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House extension costs vary hugely depending on the size, type and complexity of you project and where you live. We explain how much house extensions cost, including all the fees you may need to pay, and how to save money.
The average cost of a standard 20m² single-storey house extension is £48,000 (+VAT). But house extension costs can vary significantly due to a number of factors, so you may pay much less – or more – than this.
The main factors that affect house extension costs are:
Here are some examples of how much a house extension may cost, by extension type.
A good rule of thumb for calculating the cost of an extension per square metre is to budget £1,800 – £3,000 per m², says Checkatrade.
Here are some typical price ranges for a single storey house extension. As extensions get larger, the price per metre can reduce, this is reflected in these examples:
| Extension size | Typical house extension costs |
|---|---|
| 20m² | £40,000 – £56,000 |
| 30m² | £60,000 – £84,000 |
| 50m² | £100,000 – £140,000 |
Find and compare quality local builders to undertake your extension.
Find local builders with our partner the Federation of Master Builders.
The two storey house extension costs around £1,900 per m² for a standard finish. This is cheaper per square metre than a single storey extension. This is because there are some costs for a single storey extension that won’t simply double, like the cost of a roof or foundations.
If your extension will include a new kitchen you’ll have additional costs. You’ll need to pay for the kitchen itself and you’ll also need plumbing, gas and electrics.
Garage conversions can be a cost effective of creating more living space in your home and an easy way to add value.
How much this type of house extension costs will depend on how much work needs to be done. For example, do the foundations need to be reinforced? do you need a structural engineer? and do the walls or roof need to be repaired?
The average cost of a 30m² loft conversion in the UK can range from £27,500 to £75,000+, according to Checkatrade. The cost depends on the type of loft conversion you choose, the complexity of the project, your property’s location and other factors.
Read more in our guide Loft conversion costs and how to reduce them.
Find local builders with our partner the Federation of Master Builders.
The costs and complexity of a basement extension vary dramatically depending on whether you have an existing cellar with enough headroom that can be easily converted or whether you will need to excavate to make the space workable.
Read our guide Basement extension: where do I start?
There are some professional fees you may need to factor into your house extension budget:
The government doesn’t publish a single set of standard fees for Building Regulations applications. Charges are set by each local authority or Approved Inspector and can vary significantly.
The following table gives a rough guide to structural engineer costs in 2025 for some common jobs.
| Type of work | Typical costs |
| Average hourly rate | £100 – £200 |
| Basic project | £1,000 |
| Complex project | £5,000 |
| Calculations for an RSJ (per calculation) | £200 |
| Structural engineer inspection (site visit) | £175 – £300 |
| Structural engineer report | £700 |
| Structural engineer for load bearing walls (per project) | £400 |
You can find structural engineers who cover your local area with our free tool:
A structural engineer can provide structural calculations or carry out a structural inspection on your home.
Remember to check whether your contractor will charge VAT and if so, whether it is included in quotes.
| Type of fees | Typical cost |
|---|---|
| Architect fees | 3%-15% total construction costs |
| Planning permission fees | £258 |
| Lawful development certificate | £129 |
| Building regulations | £300-£500 |
| Structural engineer (basic project) | £1,000 |
| Party wall agreement | £1,000 |
| VAT | 20% |
You will need to work out the best way of funding the cost of your house extension. There main ways to do this are:
One way to fund house extension costs is to remortgage to release equity from your home. This means taking out a bigger loan against your property in order to free up some of the cash you’ve built up in it.
Remortgaging to release equity is very common; in fact the LMS Monthly Remortgage Snapshot showed that in March 2025, 47% of people who remortgaged increased their loan size, by an average of £20,224.
However, bear in mind your loan size will increase and you may need to pay a hefty early repayment charge if you remortgage during your deal’s initial period. So make sure you get expert mortgage advice first.
Get fee-free remortgage advice from our partners at L&C. Use the online remortgage finder or speak to an advisor today.
Another option to finance house extension costs is finding a second mortgage (in addition to your existing mortgage). This is also known as a secured homeowner loan or second charge mortgage. However, these loans usually carry higher interest rates.
You might consider using an unsecured personal loan or even a credit card, depending on the sums required to pay for your house extension costs.
However, these will generally come at higher rates than mortgages and so it’s important to consider all the options, particularly for larger projects.
If you have savings, could you use these to cover some or all of your house extension costs.
There are a number of straightforward ways you can reduce house extension costs – and prevent them spiralling out of control:
The easiest way to cut house extension costs is to get multiple quotes. We recommend getting quotes from three different contractors for the work. You’ll usually need to ask at least five firms to get three quotes. Find out how to invite contractors to tender for the work.
If you’re looking for the cheapest way to build a house extension there are a number of ways to cut costs.
While if you want to convert your loft, loft conversion costs vary hugely depending on the type you choose.
So when you’re looking for the cheapest way to build a house extension, explore your options and how much they cost.
The last think you’ll want is to end up half way through your project before realising you’ll struggle to pay additional house extension costs you hadn’t factored in.
So before you start, set a budget for your whole job. Making a list of everything you would like included. Compare your quotes item by item and when pricing services and materials always check the VAT is included. We’d also recommend setting aside around 10% of the total cost for unexpected additional house extension costs.
Once you’ve designed the perfect extension, you may want the help of a professional designer to pull together the finishing touches. We’ve partnered with my Bespoke Room who are making interior design more accessible and affordable. Find out more about their room styling package from just £295.
Find a professional interior designer and get a free consultation with our partners at My Bespoke Room.
House side extensions in the UK typically cost between £1,800 – £3,000 per m².
If you’re adding a single storey extension to a semi-detached house, house extension costs are usually around £1,800 – £3,000 per m².
As a rough guide, a house rear extension costs from £1,800 – £3,000per m². But this will depend on the size, finish and complexity of the design.
– Yes, two storey extensions are often cheaper per square metre than single storey extensions because the expensive costs of foundations and a roof are shared across both floors.
– While a two-storey extension will cost more overall than a single-storey, it will be cheaper in cost compared to building two separate single-storey extensions.
Yes, a well designed extension can add value to your house, often by more than the build cost. However, this will depend on how much you spend on your house extension, location and finish.
A single storey extension in London may cost from £2,300 – £3,400 per m², compared to the UK average of £1,800 – £3,000 per m². However, the cost of a house extension in London will depend on factors such as the type, size, complexity and finish.
House extension costs in London and south east England are typically more expensive than in the north, the midlands, and in Wales and Scotland.
The cheapest type of extension is usually a garage conversion, averaging £14,250.
A loft conversion costs £27,500–£75,000+ in 2025, depending on the type.
HomeOwners Alliance Ltd is registered in England, company number 07861605. Information provided on HomeOwners Alliance is not intended as a recommendation or financial advice.
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