Building Extension Costs Per Square Foot Calculator
Quick Answer
UK building extension costs typically range from £140 to £280 per square foot, depending on the type of extension, quality of finish, and location. A standard single-storey rear extension averages around £165 per sq ft, while a double-storey extension is closer to £190 per sq ft. These figures include labour, materials, and a typical contractor margin but exclude VAT, professional fees, and planning application costs. Use the calculator below to generate an instant estimate tailored to your project.
Extension Cost Calculator
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These figures are illustrative budgeting estimates based on synthesised market benchmarks. They include labour and materials but exclude VAT, professional fees, planning application charges, and Building Regulations inspection fees. Actual costs vary by site conditions, access, groundworks, and specific design requirements. This is not a quotation.
Extension Types & Cost Overview
The cost per square foot for a building extension in the UK varies significantly depending on the type of structure. A single-storey rear extension is the most common and generally the most affordable per square foot, while double-storey and wrap-around extensions command higher rates due to additional structural requirements, scaffolding, and complexity.
Below is a breakdown of typical costs by extension type, based on a standard finish in the Midlands region. Figures are illustrative budgeting ranges.
Extension Type
Typical Cost Per Sq Ft
Example: 250 Sq Ft
Example: 500 Sq Ft
Single-storey rear/side
£140 – £200
£35,000 – £50,000
£70,000 – £100,000
Double-storey
£160 – £240
£40,000 – £60,000
£80,000 – £120,000
Wrap-around
£150 – £220
£37,500 – £55,000
£75,000 – £110,000
Conservatory / Orangery
£70 – £130
£17,500 – £32,500
£35,000 – £65,000
Loft conversion (dormer)
£120 – £190
£30,000 – £47,500
£60,000 – £95,000
Garage conversion
£55 – £110
£13,750 – £27,500
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Double-storey extensions benefit from shared foundations and roof work, making the cost per square foot more efficient than building two separate single-storey structures. Conservatories and orangeries sit at the lower end, though adding underfloor heating, bespoke glazing, or masonry dwarf walls pushes costs higher.
Garage conversions are typically the most affordable because the structural shell already exists. Most of the spend goes on insulation, flooring, electrics, and replacing the garage door with a window or patio door.
What's Included in the Cost Per Square Foot
When a contractor quotes a rate per square foot for an extension, the figure usually covers the core construction elements. Understanding what is and is not included helps avoid budget surprises later. A typical extension rate includes:
Labour: All on-site trades — groundworkers, bricklayers, carpenters, roofers, plumbers, electricians, plasterers, and decorators.
Plant and equipment: Scaffolding, skips, mixers, and small tools.
Waste removal: Disposal of excavated soil and construction debris.
Contractor overheads and margin: Project management, insurance, and profit.
Items commonly excluded from the base per-square-foot rate and charged separately include:
VAT (typically 20% on most residential extension work)
Architectural design fees and structural engineer calculations
Planning application fees (where planning permission is required)
Building Regulations application and inspection fees
Party wall surveyor costs if the extension affects a shared wall or boundary
High-spec kitchen units, stone worktops, bespoke joinery, and premium flooring
Landscaping and external works
Always request a detailed written quotation that explicitly states what is covered, what is excluded, and whether VAT has been included in the figures.
Key Factors That Influence Extension Costs
No two extensions are identical, and several variables can shift the final cost well above or below the typical ranges. Being aware of these factors helps when comparing quotations and setting a realistic budget.
Ground Conditions and Foundations
Poor ground conditions — such as clay soil, tree roots, or a high water table — may require deeper or reinforced foundations. If the existing property has shallow footings, underpinning might be necessary, adding substantially to the groundwork cost.
Access and Site Constraints
Terraced houses with limited side access often require materials to be carried through the house or hoisted over the roof. Restricted access increases labour hours and may require hand-mixed mortar or smaller deliveries, pushing costs higher.
Structural Openings and Steelwork
Creating an open-plan layout between the existing house and the extension usually involves installing structural steel beams. The size, number, and complexity of steel supports directly affect both material and installation costs.
Roof Design
A flat roof with a simple EPDM or felt covering costs less than a pitched tiled roof. Adding rooflights, lanterns, or a green roof increases the per-square-foot rate. Complex roof geometries require more skilled labour and additional structural calculations.
Glazing and Bi-fold Doors
Large aluminium bi-fold or sliding doors are a popular feature in modern extensions. A 4-metre aluminium bi-fold door set can cost £3,500–£6,500 installed, significantly raising the average cost per square foot for smaller extensions.
