More likely where the design is simple, access is reasonable, ground conditions are straightforward and fit-out is limited.
Extension cost guide
Extension Cost Guide UK
Cost per m² can help you start planning, but it does not explain the whole extension budget.
A house extension quote can change because of foundations, structural work, roof design, glazing, kitchen or bathroom fit-out, services, access, professional fees and what the builder includes. This guide explains those cost drivers so you can prepare a clearer budget before requesting quotes.
Quick planning answer
How much does a house extension cost in the UK?
As a planning guide, UK single-storey extension cost references often sit around £1,800 to £3,000 per m², with higher figures possible where the project has complex structure, premium specification, difficult access, glazing-heavy design or a kitchen and bathroom fit-out. That range should be used as context, not as a confirmed price.
The useful question is not only “what is the cost per m²?” A better question is: what does that figure include? A quote that includes foundations, structural steel, roof works, glazing, drainage, plastering, electrics, VAT and waste removal is not the same as a quote that leaves several of those items out.
More likely where the project includes normal specification, standard openings, typical services and a clearer quote scope.
More likely where the extension includes structural complexity, large glazing, kitchen fit-out, restricted access or a stronger contingency allowance.
Cost figures should be checked against suitable professional quotes. For more detail on how CostIntel handles assumptions, see our pricing data and how costs are calculated pages.
Extension type comparison
Extension types and how costs behave
The type of extension matters because the same floor area can involve very different work. A side return, kitchen extension and double-storey extension may all add space, but the cost drivers are not the same.
| Extension type | Cost behaviour | Main budget risks |
|---|---|---|
| Single-storey rear extension | Often easier to estimate than complex side or wraparound work, but still shaped by structure, roof, glazing and fit-out. | Foundations, large openings, roof design, drainage, kitchen fit-out and access. |
| Double-storey extension | Can offer better cost efficiency per m² because foundations and roof serve more floor area, but total spend is higher. | Design complexity, approvals, structure, staircase changes, bathrooms and upper-floor services. |
| Side return extension | Can be more complex than its size suggests because it often involves tight access and structural openings. | Party wall matters, drainage, rooflights, steelwork, glazing and narrow-site logistics. |
| Wraparound extension | Usually more complex because it combines rear and side extension work into one larger project. | Roof junctions, structural design, drainage, access, glazing and kitchen or living-space fit-out. |
| Kitchen extension | Often carries a higher allowance than a simple living-space extension because the shell is only part of the budget. | Cabinetry, worktops, appliances, plumbing, electrics, ventilation, flooring and lighting. |
| First-floor extension | Can depend heavily on whether the existing structure and foundations can carry the new load. | Structural checks, roof changes, access, design constraints and making good existing rooms. |
Cost per m²
Cost per m² is useful, but limited
Cost per square metre is useful for a rough first comparison. It becomes less useful when it hides fixed costs or makes two very different projects look similar.
A 20 m² extension and a 40 m² extension do not scale in a neat line. Some costs, such as design work, structural calculations, site setup, drainage changes and opening up existing walls, can appear on both projects. That is why a small extension can have a high cost per m² even when the total cost is lower.
The same applies to specification. A simple room extension and a kitchen extension might share the same footprint, but the kitchen project can add cabinetry, appliances, plumbing, ventilation, lighting, flooring and electrical work.
Main cost drivers
Main factors that affect extension cost
Extension costs move when the project needs more structure, more specification, more site preparation or more professional input. These are the areas to check before relying on any planning estimate.
Structure and groundworks
Foundations, ground condition, drainage and structural openings can have a large effect on the budget. Poor ground or large steelwork can make a modest extension more expensive than expected.
Roof, walls and openings
Roof shape, rooflights, glazing, doors, insulation and large openings all affect material choice, labour time and structural design.
Services and fit-out
Kitchens, bathrooms, utilities, heating, electrics, lighting and ventilation can add significant scope beyond the basic shell.
Access and site conditions
Tight access can slow deliveries, waste removal and machinery use. This can matter on terraced houses, side-return projects and urban sites.
Specification and finish
A basic finish, standard finish and higher-spec finish can produce very different quotes even when the extension footprint is similar.
Fees, VAT and contingency
Professional fees, approvals, VAT where applicable and contingency can change the total project budget. They may not all appear in a builder’s headline price.
