How Much Does a Double Storey Extension Cost?

Key Highlights: Double Storey Extension Costs

• Average cost per m²: £1,000–£2,500, depending on specification and location.
• Total cost for a typical 30m² extension: approximately £30,000–£75,000.
• Labour typically accounts for 40–50% of total project costs.
• A double storey extension can add up to 25% to your property's value, according to Nationwide research.
• Planning permission may not be required for rear extensions up to 3m depth under permitted development rights, though side extensions typically do need approval.

Double Storey Extension Cost Per m²

Understanding the cost per square metre — sometimes referred to as the double story extension cost — helps you set a realistic budget from the outset. Prices vary depending on the level of finish you choose and where you live in the UK, with London and the South East generally at the higher end of the scale.

Specification Level	Cost Per m²
Standard	£1,000–£1,500
Mid-range	£1,500–£2,000
High	£2,000–£2,500+

These figures cover the build itself, including foundations, structural work, and basic finishes. Costs for architect fees, planning applications, and interior fit-out are typically additional.

For comparison, a single-storey extension costs around £1,500–£2,000 per m², which means a double storey extension delivers roughly twice the floor space for a similar or lower cost per square metre. This is because the same foundations and roofing serve both floors, making it a more cost-efficient way to add space.

For a broader overview, see our guide to house extension costs. You can also learn about the different types of foundations used in building projects.

Total Cost of a Double Storey Extension by Size

Per-m² figures are useful for comparison, but most homeowners want to know how much does a double storey extension cost in total. The table below gives indicative total costs for common extension sizes, based on the £1,000–£2,500 per m² range.

Extension Size (Total Floor Area)	Estimated Total Cost
20m²	£20,000–£50,000
30m²	£30,000–£75,000
40m²	£40,000–£100,000
50m²	£50,000–£125,000

These are indicative figures. Actual costs depend on specification, location, site conditions, and the complexity of the design. Always get detailed quotes for your specific project before committing to a budget.

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Double Storey Side Extension Cost vs Rear Extension Cost

When planning a double storey extension, one of the first decisions is whether to build to the side or the rear of your property. Each option has different cost implications and practical considerations.

Side Extensions

A double storey side extension cost (also known as a double story side extension cost) can be higher than a rear extension for several reasons. Side extensions often require more structural work to integrate with the existing building, particularly where load-bearing walls are involved. If the extension is near a shared boundary, a party wall agreement is usually needed, adding to costs.

Planning permission is typically required for side extensions, which adds time and fees to the project. Side extensions can also reduce natural light to existing rooms, so you may need to budget for features like skylights or larger windows to compensate.

Rear Extensions

Rear extensions are generally simpler to build and may cost slightly less due to fewer structural challenges. In many cases, a two-storey rear extension can be built under permitted development rights, avoiding the need for a full planning application.

The main trade-off is garden space — a rear extension will reduce your outdoor area. However, features like bifold doors or open-plan layouts can help connect indoor and outdoor living spaces effectively.

Both options can work well depending on your property layout and priorities. An architect can help you assess which approach suits your home and budget.

Factors That Affect Double Storey Extension Costs

The cost of a double storey extension in the UK varies significantly from project to project. Understanding what drives costs up or down helps you plan more effectively and avoid surprises.

• Location: Labour and material costs are higher in London and the South East compared to other regions. Double storey extension costs in the UK can vary by 20–30% depending on where you live.
• Size and footprint: Larger extensions cost more overall, though the cost per m² may decrease slightly with scale.
• Design complexity: A simple rectangular extension is cheaper to build than an L-shaped or wrap-around design. Complex rooflines and structural features add cost.
• Ground conditions and foundations: Poor ground conditions may require deeper or more complex foundations, which increases costs significantly.
• Site access: Restricted access for machinery and deliveries can slow the build and increase labour costs.
• Specification and finishes: The level of finish — from standard to high-end — has a major impact on the overall budget. Premium kitchens, bathrooms, and flooring can add tens of thousands of pounds.
• Listed buildings or conservation areas: If your property is listed or in a conservation area, additional approvals and sympathetic materials may be required, increasing both time and cost.
• Structural alterations to the existing property: Removing or altering load-bearing walls, adding rolled steel joists (RSJs), or reconfiguring existing rooms to connect with the extension adds to the budget.

