Use our free extension cost calculator to find out how much your build will cost – get your answer in just a few minutes

Looking to extend? Then our free extension cost calculator will help you get an idea of costs for your build. And it takes a matter of minutes to do.

Before you go ahead and start building an extension you'll need to know how much the project is likely to cost before you commit.

That's where our free extension cost calculator can help you. By answering questions about your build in a few simple steps, our extension cost calculator will use the information provided to build a quote for you, based on the information you provide.

How to use our extension cost calculator

Lots of factors will affect the price of your quote so our extension cost calculator will ask for information on things like the size of your build, the shape, the build route, the materials you'll likely use and who will project manage it.

To answer some of the questions you'll need to have some key measurements in m² to hand, for example the gross internal area (GIA) of the ground floor of this element of your project.

Easy and simple to use, our extension cost calculator will give you an estimate for your project straight to your inbox.

Extension cost calculator

Terms of use

Using the extension cost calculator is no substitute for professional advice and judgement, particularly where a property or design proposals have unusual features or a very high specification. Professional advice on build costs can be obtained from a quantity surveyor – visit ricsfirms.com to find a firm in your area.

Future plc will not be under any legal liability in respect of any mis-statement, error or omission contained within the data used by the extension cost calculator, or the reliance any person may place thereon.

These terms and conditions are guaranteed by and constructed in accordance with English law with English courts having exclusive jurisdiction.

Extension costs vary across the UK and so it is important to enter the postcode for the property which is to be extended. Material costs tend to vary only marginally across the UK reflecting the cost of delivery over larger distances in more remote locations.

Labour rates vary significantly, with Central London being the highest, followed by Greater London, Home Counties etc. The lowest labour costs are in Northern Ireland, Wales the North East and North West.

We go into further detail below about the other variations and provide useful information on how to complete the form and understand the full cost breakdown, whatever type of house extension you are planning.

Build quality cost variations

Although the cost of many of the construction elements such as the type of foundations, and superstructure do not vary at all based on quality, the cost of external roof and wall cladding, windows and doors, flooring, fixtures and fittings, and renewables can potentially double the cost of building the same extension compared to a standard specification.

The average build cost figures used by the extension cost calculator are based on three levels of quality, Standard, Good and Excellent.

• Standard This represents a basic build quality equivalent to that offered by most speculative developers. Cavity walls: facing bricks, insulation, and 100mm block­work; concrete interlocking tiles; standard softwood joinery; studwork partitions; contract kitchen; basic sanitaryware; and radiator central heating.
• Good This is equivalent to that offered by quality developers. Cavity walls: facing bricks, insulation, and 100mm blockwork; clay machine-made tiles; high-end off-the-shelf softwood joinery; blockwork partition walls; top-of-the-range contract quality kitchen; quality sanitary­ware; and underfloor heating (UFH) downstairs.
• Excellent A very high standard. Cavity walls: bricks, insulation, and 100mm blockwork; plain clay tiles; hardwood joinery; blockwork partitions; bespoke kitchen; quality sanitaryware; UFH.

Build route cost variations

The build route you opt for will also play a part in the overall cost. If you are deciding whether or not to project manage your extension, knowing the difference in costs might help to make your decision.

• Mainly DIY plus some subcontractors: Building on a largely DIY basis, substituting around 30% of labour costs with DIY, and employing help with the rest of the building work. Materials purchased directly.
• Directly employed subcontractors: Building using trades­people hired directly. Minimal DIY involvement. Most materials purchased directly.
• Main contractor then self-managed subcontractors: Building using a main contractor to complete the structure to a weathertight stage, with the remaining work being under­taken by subcontractors with most materials purchased by the homeowner direct from suppliers.
• Main contractor: Building using a main contractor. Building in this way requires the least involvement from the homeowner.

Size and number of storeys

Most extensions are at ground level. If your project includes a loft, cellar or garage conversion, or a basement extension, you can produce a separate calculation for each element.

Many home improvement projects include multiple elements, e.g. side extension, rear extension, loft conversion, basement extension. The extension cost calculator allows you to add each of these elements separately.

