Your Property Wizard

What’s the cost of building an extension in 2021?

Planning a house extension is a big job that can seem daunting, especially with so many things to think about.

There are so many decisions to make before you even start building your extension, and so many rules and regulations to follow. That’s why it’s a good idea to get a proper handle on extension costs before you dive in.

In this guide, we’ll discuss:

  • How much an extension costs
  • What affects the cost of building an extension
  • How to save money on an extension
  • What’s involved in fitting an extension
  • How to find and hire a builder

If you want to extend your home and add to your living space, but aren’t quite sure where to begin, keep reading to find out everything you need to know.

How Much Does an Extension Cost?

Extension costs vary widely depending on factors such as the size and how you intend to use it. Smaller, less complex extensions tend to be the most affordable.

Here are some common extension types and their associated costs, from budget to mid-range and to luxury, as well as the time required to complete them:

Single-storey Extension £1,000 to £1,600 £1,700 to £2,000 £2,200 to £4,000 and up 8 to 10 weeks
Two-storey Extension £1,200 to £1,900 £2,000 to £2,200 £2,300 to £4,000 and up 12 to 16 weeks
Side Return Extension £1,500 to £1,900 £1,900 to £2,200 £2,200 to £2,500 10 to 12 weeks
Glass Extension £1,350 to £1,950 £1,800 to £2,300 £3,000 and up 10 to 12 weeks
Flat Pack Extension £750 to £1,440 £1,275 to £1,800 £1,650 to £3,600 1 to 2 weeks

For a single-storey extension, you can expect it to cost between £1,000 to £1,600 at the lower end of the price range, £1,700 to £2,000 for the mid-range and £2,200 to £4,000 and up at the luxury end of the price range. You can expect this build to be relatively straightforward and box-shaped as a single-storey.

A two-storey extension is estimated to cost £1,200 to £1,900 at the lower end of the price range, £2,000 to £2,200 for a mid-range extension and £2,300 to £4,000 and up for a luxury two-storey extension. A two-storey extension isn’t likely to cost that much more than a single-storey as you’ll only be adding extra joists and walls.

A budget side-return extension is estimated to cost £1,500 to £1,900, £1,900 to £2,200 for a mid-range extension and £2,200 to £2,500 for a luxury side-return extension.

A budget glass extension is likely to cost £1,350 to £1,950, with a mid-range glass extension costing £1,800 to £2,300 and a luxury range to cost £3,000 and up.

When it comes to flat pack extensions, a budget one is likely to cost £750 to £1,440, a mid-range is estimated to cost £1,275 to £1,800 and a luxury flat pack extension is estimated to cost between £1,650 to £3,600.

Please note that the estimates given above are for building the shell of the extension only. The estimates do not include the cost of any fittings and fixtures, since these are entirely of your own choosing.

However, our estimates do include VAT. This is because the vast majority of builders who undertake extensions are VAT registered.

Otherwise, they would only be able to complete one or two projects per year before hitting the government’s VAT registration threshold.

On average, a 30 square metre single-storey extension built on a budget costs between £30,000 to £48,000. If your finances can stretch a bit more, a mid-range extension of this size usually costs between £51,000 and £60,000. These prices exclude fitting costs.

If you want to build a two-storey extension, the costs per square metre aren’t significantly higher. Builders usually quote by the square metre, not on the number of storeys.

Both single-storey and two-storey extensions require foundations and a roof, so the main additional costs you’ll need to consider are scaffolding, extra material for the floor joists and walls, heating, plumbing, and electrics, as well as whatever fittings you choose.

On average, a 40 square metre two-storey extension can cost anywhere from £48,000 to £76,000 if you’re working on a budget or between £80,000 and £88,000 if your budget is more flexible.

A two-storey extension measuring 60 square metres typically costs in the range of £72,000 to £114,000 for a budget project or £120,000 to £132,000 for a mid-range build.

Where you live plays an important factor, too. If you live anywhere near London, it’s likely your extension will cost more.

The cost of a glass extension per square metre come in at around £1,350 to £1,950 per square metre for a budget finish; mid-range will be around £1,800 to £2,300, and a luxury spec will be in excess of £3,000 per square metre.

For a typical extension, budget for around 10 to 12 weeks of labour.

A flat pack extension is a cost-effective route, with prices for a budget option starting at £750 to £1,140 per square metre, mid-range £1,275 to £1,800, and luxury at £1,650 to £3,600. As you’d imagine from the name, the time to build is a lot shorter, too, at just 1 to 2 weeks.

