How Much Does an Extension Cost?
Find out which extensions are the cheapest, which cost the most, and how your budget would be split
A rear extension is a major project that has many elements and stages to it. Each comes with its own costs and, within each stage, these costs can vary greatly. So how do you start to figure out whether your budget will stretch to extending, or where you could scale back to make things work?
Three design experts with experience of working on a huge variety of extension types offer their takes on how much you’ll need and on what exactly your money will be spent.
Professional advice from: Sam Cooper of E2 Architecture + Interiors; Cat Hoad of Absolute Project Management; Eilir Sheryn of van Ellen + Sheryn Architects
Three design experts with experience of working on a huge variety of extension types offer their takes on how much you’ll need and on what exactly your money will be spent.
Professional advice from: Sam Cooper of E2 Architecture + Interiors; Cat Hoad of Absolute Project Management; Eilir Sheryn of van Ellen + Sheryn Architects
Which type of extension is cheapest?
The general consensus is that smaller extensions simply require a smaller budget. “And the less structural work, the cheaper, too,” Cat says.
She also points out that the more paperwork is involved, the more a project is likely to cost. So if you can avoid the need for Planning Permission and swerve triggering the Party Wall Act, this should save you money in terms of time and specialist fees.
Sam agrees the smallest and simplest box, with the minimum amount of glazing – and no non-standard windows or doors – should make for the cheapest option. However, he suggests it’s not just about size. “What is the best value extension is another question you should be asking,” he says. “This would be one that, within an efficient amount of space, achieves the biggest wow by design. By reducing the area, you have more budget to spend on higher-quality materials and fittings and you will plan and use your space better.”
“A masonry or timber-framed construction with standard windows and simple, mono-pitched roof construction is the most cost-effective,” Eilir says. “Generally, one should try to reduce the amount of expensive components and complexity of structure.”
The general consensus is that smaller extensions simply require a smaller budget. “And the less structural work, the cheaper, too,” Cat says.
She also points out that the more paperwork is involved, the more a project is likely to cost. So if you can avoid the need for Planning Permission and swerve triggering the Party Wall Act, this should save you money in terms of time and specialist fees.
Sam agrees the smallest and simplest box, with the minimum amount of glazing – and no non-standard windows or doors – should make for the cheapest option. However, he suggests it’s not just about size. “What is the best value extension is another question you should be asking,” he says. “This would be one that, within an efficient amount of space, achieves the biggest wow by design. By reducing the area, you have more budget to spend on higher-quality materials and fittings and you will plan and use your space better.”
“A masonry or timber-framed construction with standard windows and simple, mono-pitched roof construction is the most cost-effective,” Eilir says. “Generally, one should try to reduce the amount of expensive components and complexity of structure.”
Which is the most expensive?
“If your project requires extensive re-engineering to the main structure of the existing house – common if you’re opening up several walls – then the cost will be higher,” Cat explains. She also points out that how you finish the extension will make a huge difference, saying that anything bespoke rather than off-the-shelf will up costs. “One recent project spent £12,000 on doors alone,” she says.
Eilir agrees. “The more high-spec the glazing, the more expensive the design becomes,” he says. “Reduced sightlines and transparency come at a cost.”
At the same time, the opposite approach can also incur more cost. “Expressed structures – which will be on show rather than concealed – can be more expensive due to the additional craftmanship and ‘finish’ necessary,” Eilir says.
Sam takes a slightly broader angle, thinking more about the value of a design. “What’s more expensive is an extension that’s larger than it needs to be, one that devalues the house, one not built to the right quality so it’s unusable at certain times, or one that doesn’t connect to the house in the right way, so you don’t use existing spaces in the house as a consequence.” He adds that extensions with basements are expensive, too.
Read more about how to avoid ‘dead front room syndrome’ when extending.
“If your project requires extensive re-engineering to the main structure of the existing house – common if you’re opening up several walls – then the cost will be higher,” Cat explains. She also points out that how you finish the extension will make a huge difference, saying that anything bespoke rather than off-the-shelf will up costs. “One recent project spent £12,000 on doors alone,” she says.
Eilir agrees. “The more high-spec the glazing, the more expensive the design becomes,” he says. “Reduced sightlines and transparency come at a cost.”
At the same time, the opposite approach can also incur more cost. “Expressed structures – which will be on show rather than concealed – can be more expensive due to the additional craftmanship and ‘finish’ necessary,” Eilir says.
