Last Updated on March 7, 2021

Flat Roof Extension Cost – How Much Does It Cost On Average?

Zac Houghton

Zac Houghton

Construction expert and founder of Loftera.

Are you looking to upgrade your home but don’t want to face the hassle of moving to a completely new area? Thousands of homeowners in the UK are opting to extend their current home, as opposed to moving somewhere entirely different.

Doing so is not only a more cost-effective way of acquiring the space that you are after, it also takes much less time and requires less upheaval to your life.

The average flat roof extension cost in the UK is just over £50,000, although this can vary depending on the size of extension, materials used and your location. We’ll delve further into the other factors that will impact the price of your flat roof extension shortly, but the biggest variant is the size.

flat roof extension cost

Table of Contents

Flat Roof Extension Cost: A Breakdown

The easiest way to figure out your estimated final cost is to work out the costs per m2. On average, UK residents will be charged £1,750 per m2 for their flat roof, with that price fluctuating between £1,000 and £2,500 based on the above factors.

No matter the cost of your extension, your materials used should make up around half of your final cost, with the labour itself and waste removal finishing the final price.

The below table gives a brief overview of the breakdown of your costs, depending on the overall price.

Overall Cost

Materials (50%)

Labour (45%)

Waste Removal (5%)





















As you can see, the vast majority of your costs are incurred during the build itself. However, there are other fees to take into account, beginning with how much you’ll pay your architect. After all, these are the people with the vision of how your extension will look, and they charge accordingly. Most architects charge a set fee that is based on the size of the project, usually between 3% and 7% of the construction costs.

If your extension is close to a boundary wall or that of a neighbour, there is every chance that you’ll be required to adhere to a party wall agreement. Additionally, while it isn’t definite that you’ll require planning permission for this type of project, acquiring it will set you back around £200.

There are also aspects such as tree reports, flood risk assessments, and ecology reports, but most architects will factor this into their planning.

Naturally, once built, you’re going to want to decorate how you see fit, and also fit central heating. There’s no point building a brand-new room if it is too cold to sit in!

Pros and Cons to Your Flat Roof Extension

The advantages to building a flat roof extension far outweigh the cons, but we will take a look into both. We’ve already discussed how building a flat roof is a more affordable way of extending your living space, and they can look stunning when built correctly. Although you are opting for a flat roof, it will be built with a slight pitch to prevent water from collecting, removing what would be a huge disadvantage in flooding risk.

As for the disadvantages, there really aren’t many. Your ceilings will be lower than that of a pitched roof, but you’d need to be wearing stilts for that to become an issue.

Reasons to Build a Flat Roof Extension

Have we already mentioned that building an extension is a much easier way of upgrading than moving to a new house entirely? Well, it’s a point worth repeating, as it really is the case. An extension can be used for a variety of needs, such as a kitchen, bedroom, office or even a creche.

A flat roof extension is generally cheaper than a pitched roof alternative, and given the lack of disadvantages, is our go-to choice. You also leave yourself open to further development down the line, on top of the new flat roof that you are building.

In the 21st century, it is becoming increasingly popular to open up your downstairs area, and knock down interior walls in the process, in a bid to create more living space. An extension opens up an array of opportunities in this instance.

Benefits of Building a Flat Roof Extension

We have pretty much discussed some of the key benefits in opting for a flat roof extension. You save money, you don’t limit yourself for further expansion, and you have great flexibility in what it is used for.

In addition, you can bring natural sunlight into your home through an extension, which is always welcomed.

A recent Zopa survey found that the average return on investment for an extension was as high as 71%, with the average amount of profit raised coming in at £14,000. So, not only will you be reaping the benefits while living there, but you’ll be also increasing your property value massively.

Factors Impacting Your Flat Roof Extension Cost

As discussed earlier, a flat roof extension can cost anything between £30,000 and £70,000. In reality, the range can be even larger than that, so you will want to know exactly where you fall on the scale.

As is the case with most property projects, your location will have a big say in your overall price. Live in central London? Expect to pay a premium compared to someone living in Leicester.

The materials that your builders use will also come into account, but they will discuss your options with you before any work begins. The size and shape of the extension plays a huge part, as does the plumbing, electrics, lighting and flooring.

You will probably want windows fitted with your extension, and your options in this regard are plentiful. Take a read of our double glazing cost post to help you make an informed decision on your choice of windows.

To DIY or Not to DIY?

Unless you are a qualified builder by trade, we wouldn’t recommend taking on a project of this size by yourself. There is quite a difference between building a garden shed and a house extension. If done wrong, it could end up costing you an awful lot more than it’s worth. Stick with the professionals, it’s what they’re here for.

Will I Need Planning Permission For My Flat Roof Extension?

Not necessarily. If your extension doesn’t extend to beyond three metres of the rear of your property, you’ll be able to crack on without planning permission. Our advice would be to check either way, you don’t want to be stung at a later date.

We hope that you are now closer to making your decision on a flat roof extension. Our recent blog should answer some of your questions on how to finance an extension. All in all, they are a fantastic way to improve your home life, and can be done so with minimal fuss. Good luck.

Save 30% off your extension quote: