A single storey extension in Birmingham typically costs £1,800 to £2,800 per m² to build in 2026, before VAT and fees. For a 20m² rear extension, that means roughly £36,000 to £56,000 for the build alone. Once you add a new kitchen, glazing upgrades, professional fees, a contingency and VAT, the real budget is usually much higher than the headline rate suggests.
This guide explains what drives the single storey extension cost in Birmingham, what a per m² price does and does not include, and how to build a realistic budget before you speak to a builder. It also covers kitchen extensions, bifold doors and the hidden costs that often appear once work starts.
Key Facts at a Glance
| Question | Short answer |
|---|---|
| Typical build cost per m² in Birmingham | £1,800 to £2,800, before VAT and fees |
| Build cost for a 20m² rear extension | Roughly £36,000 to £56,000 |
| How the West Midlands compares | About 7% below the UK average, according to BestBuilders' 2026 figures |
| Householder planning application in England | £548 from 1 April 2026 |
| Lawful development certificate | £274, half the householder fee |
| Contingency to allow | 10%, or 15% for older homes |
| VAT on most extension work | 20% |
| Time on site for a 20m² extension | Around 14 to 18 weeks |
How Much Does a Single Storey Extension Cost in Birmingham?
Most single storey extensions in Birmingham fall between £1,800 and £2,800 per m² for the build. That range comes from 2026 guides published by the Federation of Master Builders (FMB) and Build It, which put most of the UK at £1,800 to £3,000 per m². BestBuilders' 2026 regional data places the West Midlands about 7% below the UK average, which pulls the top of the range down slightly for Birmingham.
The table below turns those rates into guide figures for common sizes. These cover the build only. They do not include VAT, professional fees, a new kitchen or decoration.
| Extension size | Typical use | Guide build cost |
|---|---|---|
| 12m² (about 3m x 4m) | Small kitchen or utility extension | £22,000 to £34,000 |
| 18m² (about 3m x 6m) | Rear kitchen-diner on a semi | £32,000 to £50,000 |
| 24m² (about 4m x 6m) | Larger open-plan kitchen-diner | £43,000 to £67,000 |
| 30m² | Full-width or wraparound extension | £54,000 to £84,000 |
Where your project lands in these ranges depends on the ground, the roof, the glazing and the level of finish. Our single storey extension service starts with a site survey for this reason. The ground under the plot is checked before any price is agreed.
Why Do Smaller Extensions Cost More per m²?
Some costs stay much the same whatever the size. Drawings, structural calculations, building control fees, skip hire and setting up the site cost almost as much for 12m² as for 24m². On a small extension, those fixed costs are shared across fewer square metres. As a result, a compact extension often sits at the top of the per m² range, even if the total is lower.
How Does Birmingham Compare With the Rest of the UK?
Location has a real effect on price, mainly through labour rates. If you have been reading national guides on rear extension cost in the UK, figures from London and the South East can make a Birmingham project look more expensive than it is.
| Region | Average cost per m² (standard spec, 20m²) | Compared with UK average |
|---|---|---|
| West Midlands (Birmingham) | £1,900 | About 7% lower |
| UK average | £2,050 | Baseline |
| Greater London and Home Counties | £2,400 | About 15% higher |
| Inner London | £2,750 | About 30% higher |
The table shows how the average rear extension cost in the UK changes by region. These are averages, not quotes. A well-specified Birmingham extension with large glazing and a high-end finish can still cost more than a basic one in London.
Single Storey Extension Cost per m²: What Does the Rate Include?
A single storey extension cost per m2 figure is a useful starting point, but two builders can quote the same rate and mean very different things. Before you compare prices, check what sits inside the rate and what sits outside it.
What the Rate Usually Covers
- Foundations and groundworks for normal ground conditions
- External walls, insulation and the roof structure
- Standard windows and an external door
- First fix electrics and plumbing, then plastering
- Connection to existing drainage
Extensions must meet current building regulations on insulation, which means walls, roof, floor and glazing all have minimum standards. If you are already opening up part of the house, it can be a good time to improve the rest of it too. Our guide to lowering energy costs and improving home comfort explains which upgrades fit well alongside building work.
