many extensions can be built without planning permission under permitted development rights, but strict limits apply.

General requirements where permission is needed:

• Extension projects forward of the front or side elevation facing a highway

• More than half the land around the “original house” is built over

• Materials differ significantly from the existing house

• Extension is higher than the current roof or exceeds eaves height limits

• Eaves above 3m within 2m of a boundary

• Includes verandas, balconies, or raised platforms

Height, Boundaries, and Materials — The Key Limits

Side extensions: 

Permitted only if single-storey, under 4m high, and less than half the width of the original house.

Single-storey rear extensions: 

Up to 6m beyond the rear wall (semi-detached/terraced) or 8m (detached). Over 4m in height requires permission.

Two-storey rear extensions: 

Require permission if projecting more than 3m, within 7m of the rear boundary, with mismatched roof pitch, or side-facing windows below 1.7m that aren’t obscure-glazed.

Other Rules Worth Knowing

The 45-Degree Rule

Planning officers often use this test to check overshadowing. If your extension falls outside a 45-degree line from your neighbour’s nearest habitable room window, it may be refused.

Lawful Development Certificate (LDC)

Even if your project qualifies as permitted development, applying for an LDC is wise. It’s written confirmation from your council that your extension is lawful.

Cost:

• Existing use or operation: £293 or the relevant full application fee

• Proposed use or operation: 50% of the full application fee

Benefit: 

Essential evidence when selling or remortgaging, preventing disputes later.

Building Regulations

Planning permission and building regulations are not the same. Even if your extension doesn’t need planning permission, it must still meet building regulations for structure, fire safety, drainage, insulation, and energy efficiency. Neglecting this risks costly corrections later.

Why Getting Clarity Early Saves Stress

Many Surrey homeowners assume planning is a hurdle. In reality, it’s a safeguard. It ensures your design is achievable, lawful, and adds long-term value.

What happens when you don’t check early:

• Delays if neighbours object or councils reject incomplete applications.

• Extra costs from redesigns or retrospective permissions.

• Stress if you realise mid-build that your extension doesn’t comply.

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What happens when you do check early:

• Clear roadmap of permissions and approvals.

• Confidence in your design and timeline.

• Smooth conversations with builders, neighbours, and the council.

Take the Next Step with Confidence

At James Williamson Architects, we specialise in guiding Surrey homeowners through extensions. From first sketches to final approvals, we remove the uncertainty around planning so you can focus on the exciting part — transforming your home.

When every approval is in place, you can finally focus on the exciting part — creating a home that feels just right.

👉 Start today with a free 15-Minute Clarity Call.👉 Or, if you’re ready for tailored planning and design advice, book a 90-Minute Strategy Session.

Book a Free 15-minute Clarity Call

Frequently Asked Questions

1. What’s the difference between planning permission and building regulations?

Planning deals with how your extension looks and its impact on neighbours. Building regs ensure it’s safe, efficient, and structurally sound. Most projects require compliance with both.

2. Do rules vary across the UK?

Yes. This guide focuses on England. Scotland, Wales, and Northern Ireland have different frameworks.

3. What is retrospective planning permission?

It’s permission sought after work is completed without approval. It can legalise the development but carries risks. Always safer to apply beforehand.

4. Do I always need permission in a Surrey conservation area?

Almost always, yes. Local authorities such as Guildford, Elmbridge, and Mole Valley enforce stricter controls. We’ll advise on specifics at the outset.

5. Should I bother with a Lawful Development Certificate?

Absolutely. For a modest fee, it provides peace of mind and avoids future disputes when selling or refinancing.