Labour Costs for a Double Storey Extension

Labour typically accounts for 40–50% of the total cost of a double storey extension. The exact amount depends on your location, the complexity of the project, and the tradespeople involved.

For a double storey extension, average labour costs fall between £1,200 and £1,800 per square metre, covering construction, electrical work, plumbing, and finishing trades.

Here is a breakdown of typical tradesperson rates:

Tradesperson	Typical Rate
Builder	£150–£250 per day
Electrician	£50–£120 per hour
Plumber	£35–£45 per hour
Plasterer	£30–£50 per hour
Decorator	£20–£50 per hour
Joiner	£35–£45 per hour
Roofer	£30–£40 per hour

To get an accurate picture of labour costs for your project, it helps to gather quotes from several builders. You can post your job on MyBuilder to receive interest from available extension builders in your area.

Material Costs for a Double Storey Extension

Materials typically account for 30–40% of the total project cost. The materials you choose affect not only the upfront budget but also long-term maintenance and energy efficiency.

Material	Typical Cost
Brick	£300 per 1,000 bricks
Concrete blocks	£1.50 per block
Roofing tiles	£0.90–£2.00 per tile
Timber	£25–£35 per m²

Choosing materials that match the existing property is important for both planning approval and visual consistency. Your builder or architect can advise on the most suitable options for your home's design and location.

Additional Costs to Budget For

Beyond the main build, there are several additional costs that can catch homeowners off guard. Planning for these from the start helps you set a more realistic budget.

Cost Item	Estimated Cost
Planning permission application	£200–£500
Building Regulations fees	£500–£1,500
Architect or designer fees	£1,500–£5,000+
Structural engineer fees	£500–£1,500
Party wall agreements	£700–£2,000
Utility upgrades (water, gas, electricity, drainage)	£1,000–£5,000
Structural changes (foundations, load-bearing walls)	From £2,000
Contingency fund	10–15% of total budget

Setting aside a contingency fund of 10–15% is strongly recommended. Unexpected issues — such as ground conditions or changes to the design during the build — are common with projects of this scale.

For more on application fees, see our guide to planning permission costs.

Planning Permission for a Double Storey Extension

Understanding the planning rules before you start can save time, money, and frustration. In many cases, a two-storey rear extension can be built under permitted development rights, meaning formal planning permission is not required. However, the extension must comply with specific rules:

• The height of the extension must not exceed the height of the existing property, including eaves and roof pitch.
• Where the extension is within 2 metres of a boundary, the eaves height cannot exceed 3 metres.
• The extension may project up to 3 metres from the original rear wall, provided there is at least 7 metres between the extension and the rear boundary.
• All extensions, outbuildings, and sheds combined must not cover more than half the total land surrounding the original house.
• Materials should closely match the appearance of the existing building.
• Any upper-floor side windows must use obscured glass and be fixed shut, unless the opening is at least 1.7 metres above the internal floor level.

Planning permission is required for side extensions, front extensions, and any rear extension that exceeds the limits above. If your property is in a conservation area, a national park, or is a listed building, additional restrictions apply.

For a two-storey extension, appointing an architect is strongly recommended. They can help maximise the potential of the space while ensuring the design meets all relevant regulations.

You can check whether your project falls under permitted development on the Planning Portal.

How Much Value Does a Double Storey Extension Add?

A double storey extension can significantly increase your property's value, particularly when it adds usable rooms such as bedrooms and bathrooms.

Research by Nationwide suggests that adding floor area through an extension can increase property value by up to 25%. Family homes tend to benefit most when extensions increase the bedroom count, and adding a bedroom and bathroom upstairs often delivers the strongest return.

That said, the actual return depends on several factors:

• The local property market — in areas with high demand, the uplift is typically greater.
• The quality of the build — a well-finished extension adds more value than a poorly executed one.
• Whether the extension suits the area — an extension that is in keeping with neighbouring properties is more likely to add value than one that looks out of place.