What the Rate Often Leaves Out
- VAT at 20%
- Architect or designer fees and structural calculations
- Planning and building control fees
- A new kitchen, bathroom or utility fit-out
- Bifold doors, roof lanterns and other glazing upgrades
- Flooring, decoration and new furniture
- Party wall costs, if they apply
Extension Build Cost Breakdown
An extension build cost breakdown helps you see where the money actually goes. It also makes it easier to compare quotes line by line rather than only on the final total.
The Build Itself
The build usually takes the largest share of the budget. It covers digging and pouring foundations, building the walls, forming the roof, fitting windows and doors, running services and plastering. Ground conditions can change this quickly. Clay soil near trees, for example, often needs deeper foundations than a standard design allows for.
Professional and Approval Fees
| Item | Typical cost | Notes |
|---|---|---|
| Drawings and design | £1,500 to £5,000 | Simple designs cost less. Source: Build It, 2026 |
| Structural calculations | £600 to £1,200 | Higher if steel beams are complex. Source: BestBuilders, 2026 |
| Householder planning application | £548 | England fee from 1 April 2026. Only needed if permitted development does not apply |
| Lawful development certificate | £274 | Optional proof that a permitted development extension is lawful |
| Building control | £700 to £1,400 | Depends on the council or approved inspector and extension size |
| Party wall surveyor | £700 to £1,200 per neighbour | Agreed surveyor. Two surveyors can cost £1,500 or more |
Whether you need full planning permission depends on the size, height and position of the extension.
Worked Example: A 20m² Kitchen Extension on a 1930s Semi
The figures below are an illustration built from the published ranges in this article. They are not a quote for any real project.
| Item | Allowance |
|---|---|
| Build at £1,900 per m² (20m²) | £38,000 |
| Aluminium bifold doors, four panels | £5,000 |
| Steel beam to open into the existing house | £2,000 |
| Roof lantern | £3,000 |
| Mid-range fitted kitchen | £10,000 |
| Drawings and design | £3,000 |
| Structural calculations | £800 |
| Building control | £900 |
| Lawful development certificate | £274 |
| Party wall surveyor, one neighbour | £1,000 |
| Contingency at 10% of build items | £5,800 |
| Total before VAT | £69,774 |
| VAT at 20% on builder and professional charges | £13,720 |
| Estimated total | £83,494 |
The headline rate in this example was £1,900 per m². The finished project works out at about £4,175 per m², once everything is included. This gap is the main reason extension budgets go wrong.
What Is the Cost of an Extension With Bifold Doors?
Bifold doors are one of the most popular upgrades on a Birmingham kitchen extension. They open the room to the garden and bring in a lot of light. They also add cost in three ways: the doors themselves, the structure above them and the insulation rules for glazing.
| Bifold doors, supplied and fitted | Three panels | Four panels | Five panels |
|---|---|---|---|
| uPVC | £1,950 to £2,800 | £2,800 to £3,500 | £3,550 to £4,250 |
| Timber | £2,000 to £2,800 | £3,050 to £3,450 | £3,700 to £4,200 |
| Aluminium | £3,150 to £4,650 | £4,300 to £5,700 | £5,250 to £7,200 |
A wide opening also needs a steel beam above it to carry the load. Supplying and fitting a beam typically adds £1,500 to £2,500, plus the engineer's calculations. So the full cost of an extension with bifold doors is usually several thousand pounds more than the door price alone.
The 25% Glazing Rule
Building regulations limit how much glazing an extension can have before extra checks are needed. Approved Document L sets the guide as 25% of the extension's floor area, plus the area of any existing windows or doors the extension covers. A 20m² extension would therefore allow about 5m² of windows, doors and rooflights, plus any openings it replaces.
