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myFlat Roof Extension Cost
At a Glance
- The average flat roof extension cost is £50,000.
- Most jobs take 8 to 10 weeks to complete.
- Flat roof extension cost breakdown by type and size.
- How long a flat roof extension takes and what's typically involved.
- How to find builders near you using MyJobQuote.
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Want to know the cost of having a flat roof extension installed?
On average, building a standard flat roof extension of around 25 to 30m² will cost approximately £50,000. However, costs can vary depending on the materials and complexities of the build, along with your location.
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In this guide, we'll look at the cost of installing different-sized flat roof extensions and explore other related subjects ranging from what's involved in installing flat roof extensions to what you should consider before hiring contractors for the job.
£50,000
Table of Contents
- How Much Does a Flat Roof Extension Cost?
- Labour Cost of a Flat Roof Extension
- Supply Only Cost of a Flat Roof Extension
- Additional Flat Roof Extension Costs
- What Impacts Flat Roof Extension Costs?
- What's Involved in a Flat Roof Extension?
- Interior Finishing Costs of a Flat Roof Extension
- Types of Flat Roof Extension
- How Much Does It Cost to Insulate a Flat Roof Extension?
- Flat Roof vs Pitched Roof Extension Cost
- Checklist: Hiring a Contractor to Build a Flat Roof Extension in the UK
- FAQs
How Much Does a Flat Roof Extension Cost?
Flat roof extension costs can vary depending on the size, style and complexities of the roof extension. As such, this can be anything from £25,000 to £85,000 or potentially even higher in more substantial flat roof house extension projects.
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So, what is the cost of a flat roof extension?
Flat roof extension costs generally scale with the size of the build, working out at around £1,200 to £2,500 per m² for materials and labour combined. The table below shows how total costs typically break down for common extension sizes, from 20m² up to 40m².
What sort of drainage systems can you get for flat roofs? We're thinking of a flat-roof extension but I don't know what the best option is to keep it draining well.
"Scuppers are the most effective drainage solution for flat roofs. With this system, large square openings are made along the edge of the roof that shoot the water away from the side of the building."
Flat Roof Extension Prices
The cost of a flat roof extension depends largely on the size of the installation, with estimated prices listed below (including materials and labour fees). However, the actual cost may vary, depending on factors such as ease of access and the extent of work that needs to be actioned.
Location also impacts the price, as labour fees tend to be higher in areas such as London and the South East.
| Size of Roof | Average Cost |
|---|---|
| 20m² | £25,000–£45,000 |
| 25m² | £30,000–£55,500 |
| 30m² | £35,000–£65,000 |
| 35m² | £40,000–£75,000 |
| 40m² | £45,000–£85,000 |
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Labour Cost of a Flat Roof Extension
The work is likely to last between 8 and 10 weeks. However, the work could be shorter or longer. An approximate average labour cost for this work would be £21,000 to £26,000, but it would depend on the size of the flat roof extension.
The cost of labour and work duration would vary depending on the size of the flat roof extension and the ease of access. Beyond that, where you live also matters when it comes to the cost of labour specifically.
| Size of Roof | Labour Only Cost |
|---|---|
| 20m² | £16,000–£20,000 |
| 25m² | £20,000–£25,000 |
| 30m² | £24,000–£30,000 |
| 35m² | £28,000–£35,000 |
| 40m² | £32,000–£40,000 |
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Supply Only Cost of a Flat Roof Extension
Materials make up a significant part of a flat roof extension's total cost, and the roof covering itself is only one part of that. Foundations, external walls, windows, doors, and insulation typically account for a larger share of the overall material cost. This section breaks down the main supply-only prices, useful if you're comparing quotes or sourcing some materials yourself.
| Material | Typical Supply Cost |
|---|---|
| Foundations and Groundworks | £80–£300 per m² |
| External Walls (Brick and Block) | £90–£160 per m² |
| Roof Structure (Timber Joists and Decking) | £25–£45 per m² |
| Roof Covering (Felt, EPDM, or Glass Reinforced Plastic) | £30–£100 per m² |
| Insulation | £18–£30 per m² |
| Windows and Doors | £150–£400 per window |
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Foundations usually need to be more substantial for an extension than routine groundworks, and costs vary significantly depending on ground conditions and the depth required. Costs are normally priced by trench length, depth, and concrete volume rather than a flat rate per m². See our guide to laying foundations for more detail.