Things get a bit more complicated when you start thinking about kitchen and bathroom extensions.

Planning permission needs to account for plumbing, and then the plumbing actually needs to be installed. To ensure your extension is safe and stands the test of time, it’s a good idea to get this done properly.

For a bathroom, you’ll need to add about £5,000 to the cost of building your extension’s shell. However, these costs can vary depending on the bathroom suite and finishes you choose.

A kitchen will cost you more than a bathroom, adding on around £10,000 to your fee for a low- to mid-range kitchen.

In addition to the costs of building the shell of your extension, you’ll need to consider the cost of any fittings.

If you’re building a 20-square metre kitchen extension, you should allow an extra £2,600 to £6,200 on top of the building costs if you’re on a strict budget (including labour, but excluding appliances).

If your budget is higher, you could pay anywhere from £5,600 to £12,000 on top of the cost of the extension (including labour, but excluding appliances).

Bathroom extensions tend to be smaller. The average costs of fitting a new bathroom suite range from £2,750 to £7,000 for a high specification finish.

Are you ready to get started on your house extension? HouseholdQuotes can help you find the right builders.

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What Affects the Cost of Building an Extension?

Quite a few factors come into play when looking at the overall cost of an extension. With the price per square metre of construction so high, it’s good to know what else might ramp the costs up for your project.

The Design and Planning

The cost of hiring a surveyorstructural engineer and architect will all need to be considered within your project. Getting hold of the relevant planning permission (if required), building regs approval and party wall agreements will also factor in, costing both time and money.

The Extension Size, Shape, and Height

Typically, the size, shape and height of your extension will bring your costs up. The larger you go, and with more premium materials, the higher your cost will be.

Similarly, the type of building materials – brick face, timber clad, glass – will affect your price.

The Necessary Groundworks

The groundwork, such as digging a foundation, improving drainage, or underpinning will all play into your final cost.

If you’re looking to build upon an area of uneven terrain, expect your prices to be higher than if you were building onto something existing and flat.

Any Tricky Trees

Trees can sometimes be even trickier than the extension itself. Many trees are protected by Tree Preservation Orders (TPOs), so make sure you take any tree into account when you are requesting planning permission.

If you remove or alter a tree without the correct permission, you could end up in big trouble.

Any Building Site Constraints

Do not forget to factor in additional costs for your site. If you have a complex site, you’ll need to ask your builder to factor in any extra costs.

For this, issues may include:

  • Soil type that demands a specific building technique or material
  • Changes to the existing structure – any changes to steelwork or walls or will see additional costs
  • Ease of access to the actual site (your house!) – if it’s difficult to manoeuvre tools and materials, you should plan ahead so work isn’t delayed
  • If you’re moving drainage, pipework, gas meters and so on this will require extra planning and cost

The Windows and Doors

Windows and doors can be pricey and easily increase the cost of even a modest extension.

Choosing larger windows, triple glazing, or bi-fold doors can further increase your costs.

If you want to bring light but keep costs down, ask your builder whether a small bit of bespoke glazing could work in combination with standard-sized windows and doors.

The Fittings and Fixtures

As briefly mentioned above with kitchen and bathroom extensions, it’s all the little extras that can sometimes add up and lead to a hefty bill.

If you’re happy with simply painted walls, carpet or engineered wood floors, and standard lighting and electronics then your costs will be relatively low.

More luxurious touches such as bespoke flooring or tiling and fitted joinery can add a unique look to your extension but also increase the costs.

Finding the Right Tradespeople

Depending on the type of work you’re having done, you may need different types of tradespeople involved in your project. Builders, plumberselectricians, heating engineers, painters and decorators – they all come with their own skill sets and their own pricing strategies.

It’s good to note that small vs large contractors will offer very different prices. Where possible, opt for the smaller companies as they won’t have large overheads to recoup with their pricing, and will generally cost you less than the bigger national companies.

Your Location

An unfortunately inescapable fact is that your location will dictate your price – living in London will ramp up your project costs considerably.

Also, remember that if you don’t have off-road parking available at your home or site, you will be expected to pay a parking permit for your contractors over the duration of their stay.

Consulting Home Insurance

You should inform your home insurance provider of the building work, as your insurer may need to adjust your cover and premiums accordingly.

How Can I Save Money on an Extension?

Comparing quotes is a great way to potentially reduce the cost of your home’s extension. HouseholdQuotes can help you get quotes from multiple builders near you, so that you can find someone that suits your budget.

