How Much Does It Cost to Build an Extension?

How Much Does It Cost to Build an Extension?

John Davies Last updated: July 5, 2023

Building an extension on your home can be a great way to add value and space to your current home. You have to weigh up what is best, would it be best to put the money into buying a house which already had what you want, or if it’s worth it for you. Building an extension just to add value more often than not, isn’t cost-effective. If you want to learn how you can add value to your home, check out our full guide on it.

There are a lot of things to consider when looking at adding an extension, I’ve gone ahead and listed them all below so you can go through them, however, if you’re like me and just want to find out a rough cost, then take a look at the calculator below.
This article is long, but it has virtually everything you need to consider when it comes to building an extension minus the decoration, plastering etc. So without further ado, let’s jump in.

Extension Cost Calculator


Prices Per Square Meter

How Much Does An Extension Cost – UK Breakdown

There are a few initial things that you need to figure out before you can begin the actual construction. These steps are mostly about planning and finding out how you want the extension done. The following should guide you on the initial expectations before construction begins:

Get a Plan

You must have an idea of where the construction will happen depending on the type that you need. However, a plan is still required so you get the total measurements and it will give you an idea of how much it will cost. It also allows you to come up with a rough budget estimate for your building. Depending on how complex the project is, you can choose to draw the plan yourself or get a professional to do it. It saves money when you do it yourself for a simple extension such as a bathroom/kitchen extension. Getting an architect to do it will set you back between £1,000 and £3,500.

Find Out About Permits

You may not always need a permit depending on the type and size of the house extension that you are building. In most cases, you will not be required to get planning permission which costs £206 from your local planning portal. However, it is advisable to get the development certificate which usually costs half of the planning permission.
There are more permits that you may need that cost between £300 and £800, but you will get the full details when informing about your planned extension. Remember also to inform your insurance provider so if there are any adjustments you will know about them.

Contractors/ Tradesmen

Decide whether you want a contractor to handle the project or local tradespeople. This is important since it dictates how much you will be involved in the job. It as well gives an idea of how much it will cost. With a contractor, you can depend on a good one to handle every part of the project adequately. However, contractor services may subject you to 20% VAT which is an extra cost. Using local tradespeople is also a good option, but you should identify a project manager to help with the group. If they are not VAT registered that could save you from the added cost.

Building Materials

The cost of building materials can bring the overall cost of your project to be significantly high, which is why you should take charge from the start. An easy way to go about this is to request a list of all the materials needed and source them yourself. In this way, you can compare different costs from various suppliers and ensure that you get the best value for your money. This ensures that you control how much you will spend in the construction to avoid any wastage.

Types of Extensions

There are different types of extensions that you can build for your house, and each one of them has a varied price range. The price is determined by the size of the extension and other additions that you may need for the job. The following are some of the common types of extensions and how much it will cost you per square meter.

Bathroom or Kitchen Extension Cost

One of the most significant determiners for the cost of building an extension is where you live. Generally, it will cost you less in the rest of the UK than it would in London and the South East. The total cost of doing construction in these two areas may cost up to 40% more. Note this as it will remain a constant throughout this price guide.
If you want a bigger kitchen or bathroom, you can get it pretty affordably at a total of about £5,000 for a small bathroom with basic finishes. On the higher end, you could spend approximately £8,000 that is if you need fancier tiles, a large bathtub or detailed décor.
The kitchen is a bit costlier because you will need electrical and plumbing work is done. On average, set aside approximately £10,000 minimum for this project and about twice that on the higher side.

Editors Note: Find out how much a new kitchen would increase the value of your home here.

Single Storey/Bungalow Extension Costs

If you have all your plans and permits sorted out, then it will cost you about £1,600 per meter square for basic finishes and affordable labour. On the higher end, a single storey or bungalow extension will set you back approximately £2,300 with excellent finishes. Remember that your choice of material is critical as you need a sturdy building that won’t be a weak link to your home.

