How Much is a Flat Roof Extension?

Adding an extension is a great way to improve your home. There are so many potential things that you can do with the additional space. Compared to other types of extension, a flat roof extension tends to be quite an affordable option.

Flat roof extensions offer a lot in terms of additional space, and they're a great alternative to moving to a larger home. This is important if you love your current house but want to accommodate your growing family.

Compared to pitched roofs, flat roofs are much easier to install, and they give you the option to build a terrace above your flat roof extension at a later date.

flat roof extension

Plus, any extension will add value to your home, which is perfect if you ever decide to sell your house in the future.

We will also go over the timeframes that you can expect with this type of work, as well as the different types of extensions and the benefits of choosing a flat roof extension.

An extension is a very large and extensive job. With this in mind, you can never expect any extension to be cheap.

However, investing in your home is always a great decision for you, your family, and potential buyers in the future. The average cost that you can expect to pay for a flat roof extension is around £30,000-£75,000.

The price will depend on a variety of factors, including the size of the extension, the ease of access, the materials used, and your location in the UK.

Those based in and around the London areas can expect to pay more overall for this type of work than those in the North or other parts of the UK.

A flat roof extension will usually cost around £1,200-£2,500 per square metre on average. If you know the size of your extension plans, then this should give you a better idea of how much you might expect to pay for the work.

What are the Supply Costs for a Flat Roof Extension?

While it’s not recommended that you complete a flat roof extension yourself, you may be able to do some parts of the work yourself as a DIY project.

bitumen flat roof installation

Also, if you would prefer to buy the materials separately and then find a contractor at a later date, then you may be interested in learning the supply costs for a flat roof extension.

Flat Roof Extension Prices

Felt roofing materials £2-£10 per square metre
Rubber roofing materials £9-£12 per square metre
Fibreglass insulation materials £6-£10 per square metre
PIR insulation boards £5-£15 per square metre
Spray foam insulation £5-£12 per spray can
Adhesive and sealant £40-£60
Edgings and drip edge for roof £100-£200
Roofing trims and batons £90-£180
Flooring installation £7-£60 per square metre
Radiators £60-£300 per radiator
Windows £300-£800 per window
Internal doors £50-£200 per door
External doors £300-£500 per door
Lighting £40-£100 per light fitting
Painting and decorating £300-£600
Get Instant Quotes

Compare quotes and get a price for any job around the home or garden. No obligations. Completely free.

Get a Price for your Job

What are the Additional Costs of Extending a Flat Roof?

When it comes to a flat roof extension, there may be some additional costs that you may have to budget for. In the section below, we’ll go over some of the additional jobs that you may want completing while getting a flat roof extension.

Exterior House Painting

Whether your home is currently painted or not, it’s a great idea to get the whole house painted when you’ve had a flat roof extension built.

painting home

When you get an extension, the new part of the building may look a slightly different colour than the rest of your house since all of the materials will be brand new and not weathered.

The average cost to paint the outside of your home is around £500-£1200, depending on the size of your home and the quality of the paint.

Garden Waste Removal

When you get a new extension built at the back of your home, you may find that the work generates a lot of garden waste. Garden waste needs to be disposed of separately from your normal waste.

If you have a lot of it, you will need to hire a professional to remove this for you. The average cost of garden waste removal is around £100-£200.

Underfloor Heating

Underfloor heating is a great energy-efficient way for you to heat your new extension room. While in the process of the extension build, this is a great time to lay the underfloor heating so that you don’t have to pull the flooring up at a later date.

The average cost to lay new underfloor heating for your extension is around £500-£2000, depending on the size of the room and the type of underfloor heating that you choose.

Skip Hire

A skip is an important part of having building work completed on your home. There will be a lot of waste when installing a flat roof extension, so it's essential for you to hire a skip for the work.

skip hire

The average cost for skip hire is around £250-£450, depending on the size of the skip and the length of time you need to hire it for.

Tradesmen Costs for Extending a Flat Roof

When it comes to installing a flat roof extension, you will need to hire many different contractors to complete the work. Below is a table showing the daily and hourly fees you can expect to pay for various contractors.

Builder £20-£35 £100-£180
Tiler £20-£40 £100-£200
Plumber £35-£45 £100-£200
Electrician £50-£100 £150-£300
Plasterer £30-£50 £120-£200
Decorator £20-£50 £100-£200
Joiner £35-£45 £100-£200
Roofer £30-£40 £100-£180
Kitchen fitter £20-£30 £100-£180

How Long Does It Take to Build a Flat Roof Extension?

If you are having a flat roof extension built, you’re probably wondering about how long the work will take to complete.

flat roof extension

The timescales for an extension will vary depending on several factors such as the size of the extension, the number of contractors working at any one time, and any additional work that you are having completed at the same time, such as a bathroom installation or kitchen installation.

On average, you can expect a flat roof extension to take anywhere between 8-14 weeks. It's a good idea to speak to your contractor before the work starts so that you can get a better idea of the timescales.