For the roof covering itself, EPDM rubber costs £45 to £70 per m², felt roofing costs £30 to £45 per m², and GRP (glass reinforced plastic) fibreglass typically costs £70 to £100 per m². Rubber roofing is more durable than felt, which is reflected in the higher price.
Window supply costs vary by frame material and size, with uPVC at the more affordable end and timber or composite frames at the premium end. This is a supply-only figure, excluding fitting, waste disposal, and certification.
Altogether, these materials typically total £15,000 to £30,000 or more for a full-size extension, depending on specification and size. Supply costs tend to be lower overall than labour costs, though this depends on the materials you choose and how much a contractor sources on your behalf.
Additional Flat Roof Extension Costs
Beyond the core build, several additional costs can affect your final flat roof extension budget.
- Planning Permission — If your extension falls outside permitted development rights, a planning application typically costs £258.
- Building Regulations — Approval is separate from planning permission and usually costs £300 to £1,000, depending on whether you submit a Building Notice or Full Plans application.
- Structural Engineer — A report confirming foundations and load-bearing calculations typically costs £500 to £1,500.
- Party Wall Agreement — If your extension is close to a boundary or shared wall, this typically costs £700 to £1,500 per neighbour.
- Scaffolding — For a single storey extension, this typically costs £900 to £1,500, depending on how long it's needed. See our guide to the cost of hiring scaffolding for more detail.
- Waste Removal — A builder's skip typically costs £200 to £400 for a two-week hire, unless it's included in your contractor's quote. Find out more about waste removal costs.
- Roof Lanterns and Glazing — Adding natural light this way can add £2,000 to £6,000 to your overall cost, depending on the size and specification. See our guide to roof lantern costs.
- VAT — Most flat roof extension work is subject to 20% VAT. Always check whether quotes you receive include this.
What Impacts Flat Roof Extension Costs?
There are many cost-affecting factors that can shape the price of a flat roof extension.
- Type of Flat Roof Extension — The materials used are a big cost factor in a home extension. A felt flat roof is among the cheaper options, whereas rubber, fibreglass, and GRP roof options are more middle-ground choices. The design style, whether a modern flat roof extension with a lantern, overhang, or single storey extension versus a double storey extension, will also shape the overall cost of a flat roof extension installation.
- Area of Installation — The larger the work area, the more expensive the materials will be. Beyond that, a larger work area would mean the job would take longer, and this may also increase the labour cost.
- Location of Property — Labour costs differ throughout the UK and are an important consideration for your budget. Labour prices tend to be higher in the South East and London in particular, and generally lower in places like the north of England, Scotland and Northern Ireland.
- Ease of Access — The more accessible the workspace, the easier the job will be, which should reduce both the time and overall expense of your flat roof extension.
- Drainage — Flat roofs require a slight slope or slight pitch to ensure effective drainage and prevent water pooling. The minimal slope needed will affect how the roof is designed and constructed, and a more complex drainage solution will increase the overall cost.
- Glazing and Natural Light — Adding skylights, roof lanterns, or lantern roofs is a popular way to bring natural light into your flat roof extension and create the illusion of extra space. Glazing options and light wells will add to your overall flat roof extension cost, but can also improve energy efficiency and help to reduce energy bills.
What's Involved in a Flat Roof Extension?
The process of building a flat roof extension is long and complex and the methods can vary depending on the type of flat roof extension among other factors. We'll look at an approximate guide to the process.
- Hiring Professionals — You may want to hire a company or even a contractor who, in turn, can assemble the rest of the labour required. Otherwise, you could contact all necessary professionals separately from a builder to an architect to an electrician. However, this would involve a ton of organisation and planning.
- Foundations — After the area has been surveyed, plans put in place and materials purchased, the foundation will be the first focus of the actual project. Professionals will handle this phase, ensuring a solid and compliant base is laid for the rest of the structure.
- Construction — Following the foundations, the exterior walls will be built, then the roof can be installed. Once the structure is sealed, interior work begins — usually starting with plastering or painting. You may also need to install roof insulation depending on building regulations.
- Finishing Touches — After this, other interior installations such as pipework and electrics can be carried out, as well as hanging doors and introducing furniture and any personal elements.
- Clean-up — Once the job is finished, waste removal expenses can come into play unless already part of the overall cost. A skip may be used, or those you hire may initially offer to take care of this for you. Make sure to have a plan in place for waste removal before work gets underway.
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Interior Finishing Costs of a Flat Roof Extension
Depending on what you intend to use it for, once your flat roof extension has been built, you might have a range of other tasks to consider.
