How Much Does a Double Storey Extension Cost in 2026?
Our expert guide explains what a double storey extension costs in 2025, why quotes differ by tens of thousands, and how to budget confidently before you speak to a builder.
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Our expert guide explains what a double storey extension costs in 2025, why quotes differ by tens of thousands, and how to budget confidently before you speak to a builder.
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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 £2,800 per square metre, but your total budget will depend on the size, design, location, and specification of your project.
The total cost of a double storey extension increases with size, but not always in a straight line. Fixed costs mean larger builds often offer better overall value.
A double storey extension costs 30-50% more overall than a single storey build, but offers better value per m².
A double storey extension costs 30-50% more overall than a single storey build, but offers better value per m². It makes smarter use of your investment, especially if you’re already paying to extend the foundations, roofline, and utilities.
A double storey extension is priced in stages rather than as one single figure. Each phase of the build carries its own costs, and understanding where your money goes helps explain why quotes can vary so widely between projects.
Our breakdown below shows how an average budget is typically distributed across a standard double storey extension.
Converting an existing single storey extension into a double storey typically costs £35,000 to £70,000+ in 2026. As a rule of thumb, conversions offer better value when the original extension was well built and properly approved. Where foundations or walls fall short, rebuilding may be the more sensible long-term option.
Whether a conversion is feasible depends almost entirely on the existing structure. Before any design work begins, a structural engineer will usually assess:
If the foundations are not adequate, it may still be possible to proceed, but underpinning or partial reconstruction is often required. This adds time, disruption and cost.
Two conversions of similar size can end up tens of thousands apart. The difference usually comes down to the amount of enabling work needed before the new storey can be built.
Common cost drivers include:
Converting an existing single storey extension makes sense when the foundations and walls are strong, properly approved and require minimal strengthening. In these cases, reusing the structure can reduce groundwork and keep costs lower.
Rebuilding from scratch is often a better value when foundations are weak, layouts are awkward or extensive reinforcement is needed, as starting again can offer more design freedom and fewer long-term risks. An early structural assessment is the quickest way to confirm which option is more cost-effective for your home.
In most cases, yes. Adding a second storey changes the property's height, massing, and external appearance, which usually requires planning permission.
Even where Permitted Development might apply in theory, councils assess two storey additions more strictly due to overlooking, daylight impact and the relationship with neighbouring properties.
It is best to assume planning permission will be required and factor this into both your timeline and budget.
A two storey side extension costs £65,000 to £110,000+ in 2026, depending on size, structure and access. It can offer strong value where side space allows, as it adds usable space on two floors without reducing garden size.
Costs rise where significant steelwork is needed, access is restricted, or the extension must tie carefully into the existing roof and neighbouring boundaries.
A side extension delivers better value per square metre than a rear build, particularly for adding bedrooms upstairs and expanding kitchens or living space below. Whether it is worth it depends on the width available, planning constraints and how much internal reconfiguration is required.
Average prices only tell part of the story. In reality, double storey extension costs can vary widely based on how your home is built, how complex the design is, and how easy the site is to work on.
The factors below explain where those differences come from and why two similar-looking projects can yield very different quotes.
The extension's size has the greatest impact on the total cost. Larger builds cost more overall, but they usually work out better value per square metre. Straight rear extensions are the most economical to construct, whereas L-shaped, wraparound, or stepped designs increase labour time, materials, and structural complexity.
Estimated cost impact: +£15,000 to £40,000 depending on footprint and layout.
Structural changes can quickly push costs beyond the headline figure. Removing load-bearing walls, installing large steel beams, forming wide-open-plan spaces, or building over drains all require engineering design and on-site labour. Homes with older construction methods often need more reinforcement than modern builds.
Estimated cost impact: +£5,000 to £20,000 depending on the level of intervention required.
Where and how the build takes place directly affects pricing. Labour and material costs in London and the South East are typically higher than the national average. Restricted access, party wall constraints, limited storage space, or the need for manual handling can all slow progress and increase labour hours.
Estimated cost impact: +10-25% on the total build cost.
The roof is a major cost variable. Simple pitched roofs are generally more cost-effective than flat roofs with parapets, complex hips or valley junctions. If the new extension requires alterations to the existing roof structure, additional steelwork, or awkward junction details, costs will increase accordingly.
Material choices affect both appearance and budget. Brickwork to precisely match the existing house is often more expensive than rendered finishes. Internally, features such as bifold doors, large glazed openings, underfloor heating, bespoke joinery, and premium bathrooms can significantly increase the final cost if not planned early.
Planning conditions, conservation considerations or party wall agreements can add professional fees and extend timelines. Additional drawings, reports or design revisions all carry costs before construction even starts, particularly in sensitive planning areas.
The more the extension alters the existing house, the higher the cost. Relocating kitchens or bathrooms, adjusting staircases, or reworking circulation spaces often involve labour, plumbing, and electrical work that are often underestimated at the early quote stage.
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.
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.
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.
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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.
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 a garage loft conversion.
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.
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.
When planning a double storey extension it's essential to understand the planning permission requirements, permitted development rights, and building regulations that apply.
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.
While permitted development rights allow certain types of extensions without full planning permission, double storey extensions must meet strict criteria to qualify:
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.
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:
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.
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:
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.
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.
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.
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.
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.
£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.
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.
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.
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.
