Flat Roof Extension Cost

Want to know the cost of having a flat roof extension installed?

On average, building a standard flat roof extension of around 25–30m² will cost approximately £50,000. However, costs can vary depending on the materials and complexities of the build, along with your location.

Just need a quote?

If so, we can help! We have a range of flat roof tradesmen ready to offer you a free quote!

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.

How Much Is a Flat Roof Extension?

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.

So, what is the cost of a flat roof extension?

The average cost of fitting a flat roof extension is £25,000 to £45,000 for a 20m² extension, £30,000 to £55,500 for a 25m² extension, £35,000 to £65,000 for a 30m² extension, £40,000 to £75,000 for a 35m² extension, or £45,000 to £85,000 for a 40m² extension.

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.

Labour Costs and Timescales

The work is likely to last between eight and ten weeks. However, the work could be shorter or longer. An approximate average labour cost for this work would be £20,800 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.

Supply Only Costs

We'll now look at the cost of materials only for installing a flat roof extension. This is as if you were to purchase the materials directly from a retailer.

Starting with the actual roof, the supply costs for an EPDM rubber roof are about £45 to 70 per m². For felt roofing, expect to pay between £30 and £45 per m².

Then you must consider the various other material costs for constructing a flat roof extension which would run into the thousands if not tens of thousands of pounds.

The supply costs of a flat roof extension tend to be lower than the cost of labour, but this will depend on a range of factors.

Additional Costs

When having a flat roof added to an extension, you may wish to pay for extra jobs to be undertaken around the same time. Let's have a look at some popular examples.

Gutter Replacement

While working on the roof, it's an ideal time to replace guttering where required. Prices vary depending on the type of property and the amount of guttering required, although as an average cost, expect to pay as follows:

• Plastic — £50 – £110
• Cast Iron — £460 – £920
• Copper — £650 – £1,250
• Steel — £390 – £790
• Zinc — £490 – £970
• Aluminium — £570 – £1,100

Replacing Roof Tiles

If your house roof needs to be freshened up, this would be a good opportunity to replace roof tiles.

To replace several roof tiles:

• Clay Tiles — £150 to £190
• Concrete Tiles — £180 to £220
• Slate Roof Tiles — £200 to £250
• Replace All Roof Tiles — £10,000 to £20,000

Fitting Fascias and Soffit Boards:

While installing a flat roof extension, it's the ideal opportunity to consider the cost of capping fascias soffits

• Capping a 2-Bedroom Home — £770
• Capping a 3-Bedroom Home — £950
• Capping a 4-Bedroom Home — £1,325
• Capping a Bungalow — £700
• Capping a Single 15m Wall — £290

As a guide, capping fascia and soffit boards to the front and gable end wall, based on 24m length in total is approximately £445.

Find Tradespeople, compare up to 3 quotes!

It's FREE and there are no obligations

Get started

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.

Ask a Trade

Got a question that only a tradesperson can answer? We have thousands of trades ready to answer any question you may have.

Ask your question

Interior Finishing

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 EDPM.

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

Also known as BUR, built-up roofing is a popular choice for low-slope roofs. They are 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 Do You Insulate a Flat Roof Extension?

There are several steps you’ll need to take. They are as follows.

• Lay 1.2cm of OSB board or plywood along the timber joists as a base
• Fit a vapour control layer. The purpose of this is to lower the danger that moisture/condensation will compromise the structure of your flat roof
• Make markings on the vapour control layer at the point at which fixings will pierce through the membrane. To be more specific, you want to do this at the rafter centres and correct axial spacing
• Add butyl tape at the points at which the fixings will pierce the vapour control layer. The reason for this is to ensure the tape forms a seal after penetration
• Next, you’ll want to fit rigid insulation board (the thickness will depend on the type of flat roof and other factors). The exact nature of the rigid insulation board would depend on the height available based on guttering, upstands as well as adornments
• Add the next layer of OSB board or plywood
• Use helical fixings to fix the deck together. You need to pierce every layer of the roof building up. This includes the flat roof insulation
• Lastly, add a waterproof membrane

Building Regulations & Planning Permission for a Flat Roof Extension

Planning permission isn't generally required for a flat roof extension, as they largely fall within permitted development rights. The exceptions being if you live in a listed building or conservation area, the extension requires structural changes or changes the appearance of the property.

If in doubt, it's advisable to contact your local planning authority. Submitting a planning application typically costs around £200 (there may be local differences), and decisions are usually made within eight weeks.

For more information, visit the Planning Portal. Please note that rules for England and Wales may differ from the rest of the UK.

To comply with building regulations, the roof should have a minimum fall of 1:40 to ensure effective rainwater drainage, usually guiding excess water towards the edge of the roof.

The structure must be capable of withstanding high winds and strong enough to be safely walked on if access is required, and a waterproof membrane should extend at least 150mm (15cm) up any adjoining walls.

In addition, if more than 25% of the roof area is being replaced or extended, the thermal insulation must also be upgraded to meet the requirements of Part L of the Building Regulations.

Flat Roof Extension vs Pitched Roof Extension

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

Which 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 Roofer for Installing a Flat Roof Extension

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.