Internal Finish Level
A basic trade finish (plastered walls, concrete floor ready for floor coverings, basic electrics) sits at the lower end. A fully finished extension with engineered wood flooring, underfloor heating, recessed lighting, and bespoke cabinetry sits at the premium end of the pricing spectrum.
Regional Pricing Differences Across the UK
Where you live has a measurable impact on extension costs. Labour rates in London and the South East are typically 20–30% higher than in the North of England, Scotland, or Northern Ireland. Material costs vary less dramatically but delivery charges and local merchant pricing do create some regional variation.
Region
Approximate Cost Multiplier
Single-Storey Range (per sq ft)
London & South East
1.20 – 1.30
£170 – £260
South West & East of England
1.00 – 1.10
£145 – £220
Midlands
0.95 – 1.05
£135 – £210
North of England
0.85 – 0.95
£120 – £190
Wales
0.85 – 0.95
£120 – £190
Scotland
0.85 – 0.98
£120 – £195
Northern Ireland
0.80 – 0.90
£110 – £180
These multipliers reflect broad market benchmarks and should be treated as illustrative. A high-spec extension in Manchester can still cost more per square foot than a basic extension in outer London. Contractor availability, local competition, and the specific postcode all influence the final price.
Planning Permission & Building Regulations
Many single-storey rear extensions fall within permitted development rights, meaning formal planning permission is not required — provided the extension meets specific size, height, and boundary criteria. However, all extensions must comply with Building Regulations, regardless of whether planning permission is needed.
A planning application fee in England currently stands at £258 for a full householder application (as set by the relevant planning authority and subject to change). Building Regulations application charges vary by local authority and the scope of work; a typical full plans application for a domestic extension costs between £250 and £550, with an additional inspection fee.
If your property is listed, in a conservation area, or subject to an Article 4 direction, permitted development rights may be restricted. Always confirm with your local planning authority or a qualified planning consultant before committing to a design.
Important
Even if your extension qualifies as permitted development, obtaining a lawful development certificate from the local authority provides formal confirmation of the legal status. This is valuable when selling the property and for Building Regulations records.
Example Cost Scenarios
The following scenarios are illustrative examples designed to show how costs vary with different project parameters. They are not quotations from specific completed projects.
Scenario 1: Standard Single-Storey Rear Extension — Midlands
Area: 270 sq ft (approx. 25 sq m)
Type: Single-storey, pitched tiled roof, one rooflight, 3m bi-fold doors
Finish: Standard, including mid-range kitchen fittings
Estimated cost: £42,000 – £52,000 (approx. £155–£193 per sq ft)
Excludes: VAT, professional fees, planning and Building Regulations charges
Scenario 2: Double-Storey Side Extension — London
Area: 430 sq ft (approx. 40 sq m) across two floors
Type: Double-storey side extension, flat roof with lantern
Finish: Premium, underfloor heating, aluminium windows
Estimated cost: £90,000 – £120,000 (approx. £209–£279 per sq ft)
Excludes: VAT, professional fees, planning and Building Regulations charges
Scenario 3: Garage Conversion — North of England
Area: 180 sq ft (approx. 16.7 sq m), integral garage
Estimated cost: £10,500 – £17,000 (approx. £58–£94 per sq ft)
Excludes: VAT, Building Regulations fees
Common Mistakes That Increase Costs
Skipping a detailed site survey before design. Undiscovered drains, manholes, or boundary discrepancies can force expensive design changes mid-project.
Assuming all quotations are comparable. One contractor may include plastering, skirting, and decoration; another may price to a shell finish only. Always check what is covered line by line.
Ordering materials too late. Lead times for steel beams, bespoke glazing, and specialist roof systems can delay the programme, increasing site overheads and labour costs.
Under-budgeting for the internal fit-out. The base build cost often excludes floor coverings, lighting, decorating, and joinery. A separate contingency of 10–15% for these items is sensible.
Overlooking VAT. Most residential extension work attracts 20% VAT. On a £60,000 build, that is an additional £12,000. Reduced-rate VAT may apply in limited circumstances, such as converting a property back from commercial to residential use.
Not checking whether the existing boiler and heating system can handle the additional space. Upgrading the boiler or adding extra radiators adds cost that is often forgotten at the budgeting stage.
Money-Saving Tips for Your Extension
Keep the footprint simple. Square or rectangular floor plans with a straightforward roof design cost less to build than complex shapes with multiple corners, setbacks, or split levels.
Align with standard material sizes. Designing dimensions around standard plasterboard sheets, timber lengths, and brick module sizes reduces cutting waste and labour time.