Cost factor table
Extension cost factor table
Use this table before requesting quotes. It helps identify which parts of the project need checking, pricing or clearer specification.
| Cost factor | How it affects the budget | What to check before quotes |
|---|---|---|
| Footprint and layout | Size affects the base cost, but layout shape can also change structure and roof complexity. | Confirm floor area, shape, room use and whether existing rooms are being altered. |
| Foundations | Ground condition, depth and nearby drains or trees can affect foundation design and cost. | Ask whether foundation assumptions are included or provisional. |
| Structural steel | Large openings and open-plan layouts may need beams, calculations and careful installation. | Check whether structural engineer input and steel installation are included. |
| Roof design | Flat roofs, pitched roofs, rooflights and complex junctions can change labour and material allowances. | Confirm roof type, covering, insulation, drainage and rooflight specification. |
| Glazing and doors | Bifold doors, sliders, roof lanterns and large windows can be major cost items. | Check product allowance, installation, lintels, thresholds and finishing work. |
| Kitchen or bathroom fit-out | Fit-out can add cabinetry, appliances, sanitaryware, plumbing, electrics, extraction and tiling. | Confirm whether the quote includes supply, installation or only basic services. |
| Drainage and services | Moving drains, heating, electrics, water and waste pipes can change the scope. | Ask what has been allowed for existing drains, service routes and new connections. |
| Access | Restricted access can affect labour time, waste removal, deliveries and machinery. | Confirm how materials and waste will move through the site. |
| Ground condition | Unknown ground condition can create uncertainty before work starts. | Ask what happens if deeper foundations or extra preparation are needed. |
| Specification level | Finish level affects materials, detail work, labour time and quote inclusions. | Compare quotes against the same specification wherever possible. |
| Professional input | Design, structural calculations and specialist advice may sit outside the builder’s quote. | Check which fees are included and which must be budgeted separately. |
| Planning and building control | Some projects may need applications, approvals or inspections. | Check requirements with your local planning authority, building control or suitable professionals. |
| VAT | VAT can change the total cost if it is not included in the headline quote. | Ask whether all figures are inclusive or exclusive of VAT. |
| Contingency | Uncertainty can justify a planning allowance above the base estimate. | Decide whether the budget includes a realistic contingency for unknowns. |
| Quote exclusions | Excluded items can make a quote look cheaper than it is. | Ask for exclusions in writing and compare them carefully. |
Quote inclusions
What may be included in an extension quote
Extension quotes can vary because they include different levels of detail. A cheaper quote may not be cheaper if it leaves out items that another quote includes.
Structure
Foundations, walls, structural steel, roof structure, insulation and large openings.
Shell finish
Windows, doors, rooflights, plastering, basic electrics, heating points and making good.
Fit-out
Kitchen, bathroom, utility, flooring, decoration, lighting, appliances and ventilation.
Site costs
Waste removal, skips, access, welfare, protection, scaffolding, VAT and provisional sums.
Ask each contractor what is included, excluded and provisional. A headline price is easier to compare when the scope is written clearly.
Common missed costs
Costs people often miss when budgeting for an extension
Many extension budgets start with construction cost and miss the items around it. Some of these costs may be small on their own, but together they can move the total planning budget.
Design and technical input
Architectural drawings, measured surveys, structural engineer input, energy details and specialist advice may need a separate allowance.
Approvals and checks
Planning applications, lawful development certificates, building control, party wall matters and drainage checks may apply depending on the project.
Living disruption
Temporary kitchen arrangements, storage, protection, cleaning and disruption during works can affect practical planning.
Existing-room changes
Opening up walls, changing floors, moving radiators, repairing ceilings and making good existing rooms can add scope.
Finishes after the build
Decoration, flooring continuation, skirting, lighting upgrades and final detailing may not be fully included in a basic quote.
Outside repair work
Landscaping repair, patios, drainage channels, paths and garden reinstatement can be needed after construction access and excavation.
Approvals and checks
Planning, building regulations and professional checks
Some extensions may need planning permission. Some may fall under permitted development, subject to limits and conditions. Building regulations may still apply even when planning permission is not needed.
Party wall matters, structural changes, drainage, build-over issues and safety-critical work should be checked with suitable professionals where needed. CostIntel does not provide legal, planning, structural or building-control advice.
If there is doubt, check the project with your local planning authority, building control body or a suitable professional before relying on a planning estimate.
Budget planning
How to build an extension budget
A realistic extension budget should include more than the shell build. The safer planning approach is to separate the budget into core construction, fit-out, professional input, approvals, VAT where applicable and contingency.
Illustrative extension budget stack
Construction
Fit-out
Fees and checks
Contingency
Making good
This is not a percentage model. It is a visual reminder that the final budget is made from several categories, not one construction figure.