Two Storey Extension Costs

Using the price of a single-storey extension, you can come up with a reasonable estimate of how much it will cost you for twice that. However, since you are not precisely doubling everything, you will only add about 50% to the construction of a two-storey extension cost.
So if you add 50% of £1,600, you should expect to pay about £2,400 on the lower end, and approximately £3,450 per square meter for high end finishes in your home. Remember that this is only a base charge; more factors could make the cost higher, including VAT.

two storey extension


This is one of the more straightforward extensions to do since it doesn’t require much. However, the cost of things such as window glazing and type of glass used can make the price shoot up significantly. Expect to fork out about £850 on the lower end that is assuming you use affordable materials such as UPVC. If you use brick as well as double or triple glazed windows and doors, you may end up paying up to £1500 per square meter.
The great thing about a conservatory extension is you can minimalize the construction and still get a pretty excellent structure. For example, instead of adding three walls, you can do two for something small. Another piece of good news is you won’t require any permits for this type of extension.

Loft Conversion/Extension Cost

When you want to do a loft extension, the first thing that you must do is check with local authorities on the height restrictions for the area. Many of these restrictions were removed, but depending on the location of your home, you will need the confirmation.
The cost of extending your roof for the attic will cost you between £1,300 and £2,500 per meter square. The difference in price depends on the kind of roof that you need and whether you will erect any walls, so ensure when getting a quote, you have a clear plan. You also need to account for windows, Velux windows cost more than ordinary.

Challenges That May Affect the Cost of Building an Extension

Other than the location of your extension, there are other factors that you must pay attention to so you can have a handle on the full costs of the project. While getting the quote, you should ask about the following and how they affect the overall cost depending on what you want.

Type of Soil

The soil in the location where you want to construct your extension will determine a few things. One it will dictate the construction style that will be used and how long it will take, especially concerning the foundation. A strong foundation is critical to a sturdy house, so it needs careful construction.
The soil can be different from the one under your current house, so don’t be alarmed when this is pointed out. Soil with poor drainage is complicated to work with; hence it may cost you more to fix. Loose earth is also an issue since it may not be strong enough to hold up the house if not well constructed.

Doors/ Windows

You will likely want the same kind of windows and doors as your current house or even an upgrade. In places where you get glazed doors and windows, it will likely cost you more, especially when you need them double and triple glazed. You may need to upgrade from your current option if the extension will probably get hit by stronger wind and snow. As a result, this will significantly impact the overall cost of your house extension.

Restructuring- Walls

Sometimes creating an extension is not straightforward, especially when it is a direct part of the main building. This means you will have to make some changes to the existing house so that it can fit with the new structure coming up. In such a case, you may have to tear down some walls, do steelwork and in some cases bring down part of the roof. Such work can take more time as well as cost more so you can expect that your final quote may be much higher than the figures given above.
To help bring these costs down, you should try as much as possible to reuse, especially for anything that you may have taken from the main house. You should also have a good idea of the materials needed and how to get the same excellent quality at affordable prices. Alternatively, ensure that you get an architect that can come up with a plan that requires minimal changes to the existing house.

Drainage/ Piping/ Gas

This is one area that may be impossible to avoid, and so it is a big consideration regardless of the type of extension that you want to build. The drainage needs to be done correctly; otherwise, it will cause you troubles soon and your insurance may not cover such damages. Ensure that you have a clear plan of what is needed in the new structure, so you know where the connections will be made.
In case you need gas in the extension, you also have to make the changes, and it is always best when a professional does it. This means that you may have to call your gas provider for this; hence it will be an extra charge to consider.
You should also consider if your current central heating would be powerful enough to heat the new space you’re adding. In most cases, it should be fine, but it’s something else to consider.


If your compound is not very big and access is restricted in some way, this will cost you in terms of time. You may have to make different arrangements to ease movement in and out of the construction site. This is something that your contractor or tradesman should be able to identify when doing an initial site visit.
Another thing that may prevent easy access is if there are frequent movements by your family through the construction area. This is why in some cases; you may have to make different arrangements, so you get back home after the construction is completed.


If you have any kind of home insurance, you must check what your provider can cover in terms of the on-going construction and after the structure is completed. One change that you may need is taking a non-negligence policy that is important when you need a way to handle any damages that may come about the new extension. It will cost you about £1,000 yearly and will cover any issues in your property as well as your neighbours as a result of the building activities.
It is essential to inform your insurer well before you start the renovation. If your extension cost is significantly more than £100,000, you may need to take a new renovation policy. This will cost you more so you need to budget accordingly.