You should factor these into your overall costings, so you don't go over budget.
Electrics and Plumbing
Installing services such as electrics and plumbing will be necessary if the flat roof extension is to be part of your living space.
You'll need to consider radiators, sockets, and other elements that you'd typically find in a living area.
For example, you may want to explore the cost of spotlights and downlights to illuminate your new space, or perhaps the overall new bathroom cost if that's going to be part of the extension.
Insulation and Ventilation
To keep energy costs down, you should consider adding effective insulation to your flat roof extension. At the same time, you need to check whether there is adequate ventilation to stop damp building up in the space.
As an example, the typical cost of installing floor insulation can be anywhere between £210 and £950 for a 15 square metre room.
Plastering or Plasterboarding
If you're confident and have the right skills, you might be able to undertake any plastering or plasterboarding yourself to keep costs down.
You can use a plastering an indoor wall cost calculator to figure out how much it could cost to get a professional to do the job.
Painting and Decorating
Some of the more personal touches to any flat roof extension will be found in the way you choose to decorate the interior.
You can reduce the costs of painting and decorating by doing some of the work yourself, although you may want to hire a professional for that perfect finish.
Types of Flat Roof Extension
If you're unsure what type of extension flat roof to go with, this section is for you. We'll look at the features and pros and cons of various options in terms of the actual roofing material used.
EPDM
EPDM rubber roofing is a popular choice for flat roofs. Aside from being durable, EPDM requires minimal maintenance and offers a nice aesthetic. It is also relatively eco-friendly and easy to repair.
On the other hand, installation can be tricky, it is relatively damage-prone and you may find it more difficult to find someone to install an EPDM roof than other options.
Pros
- ✔ Lengthy lifespan of 30 to 50 years
- ✔ Highly weather-resistant
- ✔ Lightweight material
- ✔ Fairly low maintenance
Cons
- ✖ More expensive upfront.
- ✖ Installation can fail if not done properly.
- ✖ Not the most aesthetically pleasing option.
- ✖ At risk of punctures.
Roofing Felt
This option is fairly affordable, suited to various homes, easy to repair, and visually appealing.
With that being said, it is not well-suited to footfall, requires a professional installation and is fairly damage-prone. You may need to remove and replace roofing felt more often than you would with EPDM.
Pros
- ✔ An affordable option.
- ✔ Repairs can be patched easily.
- ✔ Straightforward installation for experienced contractors.
- ✔ Quick to install.
Cons
- ✖ Only lasts around 10 to 20 years.
- ✖ Susceptible to UV rays and other damage.
- ✖ Needs regular maintenance.
- ✖ Patch repairs can look untidy over time.
Built-up Roofing
Built-up roofing (BUR) is a popular choice for low-slope roofs. It is a great option if you want a flat roof that is water-resistant and protects against UV rays or a type of roofing that provides fire resistance.
However, BUR can be time-consuming to install and unless you go with the cold bitumen processes, installation comes with hazardous fumes. As a result, it should be left to a suitable professional. BUR is also prone to wind-related damage.
Pros
- ✔ Decent lifespan of 20 to 30 years.
- ✔ Can have effective fire resistance.
- ✔ Multi-layer design improves durability.
- ✔ Great for large areas.
Cons
- ✖ Heavy and so may need structural reinforcement.
- ✖ An expensive option.
- ✖ Complex to install.
- ✖ Future leaks may be difficult to treat.
How Much Does It Cost to Insulate a Flat Roof Extension?
Insulating a flat roof extension properly is essential for meeting building regulations, and it can help reduce your energy bills. For general pricing across different roof types, see our guide to roof insulation costs. Below is how this typically breaks down for a flat roof extension specifically.
Warm Roof Insulation
A warm roof places the insulation above the roof deck, below the waterproof membrane. This is the most common approach for new flat roof extensions, as it reduces the risk of condensation. A warm roof typically costs £60 to £90 per m² to install, including insulation boards, a vapour control layer, and the waterproofing membrane.
Cold Roof Insulation
A cold roof places insulation between the joists, below the roof deck, with a ventilated air gap above. This method is cheaper, at around £40 to £65 per m², but it requires careful ventilation to prevent condensation and damp building up over time.
Whichever method you choose, a professional installer will need to fit a vapour control layer, seal any fixing points, and ensure the insulation meets Part L of the building regulations if more than 25% of the roof is being replaced.