Use the Renovation Budget Planner to organise extension allowances before requesting quotes.
Quote comparison
How to compare extension quotes
Quote comparison should focus on scope, inclusions and assumptions, not headline price alone. A lower quote can become expensive if it leaves key items provisional or excluded.
| Quote item | Why it matters |
|---|---|
| Drawings and specification | Quotes are easier to compare when they are based on the same information. |
| Site preparation | Protection, strip-out and access arrangements can affect time and cost. |
| Groundworks | Foundations, drains and excavation can change once site conditions are known. |
| Structural steel | Large openings may depend on calculations, supply, lifting and installation details. |
| Roofing | Roof structure, covering, rooflights, insulation and drainage should be clear. |
| Glazing | Doors, windows, lanterns and sliders can vary greatly by specification. |
| Services | Electrics, heating, plumbing, ventilation and drainage may be quoted at different levels of detail. |
| Fit-out | Kitchens, bathrooms, flooring, decoration and lighting may sit outside a shell quote. |
| Waste removal | Skips, haulage and disposal can be included, excluded or provisional. |
| VAT | Compare prices on the same VAT basis so the totals are not misleading. |
| Provisional sums | Provisional sums can change when the final scope or product choice is confirmed. |
| Exclusions | Clear exclusions help avoid assuming that an item is included when it is not. |
| Timescale | A cheaper quote may not be useful if the timing, supervision or project plan does not suit the homeowner. |
| Payment schedule | Payment terms should be understood before work begins. Get suitable advice where needed. |
Alternative routes
When an extension may not be the right route
An extension may not be the best route if the cost, disruption, planning uncertainty or layout does not match the homeowner’s goals. In some cases, it may be worth comparing other scenarios such as a loft conversion, garage conversion, garden room, internal reconfiguration or moving home.
That comparison should be treated as planning only. CostIntel does not provide valuation, mortgage, tax, investment or legal advice.
Related CostIntel pages
Related CostIntel tools and guides
These pages help connect extension planning with the wider CostIntel estimating system.
Cost Guides
Browse guides for extensions, roofing, driveways and landscaping.
CostIntel Methodology
Learn how CostIntel handles assumptions, ranges and estimate limitations.
Pricing Data
Read how pricing assumptions and cost ranges are used across the site.
How Costs Are Calculated
See how user inputs, cost assumptions and contingency can shape planning estimates.
Renovation ROI Calculator
Compare project cost and possible uplift scenarios without treating the result as a formal valuation.
Renovation Budget Planner
Build a planning budget that includes allowances, contingency and quote checks.
More cost guides
Other home improvement cost guides
Roofing Cost Guide
Understand roof repair, replacement, access, scaffolding and condition-related cost factors.
Driveway Cost Guide
Compare driveway surface types, excavation, drainage, sub-base work and quote inclusions.
Landscaping Cost Guide
Plan garden works, paving, decking, planting, drainage, access and phased landscaping.
FAQs
Extension cost FAQs
How much does a house extension cost in the UK?
House extension costs vary by size, location, specification, structure, access and quote inclusions. As a broad planning guide, single-storey extension cost references often sit around £1,800 to £3,000 per m², with higher figures possible for complex or higher-specification work.
Is cost per m² a good way to estimate an extension?
Cost per square metre can help with early comparison, but it can hide fixed costs, structural work, fit-out, VAT, fees and quote exclusions. It should be treated as a starting point, not the full budget.
Why are extension quotes so different?
Extension quotes may include different groundworks, structural steel, glazing, roof details, fit-out, waste removal, VAT, provisional sums and exclusions.
Does a kitchen extension cost more than a basic extension?
A kitchen extension often carries a higher planning allowance because the kitchen fit-out can add cabinetry, worktops, appliances, plumbing, electrics, ventilation, flooring and lighting. Actual quotes depend on specification.
Do I need planning permission for an extension?
Some extensions may need planning permission and some may fall under permitted development, subject to limits and conditions. Homeowners should check with their local planning authority or a suitable professional.
Do building regulations apply to extensions?
Building regulations often apply to extension work, even where planning permission is not needed. Homeowners should check requirements with building control or a suitable professional.
What costs are often missed when budgeting for an extension?
Common missed costs include design fees, structural engineer fees, building control, party wall matters, drainage, VAT, contingency, decoration, flooring, making good and landscaping repair.
Are CostIntel extension estimates fixed quotes?
No. CostIntel extension estimates are planning estimates only. A builder or suitable professional must review the property, drawings, specification and site conditions before confirming a quote.