How Much Does a Double Storey Extension Cost in 2025?

Average double storey extension costs in 2025

Understanding the average cost of a double storey extension is the first step in planning your build. In 2025, most homeowners can expect to pay between £1,600 and £2,400 per square metre, but your total budget will depend on the size, design, location, and specification of your project.

Below, we break down what these figures look like in real terms.

Key takeaways:

• A double storey extension offers more value per m² than a single storey.
• £1,600–£2,400/m² is a solid benchmark for most standard double storey builds.
• Factors such as a London location, bespoke design, or high-end finishes can drive costs to the top end.

Total cost for 40m² vs 60m² double storey conversion

Here’s a simple breakdown of average total costs based on two standard extension sizes:

Double storey vs single storey: What’s the price difference?

A double storey extension costs 30–50% more overall than a single storey build, but offers better value per m².

A double storey extension makes better use of your build costs, especially if you're already extending foundations, the roofline, and utilities.

Types of double storey extensions

Double storey extensions offer a versatile way to significantly increase your living space, enhancing both the ground and upper floors. Depending on your property's layout and available space, several extension types can be considered.

Below, we explore the most common types of double storey extensions, their typical uses, and key considerations.

Rear double storey extension

Extending to the rear is a popular choice for homeowners seeking to expand their living space without altering the property's frontage. This extension typically adds substantial space to both floors, allowing for larger kitchens, dining areas, or additional bedrooms. It's essential to consider garden space reduction and potential planning permissions.

Best for: Properties with ample garden space; terraced, semi-detached, and detached homes.

Side double storey extension

A side extension utilises the often-underused area beside a property, expanding the home's width. This can create additional rooms or enlarge existing ones. Planning permission is typically required, and considerations include access routes and the potential impact on neighbouring properties.

Best for: Detached and some semi-detached properties with unused side space.

Wraparound (L-Shaped) double storey extension

Combining rear and side extensions, the wraparound creates an L-shaped addition, significantly increasing floor space. This is ideal for open-plan living areas and multiple new rooms. Due to its scale, it often requires careful planning to ensure compliance with building regulations and to minimise impact on neighbouring properties.

Best for: Maximising space in properties with both rear and side extension potential.

Over-structure (Above Garage) double storey extension

Building above an existing structure can be a cost-effective way to add upper-floor space without expanding the property's footprint. Structural assessments are crucial to ensure the existing building can support the additional load.

Best for: Properties with existing single-storey structures like garages.

Front double storey extension

Front extensions can enhance curb appeal and add functional space, but are subject to stricter planning regulations to maintain street character. They often require more detailed planning applications and neighbour consultations.

Best for: Properties set back from the road with sufficient front garden space.

Side return double storey extension

Utilising the side return can widen the property, allowing for more spacious interiors. While single-storey side returns are common, extending upwards adds even more value. Planning permission is typically required, and considerations include access to light and the potential impact on adjacent properties.

Best for: Victorian and Edwardian terraced or semi-detached homes with narrow side alleys.

What affects the cost of a double storey extension?

The final price of your double storey extension will depend on more than just its size. A range of practical and design-related factors can push your costs up or down. Here’s what to consider when planning your budget:

• Size and layout: Larger extensions cost more overall but may be better value per m². A straight, rectangular layout will be cheaper to build than complex L-shapes, stepped designs, or extensions that require reworking internal walls.
• Location (London vs regional costs): Labour and material costs in London and the South East are typically 10–25% higher than the national average. Rural or remote sites may also incur higher costs due to access and travel requirements.
• Roof type and structural changes: A simple pitched roof is more affordable than a flat roof with parapets or a complex hip-to-gable conversion. If the build affects existing rooflines or structural walls, expect added engineering and labour costs.
• Materials and finishes: Opting for brick to match your home is often more expensive than rendered blockwork. High-end glazing, bifold doors, underfloor heating, or designer tiles can significantly increase your fit-out costs.
• Access and site conditions: Limited side access, sloping ground, or drains running beneath the build area may require additional groundwork, machinery, or manual labour, all of which contribute to the final price.
• Design complexity and bespoke features: Custom layouts, vaulted ceilings, rooflights, or open-plan steel structures are desirable but costly. Anything outside the standard spec will push up both design and construction budgets.

Planning permission and building regulations for a double storey extension

When planning a double storey extension in the UK, it's essential to understand the planning permission requirements, permitted development rights, and building regulations that apply.

Do you need planning permission for a double storey extension?