Source a reputable contractor through personal recommendation. Three to four detailed written quotations from local builders with verifiable recent work provide a realistic market comparison.
Consider a package deal where appropriate. Some firms offer design-and-build contracts that bundle architectural services with construction, potentially reducing overall project cost and simplifying communication.
Retain some finishing tasks if you have the skills. Painting, decorating, and basic landscaping can be done after the main contractor leaves, saving on labour costs — but only if you are genuinely competent and have the time.
Plan the project for a quieter season if possible. Some contractors offer more competitive rates during slower periods, typically late autumn or early in the year, though weather can be a constraint for groundworks.
How We Estimated These Prices
The pricing ranges presented in this article are synthesised from publicly available UK construction cost data, including published building cost guides, quantity surveyor benchmarks, and contractor pricing indices. Where exact standardised data is unavailable, figures are presented as illustrative budgeting estimates rather than official national averages.
Cost ranges reflect typical domestic extension projects of standard construction — cavity masonry walls, concrete strip foundations, timber or engineered roof structures, and mid-range internal finishes. Premium materials, complex ground conditions, listed building requirements, and projects in central London postcodes may exceed the upper end of the ranges shown.
Prices were reviewed for consistency and reflect a reasonable market picture at the time of publication. Individual quotations will vary based on site-specific factors, contractor workload, material availability, and location. Always obtain multiple written quotations and confirm exactly what is included before committing to a contract.
Frequently Asked Questions
A single-storey extension typically costs between £140 and £200 per square foot for a standard finish in most parts of the UK. In London and the South East, the range shifts to approximately £170–£260 per sq ft. The final figure depends on ground conditions, roof type, glazing choices, and internal specification.
Most contractor quotations for residential extensions are provided exclusive of VAT. At the standard rate of 20%, VAT adds a significant amount to the total project cost. Always check whether the figures you are comparing include or exclude VAT. Some specialist work, such as converting a qualifying disabled person's property, may qualify for zero-rated VAT, but this requires specific conditions to be met.
Yes, in terms of cost efficiency. While a double-storey extension costs more in total, the per-square-foot rate for the additional upper floor is lower because the foundations, main structural walls, and roof are shared. A double-storey extension typically ranges from £160 to £240 per sq ft overall, which is more floor area for a proportionally smaller incremental cost compared to building two separate single-storey structures.
Multiply the internal floor area (in square feet) by the applicable cost per square foot for your extension type and location. For example, a 300 sq ft single-storey extension in the Midlands at £165 per sq ft gives a base build estimate of £49,500. To this, add VAT (if applicable), professional fees (typically 8–15% of the build cost for architect and structural engineer combined), planning and Building Regulations fees, and a contingency of 10–15% for unforeseen items and internal finishes.
A dormer loft conversion typically costs between £120 and £190 per square foot, depending on the size, number of dormers, en-suite inclusion, and structural alterations required. Velux-style rooflight conversions without dormers sit at the lower end of this range. Loft conversions in London can reach £200–£240 per sq ft.
Generally, yes. A conservatory with a polycarbonate or glass roof and basic dwarf walls costs between £70 and £130 per sq ft. However, upgrading to an orangery-style structure with a solid roof, brick pillars, underfloor heating, and high-performance glazing brings the cost much closer to — or even above — a standard single-storey extension. The key difference is the proportion of glazing to solid wall and the thermal performance specification.
Architectural design fees for a domestic extension typically range from 8% to 12% of the construction cost, depending on the level of service — from outline design and planning drawings through to detailed construction drawings and contract administration. Structural engineer fees add a further £400–£1,200 depending on complexity. A party wall surveyor, if required, may cost £800–£1,500 per neighbour affected. Including these items, total professional fees often fall between 10% and 15% of the build budget.
Written by: Bhumi Calculator Editorial Team
Last Updated: 26 July 2026
Bhumi Calculator Editorial Team researches and maintains informational guides using publicly available data, industry references, and relevant sources. Pricing information is reviewed for consistency and updated when appropriate, but actual costs may vary by location, market conditions, and individual circumstances. This article does not constitute professional advice. Always obtain multiple written quotations and consult qualified professionals for your specific project.
This website provides general informational content and illustrative cost estimates. It does not constitute legal, financial, or professional advice. Always consult qualified professionals — including architects, structural engineers, quantity surveyors, and appropriately registered contractors — for advice specific to your project. Pricing information is based on publicly available data and synthesised market benchmarks; actual quotations will vary based on site conditions, location, material choices, and contractor availability.