Enter the number of storeys above ground for this element of your extension e.g. a single-storey extension has one storey, a two-storey extension has two storeys: ignore basements and loft conversions – the calculator allows basement extensions and loft conversions to be entered as separate elements.

Two of the largest fixed costs in constructing an extension are the groundworks/foundations/slab and roof. The specification and cost for a set footprint and roof layout will not vary whether the design is single storey, two storey, or three storey, yet the overall area of the design will increase substantially, spreading out the cost.

The average cost per square metre for a single-storey extension is therefore greater than for a two-storey extension with the same footprint (all other things being equal) whilst a three-storey design, or a two-and-a-half-storey design using the roofspace will be even better value for money in terms of lower average cost per square metre.

The savings are primarily in the cost of building the superstructure, especially the foundations, walls and roof. The cost/m² of gross floor area for external walling elements decreases as the wall/floor ratio decreases. Other one-off unit costs are spread out further on a larger extension. To maximise efficiency, the design needs to be kept simple and avoid unusually large spans for floor joists/beams, rafters etc.

TOP TIP If you want to get the maximum value from your budget, it will usually pays to build as many storeys as possible. On a very expensive site in a higher-value area, it can also pay to build down and construct a basement storey.

Entering area measurements in the cost calculator

• Enter area of the ground floor (GF): Enter the gross internal area (GIA) of the ground floor of this element of your project in m².
• Enter area of the first floor (FF): Enter the gross internal area (GIA) of the first floor of this element of your project in m². Some projects include an extension above an existing single-storey garage or other existing part of a property – this is why the calculator allows separate entries for each storey.
• Enter habitable area of the second floor (SF) (excluding rooms in roof): Enter the gross internal area (GIA) of the second floor of this element of your project in m².
• Enter new habitable roofspace area (R): Enter the gross internal area (GIA) of usable (habitable) roofspace for this element of your project in m². Do not include the area of the existing roof to be converted – the calculator allows you to enter this as a separate element of your project. The average build cost figures for habitable roofspace are reduced by 10% reflecting the fact that part of the cost of constructing the roof is already allowed for in constructing the storey below. The usable area of a roof will depend on the height of the ridge and pitch of the roof. Typically an area of 60-70% of GIA of the storey below is suitable for conversion.
• Enter new basement area (B): Enter the gross internal area (GIA) of basement space for this element of your project in m². This is the GIA of new basements below new extensions and gardens and not retro basements beneath the existing property – the calculator allows you to enter this as a separate element of your project (see below). The average build cost figures used by the extension cost calculator assume the same costs per square metre for constructing a full basement as used for calculating ground floor space. This assumes straightforward ground conditions and a water table below basement level.

Plan, shape and layout

The average build cost figures used by the extension cost calculator assume a relatively simple rectangular or square floorplan for each house extension idea.

Complexity of design is one of the biggest factors affecting build costs. The simplest and most cost-effective floorplan is square. Compared to a square plan, a rectangular plan requires more external and internal wall for the same floor area, thus increasing costs. The longer and narrower the plan, the less efficient it is to build (and the less accurate the calculator results will be).

Building straight walls is cheaper than building corners, as it slows down the build process adding to labour costs. Consequently the more angles introduced into an extension floorplan, the greater the cost, so a simple design with few corners is cheaper to build.

Angles other than 90° right angles are even more time consuming to set out and build and so will increase costs further. Curved walls are amongst the most expensive to build of all. The more complicated your floorplan, the less accurate the calculator results will be.

Converting part of the building (loft, garage, cellar)

Converting existing loft, garage or cellar space is usually more cost effective than a new above ground extension. The calculator reduces the average cost per square metre (£/m²) by 10% to reflect the economies of using the existing structure.

• Enter habitable area of loft conversion Converting existing roof space is one of the most cost-effective ways to add space, assuming the existing roof is suitable for conversion. The calculator reduces the average cost per square metre for new extensions (£/m²) by 10% to reflect the economies of using the existing structure. The average cost per square meter will depend on the complexity of alterations required to the existing roof structure, and the additional volume added to the roof with your chosen type of loft conversion to increase the floor area.
• Enter habitable area of garage conversion The calculator reduces the average cost per square metre for new extensions (£/m²) by 10% to reflect the economies of using the existing structure. The average cost per square meter will depend on the complexity of alterations required to the existing structure and assumes the walls and roof are structurally sound and suitable for conversion. If the garage requires substantial reconstruction or underpinning it can significantly increase your garage conversion costs. In this situation, it is often more cost effective to demolish the existing structure and to replace it with a new extension built to current Building Regulations.

