Rear Extension: The Complete Homeowner Guide

Often, no. Most single-storey rear extensions fall within permitted development rights under Part 1, Class A of the GPDO 2015. That means you can build without applying for planning permission — as long as you stay within the size and design limits.

Two-storey rear extensions can also be permitted development, but the rules are tighter.

If your extension exceeds the PD limits, you will need to apply for planning permission. A householder planning application currently costs £548.

The limits depend on your house type and whether the extension is single-storey or two-storey.

Single-storey rear extension

Two-storey rear extension

Rules that apply to all rear extensions under PD

These conditions apply regardless of size:

• The extension must not cover more than 50% of the curtilage (garden and surrounding land) of the original house — this includes all outbuildings, sheds, and previous extensions
• The extension must not be higher than the highest part of the existing roof
• The extension must not be forward of the principal elevation (the front of the house)
• On designated land (conservation areas, National Parks, AONBs), the extension must not extend beyond the side elevation of the original house
• Materials must be similar in appearance to the existing house (for two-storey extensions and single-storey extensions on designated land)

“Original house” means the house as it was first built or as it stood on 1 July 1948. Any previous extensions count against your allowance. Check the PD traps that catch people out →

If you want to build a single-storey rear extension deeper than the standard PD limits (more than 4m on a detached house, or more than 3m on a semi or terrace), you can use the prior approval process instead of a full planning application.

This is sometimes called the “larger home extension” scheme. It allows:

• Up to 8 metres depth on a detached house
• Up to 6 metres depth on a semi-detached or terraced house

Prior approval is not planning permission. It is a lighter process where your council notifies your immediate neighbours, and they have 21 days to comment. The council then has 42 days from the date of your application to make a decision. If they do not respond within 42 days, the extension is approved by default.

The application fee is £249.

The council can only consider the impact on your neighbours’ amenity (light, privacy, outlook). They cannot refuse on design grounds or other planning policies.

Prior approval does not apply to:

• Two-storey extensions
• Properties on designated land (conservation areas, National Parks, AONBs)
• Listed buildings
• Extensions that would be within 2 metres of the boundary and over 3 metres in height

Read our full guide to prior approval →

You will need to apply for full planning permission (£548) if:

• Your extension exceeds the PD size limits and is not eligible for prior approval
• Your house is a flat or maisonette (no PD rights under Part 1)
• Your PD rights have been removed by a planning condition on your original planning approval
• An Article 4 direction removes extension PD rights at your address
• Your property is a listed building (you will also need listed building consent)
• You want to extend forward of the principal elevation facing a highway

Every rear extension needs building regulations approval — regardless of whether it also needs planning permission. Planning permission and building regulations are two completely separate systems.

Building regulations ensure the extension is structurally safe, energy efficient, and meets minimum standards for health and safety.

What building control covers for a rear extension

• Foundations — depth and type suitable for ground conditions
• Structural design — steelwork, lintels, and loadbearing walls
• Damp-proofing — damp-proof course and membrane
• Insulation — walls, floor, and roof must meet current Part L thermal standards
• Drainage — foul and surface water connections
• Electrical work — Part P compliance for new circuits
• Fire safety — fire detection, escape routes (especially important if the extension affects the existing means of escape)
• Ventilation — Part F requirements for habitable rooms and kitchens
• Glazing — safety glass in critical locations, and Part L limits on glazing area

Two routes for building regulations approval

You can use either your local authority building control service or a private approved inspector. Both have the same legal powers and issue the same completion certificate.

Read more about building regulations vs planning permission →

Almost certainly yes for a rear extension. A structural engineer will design:

• The foundations (type, depth, and width)
• Any steelwork (RSJs/steel beams) needed to create open-plan spaces
• Lintels above new openings in existing walls
• The roof structure

Structural calculations typically cost £300–£800 depending on complexity. Your builder or architect will usually recommend an engineer, or you can find one through the Institution of Structural Engineers (IStructE).

If your extension is built on or close to the boundary with a neighbour, the Party Wall Act 1996 probably applies.

The Act applies if:

• You are building on the boundary line (Section 1)
• You are cutting into or building up against a party wall (Section 2) — for example, where your rear wall is also the boundary wall with a terraced neighbour
• You are digging foundations within 3 metres of a neighbouring building and going deeper than their foundations, or within 6 metres if the excavation would cut a 45-degree line from the bottom of their foundations (Section 6)

You must serve a party wall notice at least 2 months before work starts (Section 1 and 2 works) or 1 month before (Section 6 works). If your neighbour does not agree, you will need to appoint a party wall surveyor. This typically costs £700–£1,500 per neighbour.

Build costs vary significantly by region, specification, and what the extension contains. As a rough guide for 2026:

These are build costs only. Budget an additional £5,000–£15,000 for professional fees including:

• Architect or designer: £1,500–£5,000
• Structural engineer: £300–£800
• Building regulations fees: £400–£900
• Planning application (if needed): £548
• Party wall surveyor (if needed): £700–£1,500 per neighbour

London and the South East typically sit at the higher end of these ranges. The North and Midlands are generally lower.

Read our full cost guide →

From first idea to completion, a straightforward single-storey rear extension typically takes 5–8 months:

Many of these stages can overlap. For example, you can submit building regulations drawings while waiting for a planning decision, and you can serve party wall notices while designs are being finalised.

Before spending money on a rear extension, consider getting a Lawful Development Certificate (LDC) if you are building under permitted development. An LDC is official confirmation from your council that your extension is lawful. It costs £274 to apply and is essential when you come to sell, remortgage, or insure the property.

Without one, your buyer’s solicitor will almost certainly raise questions — and that can delay or collapse a sale.

Read why you need an LDC →

1. Forgetting the 50% curtilage rule. Your extension plus all outbuildings, sheds, and previous extensions cannot cover more than half of the original garden. Many homeowners exceed this without realising.

2. Ignoring the “original house” definition. If a previous owner added an extension, that counts against your PD allowance. The limit is measured from the original house, not the house as you bought it.

3. Starting work before building regulations approval. Even if your extension is permitted development, you still need building regulations approval separately. Building without it creates problems when you sell.

4. Not serving party wall notices. If the Party Wall Act applies and you do not serve notice, your neighbour can seek an injunction to stop work.

5. Not getting an LDC. Building under PD without an LDC is fine legally, but causes headaches down the line with solicitors, mortgage lenders, and insurers.

1. Check your property using our free PD checker to see if your extension falls within permitted development
2. Read our PD rules guide to understand the limits in detail
3. Get an LDC to protect yourself when you sell — our PD Certificate Report can help