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You can see how costs can spiral when getting an extension added to your home, so it’s important to also know the ways in which you can bring those costs down.

The devil is in the details—so plan ahead to avoid any costly last-minute changes. If you have your architect drawings complete and finalised, it can be expensive to have them changed, so make sure you really assess things at each stage of the sign-off process to make changes when it’s most easy to do so.

Similarly, if you know you want to use your extension for a certain purpose – like as a kitchen or a bathroom – it’s important everyone knows this beforehand so that certain considerations can be taken into account when building to ensure electricity, gas and water can be accessed in the space.

Don’t cut costs when it comes to the structure. Hire a surveyor and structural engineer or architect to draw up plans – you can look to save on fitting costs if you’re on a tight budget, but you shouldn’t try to scrimp on the structural integrity of your extension.

Permitting this is within your allowable home development, it might be more cost-effective to undertake a conversion over an extension when looking to add space to your home. Basements, lofts and garages are all perfect spaces to do this – so make sure you explore your options before going down one route.

Where possible, it’s best to avoid the need to move gas, electric or water pipes. If you’re planning a kitchen extension, it’s best to do it as sympathetically as possible, and have your fittings put in place where they make the most sense for your existing pipework.

Take advantage of online marketplaces, or choose off-the-shelf products from high street retailers over bespoke options. This will help you to save considerably on your fixtures and fittings. Similarly, you can buy ahead during sales or percentage-off periods at shops, which will also save you money in the long-run.

What’s Involved in Adding an Extension?

The steps will differ depending on the scope of your extension, but you can expect some, if not all, of the below to be involved when adding an extension to your home.

Firstly, you need to obtain Building Regulations or Planning Permission (if required), while also checking your leasehold agreement (if applicable) to make sure you can do what you want to do.

You can then enlist an architect to draw up plans, considering your intended use (kitchen, bathroom, study).

Make sure you consult your insurance provider to let them know of the planned work ahead of it beginning and obtain parking permits if required for your contractors to ensure they have easy access to your property.

Before work begins you may need to clear the space and excavate if necessary, including the removal of trees (if not held under TPOs or in protected areas).

Once all these elements have been completed, building work can begin.

Is a Home Extension the Best Choice for My Home?

It can feel overwhelming trying to decide whether or not undertaking such a big building project is actually the right move for you. We’ve listed the advantages and disadvantages of home extensions in the table below to help you come to the right decision.

Home extensions can add value to your home They are expensive to complete
It is more affordable than moving house Home extensions can be stressful projects to undertake
There is huge scope for creativity and personalisation You may not be able to get exactly what you want

One of the key advantages to home extensions is that they can add value to your home. Most home extensions increase the sale value of your property, meaning that if you do end up selling the house it will make you more money.

In most cases undertaking a home extension is more affordable than moving house. Instead of searching for the right property and dealing with the stress of moving, you can simply extend the house to fit your specification.

This also means that the scope for creativity and personalisation of the property is endless. You can have your dream home without having to move neighbourhoods.

Whilst a home extension may be more affordable than moving house, they are still expensive to complete and will involve huge costs depending on what work you choose to have done. Budgeting is absolutely vital for home extensions so that you don’t end up spending thousands more than you wanted to.

Another disadvantage is that home extensions can be stressful projects to undertake that are time-consuming and involve a lot of planning and you may not get exactly what you wanted (depending on building regs and planning permission approval).

How Do I Find and Hire a Builder?

Finding the right builder can be challenging. At HouseholdQuotes, we can connect you to builders in your area.

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When seeking out a contractor for your work, you should look to get three to five quotes for any extension to your home. That way you can get a feel for what the market rate is, as well as see who you get on with the most or feel is most capable of the job.

It’s important you don’t confuse an estimate with a quote. It’s fairly common for tradespeople, surveyors, and architects to offer an estimate the first time they visit your property. This estimate is a good starting point, but if you think they might be a good fit for the job, be sure to ask for a detailed quote so you can get a more precise idea of the costs.

It’s also worth making sure you have someone else with you when you get quotes. It can be helpful to have another opinion on price, timeframes, and personalities. What’s more, asking a family member or trusted friend to join you might help you feel less pressured to make a commitment on the spot.

When looking for contractors, you can ask your friends and relatives – even neighbours – if they’d had work done recently of a similar type and whether or not they’d recommend their professional to you.

This can help you in your search by speeding up the process if someone you trust has used someone who can do exactly what you need doing in your home.

