Job Costs

House Extension Cost (2020 Guide)


There are many reasons why people build extensions onto their home. They might need more space to be used as a bedroom, office, or living room. An extension could also be built so that there’s somewhere to park your car or store your tools.

And then, some people see an extension as a way of adding value to the home. If it’s done right, then you can add more value to your home, meaning more overall profit. Regardless of why you want to build an extension, you must know what sort of costs you’ll incur before you get started.

It’s important to know what your extension will cost as then you can budget responsibly. In this guide, we’ll answer all the questions you might have about house extension costs. Let’s get started.

What is the Average Cost of Building an Extension?

House extension construction

The cost of an extension will depend on several variables. Of course, the first of these variables is the type of extension that you’re getting done. The more building work that has to be done will translate into a bigger cost, as you’ll need to pay for more supplies and more labour.

You also need to keep in mind that your location will alter how much an extension costs. The reason for this is because labour costs more in some areas than others. For example, an extension in Greater London will cost significantly more than in rural Cumbria.

Another element that can impact the cost of extension is the type of building that you’re extending and where it’s located. This is because planning permission could be harder to get, and the application can cost you more than £2,000 itself.

We’re now going to have a look at some common extension projects that people get done and see how much that they cost on average.

What Does a Single Storey Extension Cost?

House renovation

A single-storey extension is a basic extension that many people get done. In terms of cost, you can expect to pay around £1,500 per square metre in most of England.

This number isn’t exact, and in some areas, it will be a little lower, while in other areas, it will be a little higher. In London, of course, it will be a lot higher, and the cost per m2 in London is around £2,000, with some jobs exceeding £2,500.

This is just the build cost of labour and materials, however. Before the extension can be built, then you’re going to have a lot of expenses to do with applying for planning permission. You’ll need an architect to design the extension too. These costs will end up adding another 10% to 20% on top of the price we’ve just established.

VAT will have to be taken into consideration too. At the moment, VAT is charged at a rate of 20%.

What does this all mean in real numbers then? Well, if you build an average-sized extension anywhere but London, then you’re looking at between £40,000 and £50,000, while if the project is in London, then the total cost will be closer to £70,000.

What Does a Two Storey Extension Cost?

House extention

If you’re building a two-storey extension, then it would stand to reason that you can just double the build cost of a single-storey extension. And that’s exactly right. The cost per square metre is still the same, but you’ll just be doing more work.

Also, the cost of the planning permission or for an architect to design the two-storey extension shouldn’t cost too much more. VAT will still be 20%, and you can’t get around VAT. You do need to be aware that the whole process will take longer, as to build a two-storey extension will require more labour time.

Again, where you live will affect the cost of the extension too. Let’s say that an extension that would cost you £40k for a single storey will now cost you around £60k for the work. Then you need to add on VAT and other costs, so you’ll be looking at a total cost of around £80k. Keep in mind that these are the average costs for an extension on a house in the UK, not London.

How Much Does a Bathroom or Kitchen Extension Cost?

If you’re planning on using a single-storey or two-storey extension as a bathroom or kitchen, you’ll add some extra costs onto your building project. Why? Because you’ll need to get some plumbing done, and there will have to be some more intricate finishings done too, such as tiling.

Depending on the size of the bathroom or kitchen, the total cost will vary. It’s a good rule of thumb to add about £5k on for a bathroom and £10k for a kitchen. None of the other costs, such as for planning permission or VAT will change, although you will incur VAT charges on the products you buy for the kitchen and bathroom.

What Is the Cost of an Extension to a Bungalow?

Bungalow house

A bungalow is a house that is only one storey high. That means that the cost of an extension to a bungalow is no different from a standard single-storey bungalow. You just need to make sure that the building isn’t listed. If it is, then it’s unlikely that you’ll get planning for it.

Can You Convert An Attic As Part of an Extension?

If you plan to convert your attic as part of an extension, you don’t need to worry about any minimum height requirements for the ceiling. There used to be regulations regarding the height of ceilings, but it’s no longer an issue.

Of course, you want to make sure that you can move around when you’re in the attic or extension, so you want the design of the building work to ensure that you won’t be banging your head on the ceiling all the time.

What Extra Costs Are Connected to an Extension Project?

The extra costs are up to you. If you want a basic room with a simple carpet and plain, painted walls, then there’s no need to worry about lots of extra costs, as these things can be fitted cheaply, and it’s possible, of course, to paint the walls yourself.

But, if you want a particular type of flooring put in or you fancy having tiled walls etc. then you’re going to have a higher extension cost. Remember, VAT has to be factored into anything you purchase for the extension.

How Much Will Windows Cost?

Men are installing a window

This is another area where the amount you pay will depend entirely on what it is that you want. At the low end of the price range, you have single-glazed windows. Then at the top end, you have double-glazed windows with hardwood frames.

You’ll have to remember that you’ll need window fitters to come out to put the windows in too, so that’s got to be added to the extension costs. Try to shop around when you’re choosing windows. There are some great deals around. It’s recommended that you use a cost calculator to determine the best price.

Do I Need to Ask Permission to Cut Down a Tree?

Yes. You can’t simply chop a tree down because it’s next to your house and will get in the way of your extension. But this will all be part of the planning permission application. If there are any trees on your land, then the local authority will know if there’s an order placed on them to prevent them from being cut down.

What Could Get In The Way of a Successful Extension Project?

House project plan concept

You can run into problems when it comes to building an extension for numerous reasons. If your house is located in a difficult-to-reach area, where access is difficult, then your labourers and delivery-people will find it harder to get the necessary tools and materials to the site.

This can push up the extension cost and also add to the amount of time it will take for the job to be done. If there’s anything related to your project that is different from usual, that’s going to push up costs. Everything from having non-standard soil on your land to the need for structural changes to the home will halt progress and make things more expensive.

How Do I Know Who to Hire to Build the Extension?

House engineer

It’s always a risk when hiring builders, especially if you have no prior experience of working with them. A recommendation from a trusted source, whether that’s a website or a friend, is a good place to start. It’s also a good idea to try to meet as many builders as possible, ask them for photos of their work, see what their thoughts are for your project, and ask them to estimate the timeframe they think the building job will take.

On top of all this, you need to ask them for a quote. They will submit this to you in writing. It’s the most important thing to consider before you choose a builder. The more quotes you get, the better. You’ll then be able to gauge what the average price is.

If one company quotes a particular amount per m2 that is significantly higher or lower than the cost per m2 that another company is offering, then you can think about why that is. This is a big project that will completely alter your home, and it needs to be done right. You don’t want to skimp, but you also don’t want to be ripped off.


The price that you’ll pay will depend on where you’re located. There are many costs to consider, including the build cost, VAT, fees for architects, planning applications, and furnishings and fittings. When you budget, you should prepare for every eventuality and ensure that you have listed every possible cost that you might incur.

When it comes to hiring builders to do the work, ask a number for quotes so that you get the best price, and only use a reputable builder. If you do all this, then you’ll get a successful extension that will transform your home for the better.