In most cases, yes. You will need to obtain planning permission for a double storey extension. These extensions often exceed the limitations set by permitted development rights, particularly in terms of size, height, and proximity to boundaries. As a result, submitting a full planning application to your local planning authority is typically required.

What rules apply under permitted development?

While permitted development rights allow certain types of extensions without full planning permission, double storey extensions must meet strict criteria to qualify:

• Depth: The extension must not extend more than 3 metres beyond the original rear wall of the house.
• Height: The extension’s eaves and ridge height must not exceed those of the existing house.
• Proximity to boundaries: If the extension is within 2 metres of a boundary, the eaves height must not exceed 3 metres.
• Materials: The materials used must be similar in appearance to the existing house.
• Windows: Any upper-floor windows in a side elevation must be obscure-glazed and non-opening, unless the opening parts are more than 1.7 metres above the floor level.
• Land coverage: The extension must not cover more than 50% of the land around the original house.

It's important to note that permitted development rights do not apply to properties on designated land (such as conservation areas) or to listed buildings. In these cases, full planning permission is required, regardless of the extension’s size or design.

Building regulations and structural approvals

Regardless of whether your extension requires planning permission, it must comply with building regulations to ensure safety, energy efficiency, and structural integrity. Key considerations include:

• Structural stability: Ensuring the new extension is structurally sound and does not adversely affect the existing building.
• Fire safety: Incorporating appropriate fire-resistant materials and escape routes.
• Thermal performance: Meeting required standards for insulation and energy efficiency.
• Ventilation and drainage: Providing adequate airflow and suitable drainage systems.

You must submit a building regulations application either to your local authority or to an approved inspector. Additionally, if your extension affects a shared wall or boundary, you may need to serve a Party Wall Notice under the Party Wall Agreement etc. Act 1996.

How long does a double storey extension take to build?

Most double storey extensions take between 14 and 20 weeks from the first day on site to completion. The timeline typically includes the following core phases:

• Groundworks and foundations: 2–3 weeks.
• Structural build: 4–6 weeks.
• Roofing and external work: 2–3 weeks.
• Internal finishes (plumbing, electrics, plastering, etc.): 4–6 weeks.

However, delays can occur due to poor weather, supply chain issues, design changes during construction, or waiting for planning or building control approvals. Hiring a reliable contractor and securing all permissions before work begins can help avoid costly setbacks.

‍Typical total build time: 14 to 20 weeks.

Is a double storey extension worth it?

For many homeowners, a double storey extension is one of the best ways to maximise space and long-term value, especially if you’re already extending your foundations. While upfront costs are higher than a single storey extension, the price per square metre is significantly lower, as you’re spreading build costs like groundworks and roofing across two floors.

You’ll gain extra bedrooms, bathrooms, or even a home office loft conversion. All of which can boost resale value and future-proof your home. If you’re settled in your location and need more space, a double storey extension is often far more cost-effective than moving house or building two separate additions over time.

Double storey extension cost FAQs

Below, our experts have answered the most common queries UK homeowners ask. From how much you’ll need to budget to whether £50K is enough, and how long the build is likely to take. Whether you’re just exploring ideas or ready to start planning, these quick answers will help you make informed decisions.

How much does a 40m² double storey extension cost?

Expect to pay between £75,000 and £95,000 for a 40m² double storey extension in 2025. Pricing varies depending on your location, build complexity, and interior spec.

How much does a double storey extension cost in the UK?

On average, most double storey extensions cost £1,600 to £2,400 per m², meaning a typical 50m² build would range from £80,000 to £120,000. Costs are higher in London and the South East.

Is £50,000 enough for a double storey extension?

£50,000 is likely below the minimum for most double storey extensions in 2025. You may be able to fund a small, basic shell build outside of London, but expect to compromise heavily on size and finishes.

Is a double storey extension cheaper than a single storey?

Not in total cost, but yes per square metre. A single storey extension usually costs more per m² due to foundations and roofing covering just one floor. Double storey builds offer more value overall if you're adding multiple rooms.

Do I need planning permission for a double storey extension?

Yes, in most cases you will need planning permission on a double storey extension. Double storey extensions don’t fall under permitted development unless they meet strict height, boundary, and design rules. You’ll likely need full planning permission from your local authority.

How long does it take to build a double storey extension?

Typically, a double storey extension takes about 14 to 20 weeks to finish once construction begins. This timeframe covers everything from laying the groundwork to completing the structure, roofing, and interior touches. Keep in mind that planning and design phases might extend the timeline by a few months before construction starts.