Flat Roof vs Pitched Roof Extension Cost
A flat roof extension is generally cheaper than a pitched roof extension. A pitched roof extension requires additional framing, more roofing material, and longer labour time compared to a flat roof extension, which means fewer materials and lower overall costs.
For a single storey extension, the roofing element alone typically costs:
- Flat roof — £5,000 to £8,000.
- Pitched roof — £9,000 to £15,000.
This means a pitched roof extension adds around £4,000 to £7,000 just for the roof itself.
A pitched roof extension can complement the existing architecture of traditional properties, and for many homeowners, that is worth the extra cost. However, a modern flat roof extension offers a contemporary look with clean lines that works well on newer builds and is a cost effective option for most budgets.
It's worth weighing up both a flat roof extension and a pitched roof extension against your existing property before making a decision.
Checklist: Hiring a Contractor to Build a Flat Roof Extension in the UK
Before hiring a professional for your flat roof extension, there are some pointers worth taking into account:
- Are they a member of a recognised trade body? The National Federation of Roofing Contractors (NFRC) and the Federation of Master Builders (FMB) are both worth looking out for.
- Are they registered with the NFRC Competent Person Scheme? Roofers on this scheme can self-certify that their work meets building regulations, so you won't need to verify compliance through your local authority or local council.
- Are they TrustMark registered? This government-endorsed scheme vets businesses for quality workmanship, trading practices, and customer service, giving homeowners extra peace of mind.
- Do they have insurance? Public liability insurance protects you if something goes wrong during your new extension.
- Will they provide an itemised quote? A full breakdown helps ensure you know exactly what is included in your flat roof extension costs and if there are any hidden costs, such as VAT, scaffolding, or additional expenses.
- Is waste removal included? Confirm that the removal and disposal of all waste generated during your flat roof extension is included in the total cost quoted.
- Do they have experience with your type of flat roof extension? Ensure they have worked with your chosen flat roofing material before and can show evidence of their work.
- Do they have positive reviews? Check their MyJobQuote profile to read feedback from previous customers.
FAQs
Do I Need Planning Permission for a Flat Roof Extension?
Most flat roof extensions fall under permitted development rights, so planning permission usually isn't required. You may need permission if your property is listed, sits in a conservation area, or if the extension changes the height, footprint, or appearance of the property beyond permitted limits.
If you're unsure, contact your local planning authority before starting work. A planning application typically costs £258, and decisions are usually made within 8 weeks. You can find more information on the Planning Portal, though rules for England and Wales may differ from the rest of the UK.
What Building Regulations Apply to a Flat Roof Extension?
A flat roof must have a minimum fall of 1:40 to allow effective drainage, be strong enough to withstand high winds, and be safe to walk on if access is required. The waterproof membrane should extend at least 150mm up any adjoining walls.
If more than 25% of the roof area is being replaced or extended, thermal insulation must also be upgraded to meet Part L of the Building Regulations. Building regulations approval typically costs £300 to £1,000, depending on whether you submit a Building Notice or a Full Plans application, and this fee usually includes the required Building Control inspections.
Is £40,000 Enough for a Flat Roof Extension?
£40,000 is enough for a smaller flat roof extension, typically up to around 25m², depending on the roofing material, finish, and your location. Larger extensions, or those with roof lanterns and higher-spec finishes, are likely to exceed this budget.
How Much Does a 3m by 4m Flat Roof Extension Cost?
A 3m by 4m extension covers around 12m², which typically costs £15,000 to £27,000 for a flat roof, including materials and labour. The exact price depends on the roofing material, ease of access, and your location.
Does Installing a Flat Roof Extension Affect My Home Insurance?
Yes, adding a flat roof extension can affect your home insurance with the potential to increase your premiums, so you should always check with your insurance provider for further review.
How Do You Run Cables on a Flat Felt Roof Extension?
This must be done by a professional in alignment with Part P of the building regulations. You should not attempt this DIY.
Can I Convert a Flat Roof Extension to a Pitched Roof Extension?
Converting a flat roof extension to a pitched roof is a significant structural project that involves removing the existing flat roof and building a new pitched roof structure in its place. Changing a flat roof to a pitched roof extension often requires formal planning permission, as it changes both the height and appearance of your property.
Can I Install a Flat Roof Extension Myself?
The majority of construction work must be done by a professional to meet building regulations and legal standards. Hazards include carrying additional weight with heavy materials, working at height, and incorrect installation, which can pose safety risks.