Going over this is possible. However, it usually means better-performing glass, extra insulation elsewhere or energy calculations to prove the design still complies. Each of these adds cost, so it is worth agreeing the glazing plan early.
Kitchen Extension Cost: Planning the Room and the Kitchen Together
Most single storey extensions in Birmingham are built to create a larger kitchen-diner. That means the kitchen extension cost has two parts: the building work and the kitchen itself. Treating them as one project usually saves money, because services can be planned before walls are plastered.
- Mid-range kitchen, supplied and fitted: £8,000 to £15,000
- Bespoke or high-end kitchen: £18,000 to £35,000 or more
- Laminate worktops: around £100 per m²
- Granite or quartz worktops: from around £300 per m²
Moving a sink, cooker or utility area also means moving water, waste and electrics. Cooker extraction needs to vent outside. If an island includes a hob or sink, services may need to run under the new floor. These details are cheaper to solve at the design stage than halfway through the build. Our kitchen renovation team works alongside the build team, so plumbing, electrics and fitting are planned as one job.
Reusing your existing kitchen can reduce the budget significantly. It is often worth asking whether units can be moved and adapted before choosing a new one.
What Hidden Costs Catch Homeowners Out?
Even a carefully planned extension can turn up surprises. These are the ones we see most often in Birmingham homes.
Ground and Drainage
- Trees and clay soil. Roots near clay can mean deeper or specially designed foundations.
- Public sewers. If a shared sewer runs under the planned extension, you will need a build-over agreement from Severn Trent Water, which serves Birmingham.
- Old drains. Existing pipes may need moving or replacing once the ground is opened up.
Services Inside the House
- Moving a boiler, flue or soil stack that sits on the wall being extended
- Upgrading an old consumer unit to handle new circuits
- Extending the heating into the new room, with radiators or underfloor heating
Neighbours and Planning
- Party wall costs. If you build on or near a shared boundary, the Party Wall Act may apply.
- Conservation areas. Parts of Birmingham, including areas of Edgbaston and Moseley, have conservation area status. Extensions there are more likely to need full planning permission and specific materials.
Living Costs During the Build
A kitchen extension usually leaves you without a working kitchen for part of the build. Budget for a temporary kitchen setup, extra takeaways, storage and possibly time away from home.
How to Budget for a House Extension: A Step-by-Step Plan
Knowing how to budget for a house extension is as important as knowing the cost per m². These steps will help you set a figure you can rely on.
- Decide what the room needs to do. A kitchen-diner, a playroom and a home office need very different services and finishes.
- Set your specification level. Standard, mid-range or high-end finishes can move the cost by more than £1,000 per m².
- Check the planning route early. Find out whether permitted development applies or whether you need a full application.
- Get a site survey before a price. Ground conditions, drainage and access all affect cost. A price given without a survey is an estimate, not a quote.
- Ask for a fixed price in writing. Check what is included and look for provisional sums. A provisional sum is an allowance for work that cannot be priced yet, and the final figure can change.
- Add a contingency. Allow 10% of the build cost, or 15% for homes built before 1950.
- Include VAT and living costs. Check whether quotes include VAT. Then add temporary kitchen and storage costs.
- Plan how you will pay. Agree stage payments linked to finished work. If you want to spread the cost, look at the finance options available, which are subject to status.
If you would like to see what happens between the first call and a fixed quote, read how our survey and quoting process works.
Planning a Single Storey Extension in Birmingham
A good extension budget starts with honest numbers. In Birmingham, plan for £1,800 to £2,800 per m² for the build. Then add the kitchen, glazing, fees, contingency and VAT to see the real total. In our 20m² kitchen extension example, that took a £38,000 build figure to around £83,500 in total.
Greentek Energy Ltd designs and builds single storey extensions with its own in-house team. We handle building control through to sign-off and agree a fixed price in writing before any work starts. Because we also cover insulation, heating and renewables, the new space can be built to stay warm and efficient from day one. We work across Solihull and Birmingham and the wider West Midlands. If you are planning an extension, book a free site survey and we will give you a clear, fixed price.