• Enter habitable area of cellar conversion Converting existing cellar space is a cost-effective way to add space to a property providing there is sufficient existing headroom after allowing for the build up of the floor to incorporate insulation and damp proofing. The calculator reduces the average cost per square metre for new extensions (£/m²) by 10% to reflect the economies of using the existing structure. The average cost per square meter will depend on the complexity of alterations required to the existing structure and assumes the walls and floor are structurally sound and suitable for conversion. The more complex the alterations required – such as adding light wells, or reducing the basement floor level and underpinning the existing walls of the property, the less accurate the calculator will be.
• Enter gross internal area (GIA) of new basement space beneath the existing property This is the GIA of new retro basements beneath the existing property. The calculator allows you to enter new basements constructed below a new extension or under the garden (see above) – where costs are lower – as a separate element of your project. The cost of excavating beneath an existing property and supporting it whilst the structure is underpinned is expensive and the calculator uses a multiple of 3 x the average cost per square meter for an above ground extension. The average basement conversion costs used by the extension cost calculator assume straightforward ground conditions and a water table below basement level.

Do bear in mind for all of the above, that the more complex your project is, the less accurate the calculator will be.

Garage extension

The cost of constructing attached or detached garage space is considerably lower than finished living space. To avoid double counting, the area of an integral or attached garage should be deducted from the figure (m²) entered for ground floor area.

Cost of demolitions/knock through

Most extension projects involve alterations or demolition works to the existing property to either make way for the new additions, or to link them to integrate the new space with existing.

Enter the estimate cost of the demolitions works necessary to implement the design scheme. Our guide to knocking down internal walls has useful information to help with this.

Extra fees

• Party wall fees (with each neighbour): £1,500-£3,000 per neighbour
• Measured survey of existing property: From around £800, but average costs are in the region of £1,400 and £1,500.
• Topographical site survey: Typical cost £500 to £2,000 upwards for more complicated sites.
• Design fees: For design-only services, average costs are around 3-7% of your extension build costs. For full project management services, fees increase to 8-15% of build costs.
• Structural engineers’ fees: From £500, but average costs for a small residential project are likely to be closer to £1,000. Expect to pay more for a more larger, more complex project.
• Planning application fees: Householder application currently costs £613 in England. Check with your local authority for the relevant fees are for your application.
• Building Regulations fees: Fees will vary according to the size of your extension and the type of project. Check with your local authority building control department, or an independent approved inspector. Allow an average of £1,200.
• Site Insurance: Your current home insurance policy may not cover building work to the house. Check what you will be covered for, but for any significant work it's worth taking out a specific self-build insurance policy to cover you for all eventualities. Prices will vary depending on the size and duration of the project, but budget from £1,500.

Final considerations

• Figures are based on a survey of all the case studies that appeared in the magazine, adjusted for our own experiences and knowledge. These base figures are then updated monthly using the Rebuilding Cost Guide indices published by the Building Cost Information Service.
• Gross internal floor area is the measure we use (the industry standard). It’s the area measured to the internal face of each external wall for each floor level, including areas occupied by internal walls. Calculate the area for each floor of each element of your proposed extension project.
• The cost per square metre figure allows for a finished extension, excluding kitchens, utility rooms, cloakrooms, bathrooms and shower rooms, and external landscaping.
• VAT will be applied to all goods and services supplied by VAT registered building contractors or subcontractors, and by suppliers, at the prevailing rate, usually 20%. Some VAT concessions are made on certain categories of work – see HMRC VAT Notice 708 for details.

Once you have received your results from our extension cost calculator, you'll have a better idea of your final project budget. If costs are looking to be more than you expected, there are several ways you can try and reduce the cost of your extension.

One key reminder is that whether you're adding a glass extension, a side return extension or a pitched roof extension, always make sure you get at least three quotes for your project so you can compare costs and get the best extension for your budget.