Sam takes a slightly broader angle, thinking more about the value of a design. “What’s more expensive is an extension that’s larger than it needs to be, one that devalues the house, one not built to the right quality so it’s unusable at certain times, or one that doesn’t connect to the house in the right way, so you don’t use existing spaces in the house as a consequence.” He adds that extensions with basements are expensive, too.
Read more about how to avoid ‘dead front room syndrome’ when extending.
Will I need to pay extra for planning?
“It depends if the extension falls within Permitted Development rights,” Sam says. “If not, [it will need Planning Permission]. If it’s in a conservation area or on a listed building, extra scrutiny and therefore design expertise will be required to get it through planning.”
Cat adds that the cost will depend on the sort of planning consent required. “A Lawful Development Certificate or similar will be cheaper than full Planning Permission,” she says, while Listed Building Consent can be complex, so may add cost.
Although a project may not require Planning Permission, Eilir says he would always advise homeowners receive a Lawful Development Certificate from the local authority.
Find the perfect architect or design and build company for your extension in the Houzz Professionals Directory.
“It depends if the extension falls within Permitted Development rights,” Sam says. “If not, [it will need Planning Permission]. If it’s in a conservation area or on a listed building, extra scrutiny and therefore design expertise will be required to get it through planning.”
Cat adds that the cost will depend on the sort of planning consent required. “A Lawful Development Certificate or similar will be cheaper than full Planning Permission,” she says, while Listed Building Consent can be complex, so may add cost.
Although a project may not require Planning Permission, Eilir says he would always advise homeowners receive a Lawful Development Certificate from the local authority.
Find the perfect architect or design and build company for your extension in the Houzz Professionals Directory.
What other costs should I consider?
“Interior design,” Cat says. “Someone has to do this, even if you ‘just’ want, for example, a simple bathroom.” She highlights this should cover electrics, as changes after plastering or structural work – ie, to add power in or move cables – can be expensive. She also advises homeowners check whether a ‘second fix’ – floor fittings, window coverings, light fittings – is included in the price quoted.
You’ll also need to take into consideration any other work that needs to be done to the ground floor and garden as a result of the extension, Sam says. “Other common problems include structural defects in the existing foundations,” he says, adding that drains are also often a problem that require build-over licences from the water authority or expensive diversions and upgrades.
“Control of these is all in the preparation,” he says. “Spend that early money on surveys and other investigations your professionals may request, so you can control the risks as much as possible.”
“Interior design,” Cat says. “Someone has to do this, even if you ‘just’ want, for example, a simple bathroom.” She highlights this should cover electrics, as changes after plastering or structural work – ie, to add power in or move cables – can be expensive. She also advises homeowners check whether a ‘second fix’ – floor fittings, window coverings, light fittings – is included in the price quoted.
You’ll also need to take into consideration any other work that needs to be done to the ground floor and garden as a result of the extension, Sam says. “Other common problems include structural defects in the existing foundations,” he says, adding that drains are also often a problem that require build-over licences from the water authority or expensive diversions and upgrades.
“Control of these is all in the preparation,” he says. “Spend that early money on surveys and other investigations your professionals may request, so you can control the risks as much as possible.”
How much budget should I keep aside and for what?
All agree that between 10% and 20% is advisable to set aside over and above the estimated costs. “We advise 10% for a straightforward project, but that would rise to 15% or 20% if there were a lot of unknowns with the condition of the building, the land it stands on, and so on,” Cat says.
Sam adds that planning, detailing and scheduling a project scrupulously is key to reducing overspend, but warns, “Projects of any size can still throw up unforeseen expenses that may not vary according to the scale of the project. For a smaller project, you therefore need a proportionally higher contingency,” he says.
Sam also says the majority of cost overruns he sees are down to clients changing – usually expanding – the brief as the project progresses, though this is not to be discouraged, he suggests. “Sometimes, requirements change over the course of a project, or you see an opportunity to change something that will cost more, but significantly increase your enjoyment of the property,” he says. “Just be realistic that this could happen to you.”
All agree that between 10% and 20% is advisable to set aside over and above the estimated costs. “We advise 10% for a straightforward project, but that would rise to 15% or 20% if there were a lot of unknowns with the condition of the building, the land it stands on, and so on,” Cat says.
Sam adds that planning, detailing and scheduling a project scrupulously is key to reducing overspend, but warns, “Projects of any size can still throw up unforeseen expenses that may not vary according to the scale of the project. For a smaller project, you therefore need a proportionally higher contingency,” he says.