A double storey extension is generally considered one of the most effective home improvements for boosting property value, but it is worth speaking to a local estate agent for a realistic assessment before committing.

Extending vs Moving: Which Is Cheaper?

Whether to extend or move is one of the biggest decisions homeowners face, and in 2026 the financial picture makes this comparison particularly relevant.

Moving house comes with significant transaction costs that add up quickly:

Moving Cost	Estimated Cost
Stamp duty	Can run into thousands of pounds, depending on the purchase price
Estate agent fees	Typically 1–3% of the sale price
Conveyancing and legal fees	Additional cost on top of the above
Removal costs	Additional cost on top of the above

• Moneyweek suggests the average moving process can cost around £1,800 in direct fees alone, but when stamp duty and agent fees are included, the total can be far higher — often exceeding the cost of a well-planned extension.
• With higher mortgage rates and stamp duty costs in 2026, extending is increasingly attractive for homeowners who are happy with their location but need more space.
• That said, moving is sometimes the better option — for example, if you need to change area for work or schools, or if your property has limited scope for extension.

How to Save on Your Double Storey Extension

There are several practical ways to reduce costs without compromising on the quality of your extension:

• Build a simple rectangular footprint. Complex shapes such as L-shaped or wrap-around designs increase structural costs significantly.
• Build both storeys at once. Adding a second storey later means paying for foundations, scaffolding, and roofing twice. Building both floors together is far more cost-efficient.
• Keep plumbing above or near existing services. Positioning new bathrooms or kitchens close to existing pipework reduces the cost of plumbing runs.
• Choose standard window sizes. Bespoke windows are considerably more expensive than off-the-shelf options.
• Get multiple quotes. Comparing quotes from several builders helps you understand the market rate for your project. You can post your job on MyBuilder to receive interest from available builders in your area.
• Plan your project for autumn or winter. Builders may have more availability during quieter months, which can sometimes result in more competitive pricing.
• Handle cosmetic finishing work yourself. If you have the skills, tasks like painting, tiling, or landscaping can be done without hiring a tradesperson.

Thinking about a kitchen as part of your extension? See our guide to kitchen extension costs for more detail on budgeting.

Find extension builders near you

Find an Extension Builder on MyBuilder

Planning a double storey extension is a big project, and finding the right builder makes all the difference. On MyBuilder, you can post your job for free and receive interest from available extension builders in your area. Browse profiles, read feedback from previous customers, and choose who to work with — all in one place.

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FAQs About Double Storey Extensions

How Long Does a Double Storey Extension Take to Build?

A double storey extension typically takes 3–6 months to complete, depending on the size, complexity, and weather conditions. Larger or more complex projects — particularly those requiring significant structural alterations — may take longer. Allow additional time for the planning and design stages before construction begins.

Do I Need an Architect for a Double Storey Extension?

While it is not a legal requirement, appointing an architect is strongly recommended for a double storey extension. An architect can help you make the most of the available space, prepare drawings for planning applications, and ensure the design complies with Building Regulations. This is particularly important for more complex projects or properties in conservation areas.

How Much Does a Double Storey Extension Cost in the UK?

A double storey extension typically costs between £1,000 and £2,500 per square metre, with total project costs ranging from around £30,000 to over £100,000 depending on the size, specification, and location. London and the South East tend to be at the higher end of this range.

Is a Double Storey Extension Cheaper Than a Single Storey?

Per square metre of additional floor space, a double storey extension is generally more cost-effective than a single storey extension. This is because the foundations and roofing costs are shared across two floors, effectively giving you twice the space for a proportionally smaller increase in cost. Single-storey extensions typically cost £1,500–£2,000 per m², compared to £1,000–£2,500 per m² for a double storey build.

Do I Need Building Regulations Approval for a Double Storey Extension?

Yes. All double storey extensions require Building Regulations approval, regardless of whether planning permission is needed. Building Regulations cover structural safety, fire safety, insulation, ventilation, drainage, and electrical work. Your builder or architect can help ensure the project meets all requirements, and inspections will be carried out at key stages during the build.