How to Save Money When Building Your House Extension

It is easy for the cost of building the house extension to become quite huge if you are not keen on cutting costs when and where you can. If you want to save money, you may have to incur different types of prices, but in the long run, only you can decide what is worth having. The following tips should help you ensure that you save as much as possible on your extension.

Start With a Budget

One of the biggest mistakes that people make is not setting a budget for the project. This is damaging to your finances since it will give you the impression that you can afford any costs that arise from the construction. This can put you in a terrible position and make you spend more than is necessary.
Start with a budget using your estimates and work with it unless it is necessary to add. Included in your budget should be a contingency budget. This also sets a limit of how much you may spend on extras and allows you to be more conscious of decisions that you may make during the construction.

Save on VAT

Usually, you must pay 20% VAT on labour and materials apart from a few situations where you can claim relief. You can claim relief if your building is zero-rated, the construction is a conversion of the current house or any changes that you make to a home that has remained unoccupied for at least two years. Usually, this is about 5% less. If you want to make further savings, using unregistered tradespeople will mean that you don’t have to pay the 20% VAT on labour.

Get Various Estimates

You must balance between getting quality work done, and affordability and the best way to do this is to compare quotes. No matter the type of workers you get for your house extension construction, you can afford to shop around starting with the best recommendations you get from neighbours, friends or family.
Use the estimates and referrals to compare what you expect from them. Remember to ask as many questions as possible, so you have a clear idea of what is on offer. You want people that understand your vision but can work without much supervision.

Start With a Minimalist Design

This doesn’t mean that the construction shouldn’t be fancy or complicated in any way; it means that it can be done without much disruption. For example, if there are any changes to be done to the main structure, they should be minimalistic, and you can get this by getting a good planner.
However, you can also do it yourself with the help of your workmen if you are not too knowledgeable so you can work out the practicability. The design should make the construction process straightforward.

Source the Materials Yourself

Many contractors use this opportunity to inflate the cost, so you must try and get everything that you need yourself. This is why it is essential to get a comprehensive quote when making your comparisons so you can also check the prices and decide what works best.
If you get your materials yourself, you can be sure that there is no wastage and if you buy in bulk, you can get discounts such as free shipping. Also, check your local providers as this will minimize added costs.

Vacate the House

Depending on how complicated the construction will be, it may be best to live elsewhere for the duration of the development. This minimizes your interference with the process that may slow down the job hence making it costlier for you. However, make sure to compare any costs that you may incur especially if you have to rent temporarily and find out the best way forward.

Supervise the Construction

Many project managers insist that supervising your construction may ensure that you get a better result and that it may cost you less. You will not have to get someone specifically for that hence saving on costs. It may also teach you a thing or two that may come in handy at a different point as well as ensuring that the construction is done fast. This may also mean that you live at the construction site for easy supervision, put this into consideration.

Get Insured

There are several insurance policies that you may need depending on what you find necessary. It is a good idea to have site insurance not to save on costs during construction. This policy covers everyone, so you don’t have to pay from your pocket in case of accidents. It is still vital even when your construction personnel have their insurance, since their policy may make you liable in case of anything. The site insurance will help to save you unnecessary battles and costs.
This policy also covers your home if you plan to vacate for the time of the construction. It ensures that everything is safe and you don’t have to worry about theft or anything happening on site. Remember also the non-negligence policy, so you and your neighbour are covered even after the construction is over.


Constructing an house extension even when it is straightforward can be a very demanding task, so you need to be mentally and financially prepared to meet everything that comes up. It is essential to have a realistic vision of what you want since this will guide you throughout the process.
If affordability of the project is important to you, then you have to be conscious about every pound that you spend. This means that you must be a bit more involved in the process from start to finish, but your wallet will thank you for it.
That said; do not get stuck in the planning process since you may have to figure out some things while the project is on-going. Once you have enough information and a reliable team, you should work to get started and stay active, so in case anything comes up, you are in a position to manage it.

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