Sam also says the majority of cost overruns he sees are down to clients changing – usually expanding – the brief as the project progresses, though this is not to be discouraged, he suggests. “Sometimes, requirements change over the course of a project, or you see an opportunity to change something that will cost more, but significantly increase your enjoyment of the property,” he says. “Just be realistic that this could happen to you.”
What should I consider when choosing a quote?
“Consider what is not in the quote,” Sam cautions. “To the untrained eye, this is not easy. Take time to interrogate the quote and ask questions: What will you be expected to provide and supply? Is VAT included (usually not)? Are fittings and interior design and finishes included? If so, what are they? What materials have been allowed for? What programme? Who will be managing the design/build? What are the contract terms?”
Cat builds on this, adding that a quote should be broken down in great detail. “[It should be] very clear about which specialists are included and how much time you’re ‘allowed’ to discuss or amend designs and be updated about progress.”
Check out how to avoid an extension that’s too hot and too bright.
“Consider what is not in the quote,” Sam cautions. “To the untrained eye, this is not easy. Take time to interrogate the quote and ask questions: What will you be expected to provide and supply? Is VAT included (usually not)? Are fittings and interior design and finishes included? If so, what are they? What materials have been allowed for? What programme? Who will be managing the design/build? What are the contract terms?”
Cat builds on this, adding that a quote should be broken down in great detail. “[It should be] very clear about which specialists are included and how much time you’re ‘allowed’ to discuss or amend designs and be updated about progress.”
Check out how to avoid an extension that’s too hot and too bright.
How is the budget distributed across a typical project?
Very roughtly, around a third of your budget should go on costs required before works start, Cat explains, while two thirds will typically be spent during the build and distributed fairly evenly throughout the works – “If you have a realistic programme of time needed to get on site,” she adds.
Sam breaks down a typical project. On a basic build of £100,000, paid to the contractor, you could expect to spend the following (working backwards) on top:
Construction phase
£30,000 – fittings and fixtures (kitchen, door handles, feature lights, downstairs toilet fittings)
£6,000 – professional fees, architect and engineer
Tender phase
£8,000 – architect fees
£700 – Party Wall Agreement
£1,500 – engineer
£750 – building control
Planning phase
£3,500 – architect fees
£150 – planning fees
Appraisal, brief development and concept
£1,500 – architect fees. “This is the lowest cost/highest value investment you will make in the duration of the project,” he says, “and should offer you the keys to unlock a spectacular project and get the highest return on your investment.”
Very roughtly, around a third of your budget should go on costs required before works start, Cat explains, while two thirds will typically be spent during the build and distributed fairly evenly throughout the works – “If you have a realistic programme of time needed to get on site,” she adds.
Sam breaks down a typical project. On a basic build of £100,000, paid to the contractor, you could expect to spend the following (working backwards) on top:
Construction phase
£30,000 – fittings and fixtures (kitchen, door handles, feature lights, downstairs toilet fittings)
£6,000 – professional fees, architect and engineer
Tender phase
£8,000 – architect fees
£700 – Party Wall Agreement
£1,500 – engineer
£750 – building control
Planning phase
£3,500 – architect fees
£150 – planning fees
Appraisal, brief development and concept
£1,500 – architect fees. “This is the lowest cost/highest value investment you will make in the duration of the project,” he says, “and should offer you the keys to unlock a spectacular project and get the highest return on your investment.”
Tell us…
What have you learned about budgeting from having an extension built? Share your tips in the Comments.
What have you learned about budgeting from having an extension built? Share your tips in the Comments.
“In London, at the bare minimum, it’s £60,000 and can go up to around £150,000,” Cat Hoad says.
“It’s important to check whether the price you’re looking at is just for the ‘shell’,” she adds. In other words, just the structural work, walls, floor, roof, and protection from the elements, but not ‘finished’, which would also include it being plastered and having its first fix electrics and plumbing.
“On top of that,” she says, “you’d also need to factor in the cost of a new kitchen and joinery, among other expenses, depending on what you’re planning.”
Sam Cooper explains his firm would initially base an estimated budget on a price per square metre. “This would be influenced by location, accessibility, complexity of the site and specification,” he says.
This would not include fixtures and fittings, which Sam explains is one of the two major areas where clients can control their budget, citing the choice of kitchen as just one example. The other, he says, is how big the extension is.
Eilir Sheryn makes a good point about remembering to factor in the VAT, which, he says, many clients can forget. “[Also add on] professional fees, application fees and a working contingency,” he says. These can all add up to around £50,000.
Eilir says his firm tends to work on extensions that cost from £150,000 to £400,000.