In this article10 sections
- House Extension Costs at a Glance
- What Does That Mean for a Real Extension?
- What a Price per Square Metre Includes, and What It May Not
- What Pushes the Cost of an Extension Up or Down?
- Costs to Budget for Beyond the Build
- How Long Does an Extension Take to Build?
- Is a House Extension Worth the Money?
- How to Keep Extension Costs Under Control
- Getting an Accurate Price for Your Extension
- Frequently Asked Questions
“How much does a house extension cost?” is the first question almost every homeowner asks, and the honest answer is that it depends on what you are building and what you are building it on. That does not mean you have to plan in the dark. With a guide price per square metre and a clear view of what moves a quote up or down, you can set a realistic budget long before the first drawing is finished.
This guide explains typical house extension costs, what those figures do and do not include, and the decisions that have the biggest effect on the final price. It is written for homeowners in Surrey and Hampshire, where we build, but the principles apply wherever you live.
House Extension Costs at a Glance
As a starting point, extensions are usually priced by the square metre of new floor area:
| Type of extension | Guide cost |
|---|---|
| Single-storey extension | £1,800-£2,500 per m² |
| Double-storey extension | £2,200-£2,800 per m² |
Where your own project falls depends on its size, the ground it stands on, the design and the standard of finish you choose.
Treat these figures as a guide for early budgeting, not a quotation. A price you can rely on only comes from a builder who has seen the house, the access and the drawings.
What Does That Mean for a Real Extension?
Multiplying the guide rate by the floor area gives a first, rough idea of the build cost:
| Extension | New floor area | Guide build cost |
|---|---|---|
| Small single-storey | 15 m² | £27,000-£37,500 |
| Medium single-storey | 20 m² | £36,000-£50,000 |
| Large single-storey | 30 m² | £54,000-£75,000 |
| Double-storey | 40 m² over two floors | £88,000-£112,000 |
These are simple sums using the guide rates above, so use them to test whether an idea is broadly affordable and no more. Two extensions of the same size can differ considerably in price once the ground, the structure and the specification are taken into account.
Remember, too, that the building work is not the whole budget. Once the kitchen, the glazing, professional fees and VAT are added, even a modest extension can easily cost £50,000 or more, while large or bespoke projects can exceed £100,000.
What a Price per Square Metre Includes, and What It May Not
A rate per square metre is a shorthand, and different people mean different things by it. Before you compare two figures, or two quotes, check exactly what each one covers.
A guide rate normally describes the building work itself: foundations, walls, roof, windows and doors, plastering, electrics and heating to the new space. The items below are often priced separately, so ask about each of them:
- The kitchen or bathroom going into the new space, including units, worktops and appliances.
- Feature glazing, such as wide sliding or bi-fold doors and roof lanterns.
- Floor finishes and decoration.
- Professional fees for drawings, structural calculations and surveys.
- Planning and building control fees.
- Party wall costs, if work near the boundary means your neighbours appoint a surveyor.
- VAT. Work to extend an existing home is normally charged at the standard rate of 20%, so always ask whether a figure includes it.
A quote that looks cheaper is not always the lower price. It may simply leave more out.
What Pushes the Cost of an Extension Up or Down?
Size and shape
A larger extension costs more in total but usually less per square metre, because the fixed costs of setting up the site, digging foundations and forming a roof are spread over more floor area. Simple rectangular shapes cost less to build than extensions with several corners, steps in the roof or awkward junctions with the existing house.
The number of storeys
A large share of the cost of any extension lies in the foundations and the roof. A double-storey extension uses the same foundations and roof footprint for two floors, so adding the second floor may cost around 50-70% more than a single storey while providing almost double the usable space. We compare the two in more detail in our guide to single-storey and double-storey extensions.
Ground conditions and drainage
What lies under the garden matters as much as what you build on top of it. Clay soils, nearby trees and sloping sites can all call for deeper or specially designed foundations. Drains that cross the site may need to be moved, and if a public sewer runs beneath or within three metres of the new foundations, the water company’s agreement is needed before you build.
Structural work to the existing house
Most extensions involve opening up the back or side of the house so the old and new spaces work as one. The wider the opening, the larger the steel beam above it and the more temporary support the house needs while it is installed. If you are planning to take out internal walls at the same time, our guide to removing a load-bearing wall explains what is involved.
Roof design and glazing
A flat roof with a single rooflight is the simplest option. Pitched and vaulted roofs, roof lanterns and long runs of glazed doors all add cost, although they are also the features that make an extension feel bright and generous. It is worth deciding early where the money will make the most difference.
What the room will be used for
A family room or home office needs little more than power, heating and good light. A kitchen extension needs plumbing, drainage, extraction, more electrical circuits and the kitchen itself, which is why kitchen extensions usually sit towards the upper end of the range.
The standard of finish
Flooring, joinery, lighting, sanitaryware and worktops come at every price. Two identical shells can end up tens of thousands of pounds apart once they are fitted out.
Knock-on work in the rest of the house
An extension often prompts other work: a bigger boiler or extra heating zone, an upgraded consumer unit, new flooring running through the old and new rooms, or redecorating the rooms the builders have had to pass through. Include these in your budget from the start.
Access
A site where materials can be delivered straight to the back garden is quicker to build on than a terraced house where everything has to be carried through the hallway.
Costs to Budget for Beyond the Build
Alongside the builder’s price, allow for:
- Design fees for measured surveys, planning drawings and building regulations drawings.
- Structural engineer’s calculations for foundations, beams and any unusual roof.
- The planning fee, if your extension needs permission. A householder planning application in England costs £548 at the time of writing, and fees normally rise each April. Our guide to planning permission for house extensions explains when you need to apply.
- Building control fees, which vary with the size of the project and who you appoint.
- Party wall surveyors, where work on or near a shared boundary leads to an award. See our party wall guide.
- A contingency, often around 10% of the build cost, for the things nobody can see until the ground is opened up or the old wall comes down.
How Long Does an Extension Take to Build?
Time on site and cost are closely linked. Build times for an extension typically range from 10-20 weeks, or longer for complex projects. Before that, allow time for design, for planning permission where it is needed (most applications are decided within eight weeks), and for structural calculations and building regulations approval.
In many cases you can stay in your home while the work is carried out. If the kitchen is out of action for part of the build, it is worth planning a temporary one.
Is a House Extension Worth the Money?
For most families the first return is the space itself: a kitchen everyone fits into, a place to work, or a bedroom that means nobody has to move schools. A well-designed extension can also add to the value of the property, particularly where it fixes a weakness in the layout, although the uplift depends on the house, the street and the local market.
If you are still weighing up your options, our comparison of a loft conversion and a home extension looks at cost, space and value side by side, and our single-storey extension ideas may help you decide what to build.
How to Keep Extension Costs Under Control
Keeping to a budget is mostly a matter of making decisions at the right time.
- Finish the design before work starts. Changes on paper cost very little. Changes on site cost time and money.
- Keep the structure simple. Put the money into the features you will notice every day.
- Think about where the drains and services already are. Keeping a kitchen or utility close to existing drainage is usually cheaper than moving it across the house.
- Choose your finishes early. Then they can be priced properly and ordered in good time.
- Ask for a detailed written quotation. It should set out what is included, what is excluded and what has been allowed for any items still to be chosen.
- Agree how changes will be handled. Any extra work should be priced and agreed in writing before it is carried out.
Getting an Accurate Price for Your Extension
A guide price will tell you whether a project is realistic. Only a quotation based on your house, your site and your design will tell you what it will cost.
At Godman Construction, we begin with an initial consultation to understand what you want from the extra space, then provide a detailed quotation tailored to your requirements. Our end-to-end service covers planning and design, the necessary approvals and the construction itself, and we match materials, brickwork and roof finishes so the extension sits naturally with your home.
We build house extensions across Surrey and Hampshire, including Woking, Guildford, Godalming and Weybridge. To talk through your plans, request a free, no-obligation quote.
Frequently Asked Questions
How much does a 20 square metre extension cost?
Using the guide rate of £1,800-£2,500 per m², a 20 m² single-storey extension works out at roughly £36,000-£50,000 for the building work. Kitchens, feature glazing, professional fees and VAT may be additional, so check what any figure includes.
Is a double-storey extension cheaper per square metre?
The guide rate per square metre is a little higher, but the total is lower than many people expect, because both floors share the same foundations and roof. Adding the second floor may cost around 50-70% more than a single-storey extension while almost doubling the space.
Do I pay VAT on a house extension?
In most cases, yes. Extending an existing home is normally standard-rated at 20%, so make sure you know whether the prices you are comparing include VAT.
Do I need planning permission for an extension?
Not always. Many single-storey extensions can be built under permitted development rights, provided they stay within strict limits on size and height. Larger extensions, and homes in conservation areas or other protected areas, are more likely to need permission.
Can I live in the house while the extension is built?
In many cases, yes. We discuss the likely disruption at the planning stage and organise the build to keep it to a minimum wherever possible.
