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How Much Does a House Extension Cost?

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House extensions can be a great way to add extra space and comfort to your property without needing to move home. They can also add value to your home if you think you might sell in the future. There’s a real excitement to be had from expanding your home but it can be difficult to determine how much you’ll actually need for it.

As with anything, a house extension should be thoroughly planned out with a set budget to avoid the possibility of it becoming a drain on your finances (and sanity). With that in mind, it’s good to go into this process with your eyes open about the possible costs involved. 

So, how much does a house extension cost?

The answer isn’t a simple one and is very much based upon the type of extension you’re considering. Here, we’ll go into a bit more depth about the type of house extensions there are and how much you should reasonably allow for each project.

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The different types of house extensions

If you’re here, you probably already have an idea of the type of house extension you’re looking to get. If you’re still considering the best option for your property, we might be able to shed some light on what goes into the different types of extension.

Here are the most common house extensions you might be considering:

  • Single storey extensions
  • Multi-storey extensions
  • Semi-detached extensions
  • Kitchen extensions
  • Bathroom extensions
  • Garage extensions
  • Loft conversion
  • Basement extensions

We’ll go into each of these in detail further on with an average price for each extension. It’s worth bearing in mind that all costs are national averages. Some locations will see higher costs for extensions, such as London and the South East.

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What goes into a house extension?

There’s a lot that goes into the price of a house extension, these additional aspects can dramatically affect the price you’re looking to spend. This is worth remembering when budgeting for your extension. Discuss the process of the extension with an expert, understand what needs to be done and if there are any requirements which could end up costing you more.

How big is the extension?

The larger the extension, the higher the cost. This is due to the parts and labour involved in actually building the extension.

Location, location, location

Following a similar trend to the housing market itself, extension costs will be higher in some areas and lower in others. London and the South East is often where you’ll see the highest costs for extensions.

Quality & design

What is it you’re looking to do? If you’re after a unique extension which requires further planning and design, this will cost more. If you’re looking for a higher quality of extension, you’ll also pay more. Before demanding the best quality materials and sparing no expense, it’s worth looking at the costs of comparable extensions and creating a benchmark for what you’re hoping to achieve.

Are you using an architect?

The benefit of using an architect is that they’ll create a bespoke plan for your extension which suits your needs perfectly. This can ease potential snags and costs to fix mistakes further down the line, though architects don’t often come cheap. Most architects charge a percentage of the final build cost or use a flat fee model.

Architects will likely create drawings and in-depth plans for your extension which will help with specifications, timelines and calculations. This makes the process a lot smoother but is likely to cost between £350 – £950 depending on the type, size and intricacy of your plan.

Planning permission

This is a common barrier for home extensions but it’s worth remembering that not all extensions will require planning permission. This handy guide from the Government website will help you understand a bit more about whether you’ll need planning permission to pursue your extension ambitions and how much it’s likely to cost.

Finishing touches

You’re not creating an extension to be an empty shell in your house, you need the finishing touches like lighting, flooring, windows, doors and joinery to make the extension feel like home. Are you looking to decorate the extension yourself or will you use a professional? The cost of this can be as much or as little as you like. This is an area where additional costs can really creep up if you don’t keep an eye on your budget. For the sake of brevity, our average costs below include simple finishing touches of a reasonable quality. If you have a specific idea of the type of finishing touches you’d like, we recommend shopping around at the early stages and calculating how much you should allocate in your budget for this.

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This is where costs can also become inflated. The structural integrity of your extension is crucial to a successful build. This means that specific locations will require more groundwork than others. Some soil types may require a unique approach. If there are any trees or drains near the build site, these may need to be removed. Speak to an expert early to understand if your extension requires any additional groundwork that will inflate costs.

House insurance

You may already have house insurance, however if you’re looking to add an extension, it’s important to notify your insurance provider. This means you may have to pay more on your premium but it will ensure that your extension is also covered within this.

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Shell vs full extensions

You’ll need to make a decision on how you’d like the extension to be carried out. There are two main options for you to choose from, shell extensions and full extensions.

Shell extension

A shell extension is exactly what it sounds like. It’s the external build of the extension without any of the touches to make it a functional room. This means, you’ll only get the external walls and a roof. You’ll then need to work on the internal walls, flooring, utilities, etc. This is a more cost-effective option in the short-term and is really only recommended if you’d like to phase the work to save on budget. A shell extension still requires a significant amount of work to be done once the builders have completed their contract.

You can expect a shell extension to cost between £750 – £1,500 per square metre.

Full extension

A full extension will allow you to have a functional space once complete. Most of the work will be included within the price of the extension, though you’ll usually have to decorate once complete as this isn’t usually included. If you’re looking to get your extension completed in one go, this is likely to be your best option. 

