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 www.ricsfirms.com to find a firm in your area.
You can get a more accurate bespoke build cost estimate for your self-build or extension project using the My Building Project Build Cost Estimating Service.
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.
How are the figures in the Build Cost Guide calculated?
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.
How do I measure my intended extension(s)?
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.
What do the figures include?
The cost per square metre figure allows for a finished extension, excluding kitchens, utility rooms, cloakrooms, bathrooms and shower rooms, and external landscaping.
Do the figures include VAT?
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 per cent. Some VAT concessions are made on certain categories of work – see HMRC VAT Notice 708 for details.
Notes on use
Extension costs vary across the UK and so it is important to enter the post code 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.
Although the cost of many of the construction elements such as the 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 house 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 (£350/1,000 or £50/m² laid), insulation, and 100mm blockwork; concrete interlocking tiles (£28/m² laid); 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 (£450/1,000 or £56/m² laid), insulation, and 100mm blockwork; clay machine-made tiles (£36/m² laid); 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 (£550/1,000 or £62/m² laid), insulation, and 100mm blockwork; plain clay tiles (£45/m² laid); hardwood joinery; blockwork partitions; bespoke kitchen; quality sanitaryware; UFH.
Your level of involvement in the project will influence the build costs. For simplicity, the four most common build routes have been identified below.
Mainly DIY plus some subcontractors: Building on a largely DIY basis, substituting around 30 per cent 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 home owner.
Size & Number of Storeys
Are You Building An Extension Above Ground Level?
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 discreet elements, e.g. side extension, rear extension, loft conversion, basement extension. The extension cost calculator allows you to add each of these elements separately.
How many storeys is this extension?
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.
The average build cost figures used by the Extension Cost Calculator take into account the relative saving of building additional storeys thus the per square metre costs used for single storey extensions are higher than for two or two and a half (roof space) storey extensions.
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. Multiple storeys also make better use of land/garden area – usually the most expensive/valuable element.
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.
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.
The average build cost figures used by the Extension Cost Calculator are based on three different size ranges that reflect the economies of scale achieved by constructing larger extensions.
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.
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 per cent 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 per cent 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 not retro basements beneath the existing property – the calculator allows you to enter this as a separate element of your project.
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 basement level.
The cost of constructing a basement is largely the cost of digging out and removing the spoil. This cost and the cost of waterproofing the structure, are mitigated to some extent by the fact that fewer windows are usually used that for above ground storeys and there is no external wall cladding (e.g. brick, or stone).
As ever the quality of fixtures and fittings will be a major factor in the final cost. There are instances where a basement makes better sense, for instance where ground conditions require very deep foundations. The cost of going down just a little further and excavating between the footings may not be that much greater. On a sloping site were the basement will only be partly below ground, it can make better use of the site that cutting or filling the slope to create a level area to build on.
The water table and ground conditions are another factor that will affect costs. If the sides of the excavation are likely to collapse, for instance in sand, and this will affect neighbouring structures, it may be necessary to create a retaining structure around the basement using concrete piling, either vibrated into place or cast in situ, and this will add to considerably to costs.
Basement walls are usually built in reinforced concrete, either precast or in-situ cast concrete, using shuttering for formwork, or hollow concrete blocks. There are many competing waterproofing systems, from textured membranes that trap ground water and direct it into a sump from where it is pumped away, to waterproof renders – sometimes both are applied together. There is little variance in costs between different systems.
Note: 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 extension.
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. The more complicated your floorplan, the less accurate the calculator results will be.
Angles other than 90 degree 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 (e.g. 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 per cent 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 per cent 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 to increase the floor area. The more complex your project the less accurate the calculator will be.
Enter habitable area of garage conversion
Converting existing garage space is a cost effective way to add space to a property. 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. The more complex your project the less accurate the calculator will be. If the garage requires substantial reconstruction or underpinning 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 per cent 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.
Add Basement Extension Beneath the Existing Property?
Enter gross internal area (GIA) of new basement space beneath the existing property in m2.
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 – where costs are lower - as a separate element of your project.
In high value areas it can make sound financial sense to extend beneath an existing property, underpinning the existing structure in the process. The cost of excavating beneath an existing property and supporting it whilst the structure is underpinning is expensive and the calculator uses a multiple of three times the average cost per square metre for an above ground extension.
The average build cost figures used by the Extension Cost Calculator assume straightforward ground conditions and a water table below basement level.
The cost of constructing a basement is largely the cost of digging out and removing the spoil. This cost and the cost of waterproofing the structure, are mitigated to some extent by the fact that fewer windows are usually used than for above ground storeys and there is no external wall cladding (e.g. brick, or stone).
As ever the quality of fixtures and fittings will be a major factor in the final cost.
The water table and ground conditions are another factor that will affect costs. If the sides of the excavation are likely to collapse, for instance in sand, and this will affect neighbouring structures, it may be necessary to create a retaining structure and this will add to considerably to costs.
Retro basement walls are usually constructed in reinforced concrete using shuttering for formwork. There are many competing waterproofing systems, from internal textured membranes that trap ground water and direct it into a sump from where it is pumped away, to waterproof renders – sometimes both are applied together. There is little variance in costs between different systems.
The cost of constructing attached or detached garage space is considerably lower than finished living space. The average build cost figures used by the Extension cost calculator for standard garage construction are as below. To avoid double counting, the area of an integral or attached garage should be deducted from the figure (m2) entered for ground floor area.
Do you have an estimate of the 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. A guide to demolition can be found here.
The standard allowances used by the Extension Cost Calculator for kitchen and utility room are below:
- Standard: £3,700
- Good: £6,800
- Excellent: £13,700
- Standard: £3,500
- Good: £4,700
- Excellent: £7,300
Don't forget the extra fees
Party Wall Fees (with each neighbour): £700-£900
Measured Survey of Existing Property: £600-£1,400
Topographical Site Survey: Typical cost £400-£800
Design Fees: Architects charge 7-15 per cent of the total build cost for a service involving design and supervision. For planning drawings from other sources expect to pay from £2,500-£3,500, plus a similar figure for building regulations drawings
Structural Engineers’ Fees: £400-£4,000 depending on scale of project
Planning Application Fees: Householder application currently costs £172. Check with www.PlanningPortal.gov.uk or your local authority what 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 £500-£1,000 for a small extension or conversion project.
Site Insurance: £500-£800. If you are moving out at an stage during the project, check whether you home insurer will cover the building during this period. If not, allow for Employer’s Liability, Public Liability, and Works Under Construction insurance. This is available as a combined policy and the premium payable is based on the value of the building contract.