In this guide, we’re going to look at flat roof extensions so you can make an informed decision about whether having one built is the right move for you. There’s a lot of conflicting information out there, but this guide will give you the facts you need to make the right choice.
At Toughened Glass Systems, we don’t sell or fit flat roof extensions, but we do talk to fitters who do, and many of our customers will have them installed. That gives us a unique perspective on the pros and cons, and who they’re best suited to. Let’s dive in!
What is a flat roof extension?
A flat roof extension is generally taken to mean an addition to your home featuring an almost horizontal roof design. We say ‘almost’ because, for drainage purposes, all roofs have to have a slight slope (usually a minimum of 10 degrees) to direct water away and stop it from pooling.
Usually made with a timber structure, a flat roof extension can add a large amount of usable space to your home for a relatively low cost when compared to other extension types.
Their sleek, modern appearance means they tend to fit in nicely with traditional UK homes, and their efficient use of space means you can add significant square footage and value to your home, usually without planning permission.
Many homeowners will design their flat roof extension to open straight out onto the garden with French doors, or turn the roof into a small terrace for even more space in the summer months.
Main benefits of a flat roof extension
Some of the main benefits of getting a flat roof extension built:
- Price: Flat roofs generally use fewer materials than pitched roofs, making extensions a bit more affordable. In our experience, your average UK flat roof extension will cost around £50,000 for materials and labour.
- Versatile space: They provide double the usable space by potentially adding a roof terrace, boosting your home’s overall functionality.
- Modern aesthetic & value: The clean, minimalist look can increase your property’s appeal and market value, a key factor for UK homeowners.
- No planning permission: Generally speaking, you won’t need the hassle of getting planning permission for a flat roof extension. We’ll cover that in a bit more detail later in the guide.
- Longevity: Some flat roof extensions made with materials like EPDM can last up to 50 years with proper maintenance.
Design and aesthetic considerations
While you might have seen some boxy-looking flat roof extensions that look a bit awkwardly plopped onto the back of a terraced house (you know the ones), these days homeowners have a lot more choice about the look and feel of their extension. Out with the felt and pebble dash, in with the sleek steel, timber, and glass.
Sleek and airy with tons of natural light and premium-looking glazing and frames, a well-designed extension can provide much-needed character to a property, and look like it was always meant to be there.
- Flexible design options: Flat roof structures offer you the freedom to incorporate large areas of glazing without structural constraints.
- Integration with existing architecture: Modern flat roof extensions can easily complement the design and architectural style of your existing property.
- Customisation possibilities: You can explore options such as mini-terraces, green roofs, rooflights, and skylights, adding extra visual appeal and functionality. Our flat roof skylight is perfect for any flat roof extension.
- The popularity of lantern roof extensions: Loved for the contemporary look and the natural light they add, you can add a lantern to your flat roof extension. More on that later in the article.
What does a flat roof extension cost in the UK?
It’s quite tricky to give any useful figures for how much your flat roof extension is likely to cost. Variables like the size of the extension, the materials you’ll use, the access the fitters will need, and any structural considerations will all wildly impact the price you’ll pay.
However, here are some ballpark figures based on the most popular consumer choices:
- According to Checkatrade, the average flat roof extension cost per m2 (metres squared) is £1,850
- The average flat roof extension comes in at about £50,000 in the UK.
Factors affecting the cost of a flat roof extension
- Roofing materials used (commonly either EPDM, felt, or bitumen)
- Any unique architectural features, such as custom glazing, roof terraces, or intricate layouts, require additional design and construction work
- If the extension requires additional structural reinforcement or bespoke engineering solutions (especially for heavier materials and roof terraces), this adds to the overall expense.
And like with any big home improvement project, there are a few extra costs to consider. Any planning fees, scaffolding, technical drawing costs or additional features like skylights and rooflights should be kept in mind.
Have a look at our flat rooflight cost calculator for a better idea of what options are available and what they might set you back.
And as always, shop around for a few quotes before deciding who to work with, and remember that home extensions can increase your property value.
Do you need planning permission to build a flat roof extension in the UK?
Typically, no you don’t.
In the UK, flat roof extensions tend to fall under permitted development rights, meaning you won’t need formal planning permission if you’re able to meet certain criteria.
If you’re planning a single-storey flat roof extension that doesn’t exceed height limits (usually 3m for terraced or semi-detached properties and 4m for detached homes) and the extension isn’t visible from a main road, you should be in the clear.
As ever, there are exceptions. If your property is listed, located in a conservation area, or subject to an Article 4 Direction, planning permission will be required regardless of size.
Always consult your local planning authority if you’re unsure.
Flat roof extension FAQs
Can I walk on a flat roof extension?
Most flat roof extensions aren’t designed to support you walking around on them. Unless they’re specifically built to be roof terraces or with reinforced decking, you should avoid walking on your flat roof.
If you opt for a roof that you can walk on, our walk on rooflights will be a great addition.
How energy efficient are flat roof extensions?
Flat roof extensions can be very energy efficient if designed and built correctly. Modern flat roof designs will use high-quality insulation and waterproofing to keep your home warm in the winter and cool in summer. For example, when insulation is installed above the roof deck (a warm roof), it helps to meet UK energy standards by reducing heat loss.
The natural light they provide also reduces electricity consumption, because you’ll need to use less electricity to light the space.
Pro tip: You should always consult your local building regulations for U-value requirements for your extension project – these are insulation-specific criteria your new extension needs to meet to be compliant.
What is a lantern roof extension? What do they cost?
A lantern roof extension is a type of flat roof extension that incorporates a glazed, raised element called a “roof lantern” into the design. Lanterns consist of a framework with lots of glazing, allowing natural light to flood the interior space.
Lantern roofs are typically used to brighten areas like living rooms, orangeries, or kitchens and can create the illusion of higher ceilings. Discover 5 stunning lantern roof extension ideas in our helpful guide.
Have a look at the roof lanterns we sell, and use our handy price calculator to see what one might cost to add to your flat roof extension project.
More helpful articles
- Best Skylights for Flat Roofs: Complete Buying Guide
- Advantages and Disadvantages of Flat Roofs
- Flat Roofs: Complete Guide
- Pros and Cons of Flat Roofing for Commercial Properties
- Flat Roof vs Pitched Roof Skylight Extensions
- Rooflight Sizes: A Complete Guide
- Can You Install a Rooflight on a Flat Roof?
- Rooflight and Skylight Cost: A Complete Guide