Full extension costs do vary based upon where the extension is based, though standard room extensions will cost around £1,250 – £2,500 per square metre. The below prices for each extension type are based on the assumption that you’re looking for a full extension.

Single storey extensions (£25,000 – £120,000)

The cost of renovating your home will depend on many factors, including its size. A single-storey ground floor extension is one of the cheapest and easiest options available. These are usually at the side or back of your property which means you may have some groundwork costs to consider depending on your location. 

The costs here are very much based upon the size of extension you’d like:

  • Single storey 20 square metre extension – £25,000 – £50,000
  • Single storey 30 square metre extension – £35,000 – £70,000
  • Single storey 50 square metre extension – £60,000 – £120,000

If you’re looking for a top quality extension, expect to pay 40% more than standard.

Two storey extensions (£70,000 – £140,000)

The biggest costs to a two storey extension comes from the foundations, safe working at height, scaffolding and structural support. This means that two storey extensions are usually less than double the price of a single-storey build. 

Two storey 60 square metre extension – (£70,000 – £140,000).

Semi-detached extensions (£1,250 – £2,500 per square metre)

Semi-detached extensions are some of the most popular additions to properties and will cost a similar amount to those mentioned above. Be aware though, if you’re conducting the work on or near a shared wall, you will be required to secure a Party Wall Agreement.

Semi-detached extension – £1,250 – £2,500 per square metre.

Bathroom extensions (£1,250 – £2,500 per square metre + £2,500 – £5,000 for plumbing & renovations)

There are two types of bathroom extensions available. Either adding space to an existing bathroom or creating a new bathroom as part of the extension. It’s important to remember that bathroom extensions are some of the most expensive options available. This is because you’re not just paying for the additional space, but also for the plumbing and renovations needed to make the bathroom functional. It’s a safe bet to budget an additional £2,500 – £5,000 onto the cost of the extension for any plumbing or work needed.

Bathroom extension – £1,250 – £2,500 per square metre + £2,500 – £5,000 for plumbing & renovations.

Kitchen extensions (£1,250 – £2,500 per square metre + £10,000 – £25,000 for plumbing, gas & electric)

As with bathrooms, a kitchen extension is going to be an expensive option. This is because there’s not just plumbing involved but gas and electrics too. If you need to have a new boiler fitted or want your existing one moved, this will also add to the expense. It’s recommended to allow an additional £10,000 – £25,000 on top of the extension cost when budgeting.

Kitchen extension – £1,250 – £2,500 per square metre + £10,000 – £25,000 for plumbing, gas and electric

Garage conversion (£7,500 – £20,000)

Garage conversions are some of the most cost-effective and easiest extensions to do. This usually involves either fully or partially converting your garage space into a functioning room. Again, if you require a bathroom or kitchen in your garage space, the plumbing and gas & electric will cost extra.

Garage conversion (15 square metres) – £7,500 – £20,000

Loft conversion (£37,500 – £60,000)

Most loft spaces are used for storage, though there’s often a lot of unused space which could add significant value to your property. Loft conversions can turn that storage space into a home office, extra bedroom or playroom.

Much of the cost involved in a loft extension is spent on making the room temperature controlled and safe from the elements. There’s also a cost required for any work carried out at height.

Loft conversion (30 square metres) – £37,500 – £60,000

Basement extensions (£100,000 – £160,000)

It’s no surprise that basement extensions are often the most expensive option. This is because there is a significant amount of groundwork required to make the room functional. This will involve moving pipes, tree roots and more. Basement extensions are highly specialised, which means you will be paying a premium for the services you need.

Basement extensions are great for adding value to your home, especially if there’s restricted space above ground. These are highly desirable assets when you do come to sell your home.

Basement extension (40 square metres) – £100,000 – £160,000

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The difference between a quote and an estimate

When looking to understand how much you’ll need to achieve the extension you want, it’s important to know the difference between a quote and an estimate.

When you start working with contractors, they will give you an estimate as to how much the extension will cost. This is only their best guess and it’s possible that the amount you pay will be higher.

The quote you’re given is an exact figure and definite price. This will have an itemised list of the work to be done, a cost breakdown and will also let you know if VAT is included or not.

Looking for an estimate for your house extension? This handy calculator from RESI can help.

How much is your extension worth?

A key reason why people add extensions to their home is to increase the property’s value for when they come to sell. If you’re wondering how much value your extension is worth, we have a handy calculator to help. Our calculator will look at the size of your extension and your location to get an accurate idea of how much value it adds to your property. Try our extension value calculator here.

Extension Value Calculator

Looking for trusted local tradespeople?

If you’re in the need of trusted local tradespeople, we can help you find the best in your area to help with your home extension.

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Last Updated: November 17th, 2021