Expanding your living space with a flat roof extension is a popular choice among homeowners in the UK. However, understanding the associated costs is essential for effective budgeting and planning. Let's delve into the intricacies of flat roof extension expenses and considerations.

How much does a flat roof extension cost?

Flat roof extension costs in the UK typically range from £20,000 to £60,000 or more, depending on the size of the extension, the materials you choose and your location. On average, expect to spend £37,000 on a basic flat roof extension of 20m².

Size of flat roof extension	Average cost
20m2	£37,000
25m2	£46,250
30m2	£55,500
40m2	£74,000

The final cost of your flat roof extension will be determined by its size, whether you choose budget or premium fixtures, and where you live. To get a more accurate idea of how much a flat roof extension costs in your area, take a look at our table further down the page.

Flat roof extension cost per m²

The cost per square metre for a flat roof extension can vary from £1,200 to £2,500 or more. You may find that the larger your extension, the less you’ll pay per square metre, as extension builders can often save money on materials when they buy in large quantities. Plus, an extension that is double the size often doesn’t take double the amount to complete, so you may save on labour costs.

Factors affecting the cost of flat roof extensions

Size and design

The size and design of your flat roof extension will have a big impact on how much it costs. Larger extensions with more intricate designs, for example those with bathrooms, kitchens or more than one room, will of course need more materials and take longer to build.

Choice of materials

The materials you choose also affects the cost of your flat roof extension. Roofing options like traditional felt or bitumen often cost less than modern alternatives like EDPM rubber or fibreglass, but they may not last as long. While it may seem like the cheaper option to go for a felt roof, you may save money in the long term by choosing a more durable material.

The materials you choose for inside your extension will also have an impact. Luxury options like hardwood flooring, stone tiles or marble kitchen worktops will of course cost more, but may improve your enjoyment of the space and the resale value of your property.

Structural considerations

If you plan to have a rooftop terrace on your flat roof extension, you may need to take the cost of additional structural reinforcements into account. Ensuring your extension can take the extra weight may require reports from structural engineers or architects and you may need to apply for planning permission.

Planning permission

Most flat roof extensions are covered under permitted development rights as long as they don't exceed 3 metres in height and are not visible from a main road. However, different rules may apply if you live in a conservation area, a listed building or even a new build estate, so it’s always worth checking with your local authority to ensure you comply with the rules. A planning application typically costs £322.

Extra features or utilities

A flat roof extension that’s just being used for extra space will cost less than one that has extra features such as kitchens, bathrooms, skylights or even solar panels. Although these features cost more, they’ll enhance the functionality of the extension and help you to enjoy the space more. Plus, if you do choose to install solar panels on your flat roof, you’ll generate your own free electricity too.

Where you live

Where you live is one of the biggest factors affecting the cost of a flat roof extension, and you can’t do much about it! If you live near London or in the south east you’ll usually pay much more than the average for an extension, while those living in Scotland, Wales and the north of England typically pay less.

Advantages and disadvantages of flat roofs

It’s important to weigh up the pros and cons of flat roofs before you decide whether to add one to your extension. Take a read through our considerations to help you make up your mind:

Advantages

Cost-effectiveness: Flat roofs are usually much cheaper and quicker to install than pitched roofs. Simple fitting and cheaper materials mean that you’ll pay far less than a pitched roof extension, which could cost as much as £90,000 for two storeys.

Maximise space: Flat roofs offer additional usable space, giving you the opportunity for a rooftop gardens, balcony or solar panels. This is particularly good if you live in an urban area where space is limited.

Accessibility and maintenance: Flat roofs are easier to access and maintain than pitched roofs, making tasks such as gutter cleaning, roof inspections, and repairs more straightforward. Maintenance costs are often lower for flat roofs due to easier access and simpler design.

Modern design: Contemporary flat roofs have a sleek and contemporary appearance that complements modern architectural styles. The more modern materials such as EDPM and fibreglass offer this look more than traditional felt or bitumen.

