How Big Can I Make My Extension?

Home extensions are a popular way to add space and increase property value. However, before starting a project, homeowners must understand the rules and limitations regarding how big an extension can be. In the UK, these regulations depend on factors such as planning permissions, permitted development rights, local council policies, and property type. This guide will help you navigate the key considerations when planning your extension.

Understanding Permitted Development Rights

In the UK, permitted development (PD) rights allow homeowners to extend their property without applying for full planning permission, provided they adhere to specific size and design restrictions.

General Rules for Permitted Development Extensions

• The extension must not exceed 50% of the land around the original house.

• Single-storey extensions must not extend beyond the rear wall of the original house by more than 3m for attached houses and 4m for detached houses.

• The maximum height for a single-storey extension must not exceed 4m.

• Double-storey extensions must not extend beyond the rear wall by more than 3m and should be at least 7m from the rear boundary.

• Side extensions must be single-storey, with a maximum height of 4m and no more than half the width of the original house.

• Materials used should match the existing house as closely as possible.

• No extensions are allowed in front of the principal elevation facing a highway.

Larger Home Extension Scheme

Under the Neighbour Consultation Scheme, homeowners can build larger extensions without full planning permission:

• Single-storey rear extensions can extend up to 6m for attached houses and 8m for detached houses.

• This applies to properties outside designated areas (such as conservation areas, national parks, and listed buildings).

• The local council must be notified, and neighbours are consulted before approval is granted.

Planning Permission for Larger Extensions

If your proposed extension exceeds the permitted development limits, you will need to apply for planning permission from your local council.

Factors Affecting Planning Permission

Size and Scale – Large or overly dominant extensions are less likely to be approved.

Impact on Neighbours – Extensions must not block light, overshadow, or invade privacy.

Design and Appearance – The extension should complement the existing property and surrounding architecture.

Use of Land – Extensions must not exceed 50% of the total land surrounding the original house.

Environmental Impact – Councils may require energy-efficient designs and sustainable materials.

Restrictions on Certain Properties

Some properties have tighter extension restrictions, including:

• Listed buildings – Require listed building consent and stricter regulations.

• Conservation areas – Extensions must maintain historical integrity and visual aesthetics.

• Flats and maisonettes – Do not benefit from permitted development rights.

Loft Conversions and Basement Extensions

Extensions aren’t limited to building outward; homeowners can expand upward with loft conversions or downward with basement extensions.

Loft Conversion Size Limits

• The maximum additional roof space allowed is 40m³ for terraced houses and 50m³ for detached/semi-detached houses.

• Roof extensions must not extend beyond the existing roof slope at the front.

• No raised platforms, balconies, or verandas are allowed without planning permission.

Basement Extensions

• Permitted development does not usually cover basement extensions.

• Planning permission may be required, especially for deep excavation or structural changes.

• Some councils impose additional regulations due to potential impact on foundations and drainage.

Building Regulations for Extensions

Even if planning permission isn’t required, building regulations approval is necessary to ensure safety and structural integrity.

Key Building Regulations Considerations

Structural Stability – Foundations must support the extension and existing structure.

Insulation and Energy Efficiency – Extensions must meet modern insulation standards.

Fire Safety – Fire-resistant materials and escape routes are mandatory.

Ventilation and Drainage – Adequate airflow and water drainage are required.

Electrical and Plumbing Work – Must comply with safety standards.

Failure to meet building regulations could result in enforcement action or difficulties selling the property in the future.

Costs and Budget Considerations

The cost of an extension varies based on size, materials, and location. Here’s a rough estimate:

• Small single-storey extension (15-20m²) – £20,000 to £40,000

• Larger single-storey extension (30-40m²) – £40,000 to £70,000

• Double-storey extension – £50,000 to £100,000+

• Loft conversion – £30,000 to £60,000

• Basement extension – £100,000+

Additional costs include architect fees, planning application fees, and building control inspections.

The size of your home extension depends on permitted development limits, planning permission requirements, and building regulations. While small single-storey extensions may not require formal approval, larger extensions, loft conversions, and basement expansions often need additional permissions. Consulting with an architect, builder, or local planning authority before starting your project ensures compliance and a smooth building process. Whether you’re extending for extra living space or adding value to your property, careful planning and adherence to regulations are key to a successful extension.

For more information you can call Express Party Wall Surveyor at 020 3633 0823 or contact us