Ensuring the Professional Is the Right Fit

First things first, a written quote is an essential part of any job. You should never accept a verbal agreement as your binding terms when undertaking any work, let alone an expensive home renovation.

With a written quote, you have something firm to refer to if needs be, simply stating what is and isn’t included in your quote, as well as for what fee and under what time frames.

Just like with a job interview, it’s good to find out your contractor’s experience, and whether or not they’ve built something like the structure you’re wanting for your home extension. Even if you have a brilliant verbatim recommendation from a friend or neighbour, if the contractor isn’t familiar with what you want, they mightn’t be the best fit for you.

Similarly, anyone can write something that sounds good on a website – but the proof is in the finished project. Asking for photos or videos of the contractor’s past work can further help to ensure you’re getting exactly what you want from your trader.

You should finally always double-check your professional has insurance to cover both themselves and you in the event of any trouble while constructing. This will also help you sidestep any cowboy traders, as they likely won’t bother with insurance and will get tetchy if you ask to see proof of it.

Final Checklist

If the answer to your lack of space at home lies in an extension, here’s our final checklist to make sure you’ve considered everything before embarking on your project.

  • Is an extension the right solution? Alternative space-making options can be loft conversions, basement/cellar conversions, as well as making use of an unloved garage. Make sure you’ve considered these avenues before settling on an extension first
  • Single or double storey? Think about the functionality of the room and what you want from it. If it’s a kitchen, make sure you consider the cost of new appliances and fixtures when looking at your budget
  • Make sure you have the correct building regulations before you get started on any work to ensure there are no hitches along the way – or wasted time and money
  • Find a contractor using HouseholdQuotes to help you save up to 40% off your project’s fee
  • Make sure you get a written quote from your trader before starting any work to safeguard you and your money
  • Enjoy your newly-created space!

Use HouseholdQuotes to find local builders and potentially save money on your home extension.


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Frequently Asked Questions

Do I Need Planning Permission or Building Regs Approval for My Extension?

Adding an extension to your home falls within permitted development, so it doesn’t require planning permission—as long as it meets certain criteria.

You will require building regulations to get started on your extension project, however, with rulings on doors/windows, drainage, electrics, walls and roofing, to name a new factors.

You can read more detailed advice on the Planning Portal website.

How Does a Bungalow Extension Differ?

Bungalow extensions can expand upwards and outwards, making them very similar to that of a two-storey house. There are a few more things to consider, however, such as removing parts of the existing roof to make room for a new storey, as well as getting planning permission for adding height onto your bungalow.

For more information, take a look at our dedicated bungalow extension page.

What Are Some Alternatives to Building an Extension?

If an extension is a step too far for you right now, there are alternative ways of adding some extra square footage into your property.

Instead of adding space outside, you can venture into the basement with a cellar conversion, or upwards with a loft conversion. If you have a garage that is more of a dumping ground than it is somewhere to store a vehicle, then turning that space into a useful room may be an option, too.

For something quite a lot like an extension, but not quite with the same level of commitment, you can opt for a conservatory or a lean-to, instead.

Could an Extension Add Value to My Home?

In short – yes. According to the insurer Hiscox, adding a bedroom could add up to 11.8% to the value of your home.

Adding a kitchen or dining room extension could increase the value of your home by up to 10.8%, while adding a bathroom could increase the value by up to 5.7% (all figures based on the average UK home value of £226,071 as priced in 2017).

Do I Need to Let My Neighbours Know I’m Planning an Extension?

If you’re planning to extend your home and live close to or are attached to your neighbour’s properties, then it’s likely that you will need to notify them of your plans. They then might request additional information from you which could take more time and could also increase your costs.

Your best bet is to keep your neighbours as informed as you can, as early on in the process as you can. Whilst they can’t stop from you building an extension, they do have the right to be aware of what you’re planning and request further information or action if they want to.

How Close to My Boundary Can I Build?

The general rule to go by is that a build that reaches 7.2 feet is considered acceptable, and anything over this will require you to speak to your neighbour. An extension of more than one storey cannot go beyond the boundary at the rear by more than 3 meters.

What Is the 45 Degree Rule?

This is a common rule used by planning offices and is assessed on both plan and elevation. An extension should not exceed a line taken at 45 degrees from the centre of the nearest ground floor window of a habitable room in an adjoining property.

If a proposed extension breaks this rule, it could be deemed unacceptable and not receive the permissions required to build.

Ready to start your project? We can help!

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