Energy efficient: If you have insulation installed into your flat roof and use energy-efficient roofing materials, you’ll reduce heat loss and lower your home’s energy bill. Plus, if you decide to install solar panels on top, you’ll lower your energy bills even further and do your bit for the environment.

Disadvantages

Drainage issues: Flat roofs have a minimal pitch (or slope), which can lead to drainage problems if not properly designed and maintained. Accumulated water on flat roofs can cause leaks, structural damage and lead to premature deterioration of roofing materials. Make sure your extension builder and roofer add a small slope to prevent this from happening to your roof as much as possible.

Lifespan and durability: Flat roofs generally have a shorter lifespan and may require more frequent repairs and maintenance compared to pitched roofs. Exposure to harsh weather conditions, UV radiation, and foot traffic can accelerate wear and tear on flat roof materials, reducing their durability over time.

Limited aesthetic options: While flat roofs offer a modern look that many love, they may not suit all architectural styles or personal preferences. There are fewer options available for flat roofs compared to pitched roofs, where you can choose from a range of tiles or slates.

Which is cheaper: a flat roof or pitched roof?

Flat roofs are almost always cheaper than pitched roofs for extensions. There aren’t as many materials required for a flat roof and they take less time to install, keeping down both labour and material costs. However, pitched roofs offer advantages such as improved drainage and some people think they look better, but you could pay tens of thousands more.

How long does a flat roof extension take?

How long a flat roof takes will depend on the size and complexity of the extension, as well as the weather conditions and availability of materials. On average, expect a single-storey flat roof extension to take 16 to 18 weeks, while two-storey extensions could take 20-24 weeks.

Here’s a basic timeline of how works should progress:

Weeks 1-4: Footings, drainage, brickwork, roofing and glazing

Weeks 5-7: Remove old external wall, add steel beams, first fix electrics and plumbing

Weeks 8-12: Plasterboarding and skimming, second fix electrics and plumbing, internal doors and skirting boards, flooring preparation

Weeks 13-18: Stud walls, flooring, install kitchens/bathrooms, painting and decorating

Do you need planning permission for a flat roof extension?

In many cases, flat roof extensions can be carried out under permitted development rights, meaning you don’t need planning permission. However, certain conditions must be met, such as adhering to specified limits and conditions regarding size, height, and location. It's advisable to consult with your local planning authority or a professional architect to ensure compliance with regulations before commencing any work.

How long does a flat roof last?

How long a flat roof lasts depends on the quality of materials, how well it was installed and how often you maintain it. If you look after your flat roof and it was fitted correctly, you can expect it to last up to 25 years.

To help extend the lifespan of your flat roof, make sure you visually inspect it regularly and get any problems fixed as soon as you notice them.

Can you walk on a flat roof extension?

Yes, you can walk on most flat roof extensions – within reason. Most flat roofs are designed to support foot traffic for maintenance purposes, such as cleaning gutters or inspecting the roof surface. However, you should take care to avoid causing damage to the roof membrane or structural components.

It's best to walk on flat roofs during dry weather conditions and to distribute your weight evenly to prevent punctures or structural issues. If you want to be extra careful, lay boards down as you walk across it.

However, if your flat roof has been designed to withstand a rooftop terrace, it should have the necessary reinforcements to make it safe to walk across.

What insulation do I need for a flat roof extension?

Without the right insulation, flat roofs can lose a lot of heat through them, potentially increasing your energy bills.

A good option for insulation for a flat roof extension is 150mm rigid PIR board which your builder will lay over a sheet of vapour barrier that sits on top of the roof deck. When installed correctly, this will protect your roof from the cold, and stop condensation from forming and potentially causing damp.

Ready to get a quote for a flat roof extension? Post your job on HaMuch now and we’ll put you in touch with local extension builders who will be happy to give you a price.

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