How Much Does an Extension Cost in 2022?
According to Check-a-Trade, the average cost of a 20m2 ground floor extension today is £37,500. Of course, averages only take us so far, as every home extension project is unique. Each one has its own context within which there are many variables.
In this article, we investigate how much it costs to build an extension and how to get the best out of the process. We consider how best to balance the costs and benefits in both the short and long term.
We’ll consider
- Your purpose in building the extension
- The type of extension
- Example costs – how much does an extension cost in the UK?
- Planning Permission
- Some important non-construction costs to remember
- The best way to approach an extension project
Why are you building an extension?
What kind of value do you seek to add?
The reason for extending your home will have a significant impact on your plans and how you go about the project. Of course, the quality of the building work should be of the same high standard, whatever your motivation, but what you expect to get out of it may differ.
An extension can add 10 to 20 per cent to the value of your home. Thus, for many London houses, an investment which is modest in comparison to the value of the property can bring considerable rewards in the long term.
Home or business? How much does it cost to build an extension?
If you are redeveloping an entire property as a business proposition, the short term gains on re-sale will be a priority. It will be important to show how much the extension improves the home. Changes to floor plans will need to result in an appealing and marketable end product. Quality of workmanship, as well as versatility in how the extra space can be used, will be essential.
The same will be broadly true if you are extending your own home, but the emphasis on a short term increase in property value may be different. You’ll be more interested in how the new rooms will be used and how it will improve the enjoyment of the property overall. This is especially the case if an extension is an alternative to selling and moving home. Adding financial value in the long term is a bonus.
The type of extension is important
What do you want out of the extension, and what do you want in it?
How you intend to use the extra space, and whether it is on one or two floors, can have a significant impact. A ground floor extension for use as a home office will usually be fairly straightforward. A ground and first floor extension including a new bathroom and large kitchen-dining room will be more complex and, inevitably, it will cost more.
The size as well as purpose will contribute to the overall cost. This is not just about the cost of materials. Whether or not you need planning permission will also affect the cost and the time involved.
Many home extensions will not need Planning Permission as they will qualify under Permitted Development Rights. We look at this in more detail later.
Use and purpose also determine internal fittings and finishes, so the cost of a new kitchen – with units and appliances and tiled flooring – will naturally be more than a simple garden room or office.
It’s not just the material costs that matter
There are several other factors to consider that influence the price of an extension. Anything which relates to the time required and materials used will impact on the cost.
Examples include the size, location and ease of access, groundworks (e.g., foundations and pipework), the quality of the materials you choose, the finishes (outside as well as inside) and any remodelling of areas in the existing home which connect with the extension. There is also the preparatory work to get done, and we look at that below.
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How much does an extension cost? Examples of likely construction costs by extension type
The tables of construction costs, (i.e., excluding other fees) in this section are indicative rather than definitive. These costs include internal finishing like partition walls & plastering, as well as external finishes, but exclude carpets and furnishings.
Approximate overall construction costs – excluding VAT |
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| Type of extension | Approximate size | Average cost |
| Small ground floor extension | 20m2 | £37,500 |
| Medium ground floor | 30m2 | £54,000 |
| Large ground floor | 50m2 | £87,500 |
It’s also helpful to consider costs per square meter. Very small extensions tend to have higher unit costs than larger projects. So, how much does an extension cost per m2?
As a rough guide, we have found from our own work that costs come out at anything from £1350 to £2500/£3000 per m2 , although there can always be exceptions.
Approximate costs per square meter by type of extensions – excluding VAT |
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| Type of extension | Quality of materials & finishes | Approximate cost per m2 |
| Ground floor single storey | Basic | £1350 – £1650 |
| Good | £1700 – £2000 | |
| Premium | £1800 – £2500 | |
| Double storey (ground and first floor) | Basic | £1250 – £1650 |
| Good | £1650 – £2250 | |
| Premium | £2250 – £3000 | |
| Side Extension | Good | £1500 – £2000 |
| Extensions with kitchens | Add an additional £6000 – £10,000 to the overall cost | |
| Extensions with bathrooms | Add an additional £2500 – 5000 to the overall cost | |
| Conservatories and Basements | Considerable variety in materials and techniques used make these very niche areas about which it is difficult to generalise. | |
Planning Permission
Many extensions do not need Planning Permission as they will qualify under Permitted Development rights. This means you can bypass the full formal application process. Beware, however, not to assume anything, one way or the other. To be certain, you should still get your development rights recognised by the local planning authority.
In broad terms, for detached houses, Permitted Development Rights apply to single storey extensions at the rear of up to 8m, or 6m on a semi-detached or terraced property. A single storey side extensions is permissible up to 4m high and up to half the width of the original building. Two-storey extensions of up to 3m are included, provided they are at least 7m from the rear boundary of the property. You can find more detailed information and guidance at the government Planning Portal.
Even when Planning Permission is not required it is always best to get your local planning authority’s agreement to that fact.
With both Permitted Development Rights and Planning Permission, there are fees payable to the local authority. For a Permitted Development Certificate this is £96 and for a Planning Permission Application it is £206.
The best way to approach a home extension project
Prepare carefully, in advance
Building a home extension, even over two floors, need not be complex or stressful. It helps to take a long view about what you will achieve and the benefits you will enjoy. Then take each step towards your target carefully and logically. At each stage, being prepared will always pay off.
Some people are tempted to try and do everything except the actual construction themselves. With a lot of time and the willingness to learn many new skills, including some potentially complex regulations, this is perfectly achievable.
Specialists have done it before
However, most people benefit from using the expert advice appropriate for each part of the project.
Having professionally drawn plans allows you to think carefully about exactly what you want and how it will look. Advice from planning specialists will smooth your dealings with your local planning authority. The services of a structural engineer will ensure your building remains safe, bringing you confidence and peace of mind.
Above all, specialists know how to get things done efficiently. Getting the services of professionals is the best way to protect both the quality of your project and overall value for money.
Divi-Design are established Architectural Designers and Planning Specialists.
We deliver
- architectural designs which are both practical and beautiful, helping you get the best out of every aspect of your project.
- a complete in-house service, which means nothing need be outsourced.
- up to date understanding of the latest planning regulations.
- personal knowledge of, and insights into, the planning policies of local authorities in and around London. We have a very high success rate in all planning applications with which we are involved.
Get in touch: call +44 (0)203 488 2828 or email info@divi-design.co.uk