Your contractor should be able to provide you with an outline of what needs to be done and how long each thing will take.

Types of Flat Roof Extensions

Traditional flat roof extensions were quite simple in that they were a type of extension on the home featuring a flat roof.

A modern flat roof extension, however, can feature additional things to make it look more aesthetically pleasing.

In the section below, we’ll take a look at some types of flat roof extensions and the pros and cons of each type.

Flat Roof Extension With Lantern

A flat roof extension with a lantern features a very large window in the centre of the roof of the extension. This is extremely beneficial if you're looking to invite a lot of light into the room.

Also, this type of flat roof extension is very aesthetically pleasing. The only issue is that the top window will need to be regularly cleaned as it may collect a lot of dirt and debris.


  • Aesthetically pleasing
  • It invites a lot of light into the home
  • Modern appearance


  • It needs to be cleaned regularly
  • Can be expensive

Overhang Flat Roof Extension

An overhang flat roof extension is basically just as the name suggests – it features an overhang as part of the roof. This is beneficial if you're planning on having large windows and doors installed on your flat roof extension, as it can help stop rain from getting into open windows or doors.

Some people will choose to have a large overhang which helps to keep the room shaded and provides a small area where you can sit undercover.


  • Provides shade from the sun
  • Great for extensions with large windows and doors
  • It helps to keep the weather out of the extension room
  • Large overhangs can provide a nice shaded seating area outside


  • Can be expensive
  • May not let in enough sunlight

Benefits of a Flat Roof Extension

Flat roof extensions can provide a lot of opportunities for your family home. They give you the freedom to completely customise your home, and they provide a great amount of space for growing families.

flat roof extension

One of the best things about choosing the build a flat roof extension is that there are so many benefits that come with this type of project.

Below is a list of the benefits of installing a flat roof extension on your property:

  • Provides extra living space
  • Increases the value of your home
  • Perfect for growing families
  • Unique and customisable area
  • Provides extra storage space
  • You can choose to add a roof terrace for the above floor
  • Some varieties allow a lot of extra light to enter the home

You can use a flat roof extension for many different things. You may already have a good idea of what you want to do with the additional space.

However, if you're unsure, take a look at the following list of flat roof extension ideas for a bit of inspiration. These are some of the most common uses for a flat roof extension:

  • Extra storage space
  • Office room
  • Home gym area
  • Additional bathroom
  • Spare bedroom
  • Additional living space
  • Utility room
  • Conservatory

These are just the most common uses for the additional space. You really aren’t limited on what you can do with the additional space provided through an extension. With a little bit of imagination, you can create whatever you want to with the additional space.

How Much Does It Cost to Remove a Flat Roof?

There are several reasons why you may want to remove a flat roof. For example, if you've moved into a new home with a flat roof extension on it, but you're not happy with it, then you may want to remove it or replace it with a new one.

flat roof extension

Removing a flat roof extension can be quite a sizeable project, depending on the type of extension and how it has been built onto the property.

The overall cost to remove a flat roof extension is around £3000-£5000. This flat roof extension price for the removal will cover all of the relevant costs involved with this type of work, such as paying the contractors, paying for all of the equipment needed, bringing down the old extension, and removing all of the waste.

Once your flat roof extension has been removed, you will then need to consider what you want to do with the open wall.

You’ll likely need to hire a builder to complete some additional building work for you to close off the open wall. A builder will usually charge around £20-£35 per hour to build a new wall in the opening.


Q: How do you run cables on a flat roof extension?

A: The electricity cables for your flat roof extension will need to run in the gaps between the joists. It can be very dangerous to install electricity cables as a DIY project.

Plus, all of the work will need to be checked over and signed off for safety reasons. With this in mind, it is highly recommended that you hire a professional electrician to install any cables and electrical wiring that runs into the flat roof extension.

Q: Do I need planning permission for a flat roof extension?

A: You won't necessarily need planning permission for your flat roof extension. If your extension does not extend beyond three metres of the rear of your property, you should be able to avoid planning permission, as it will be considered a permitted development.

It's always best to check with your local authority, as you don't want to find out at a later date. If your home is in a conservation area or if it is a listed building, you will need planning permission no matter what. Speak to your local authority for some additional advice.

Q: How often does a flat roof need to be replaced?

A: A flat roof that has been professionally sealed should last for around 20 years or more without coming into any problems. In worst-case scenarios, most people will only need to replace their flat roofs around 3-4 times at most.

This total cost will still end up being less than the cost of building a pitched roof, as pitched roofs are much more expensive.

Q: Can a flat roof be repaired?

A: The majority of flat roofs can be repaired quite easily by a professional. Some flat roof repairs are quite simple and could be completed as a DIY project. This is providing that the damage or problem is only minor as some repairs may require a complete roof replacement.

However, small leaks, small holes, flashing damage and blisters will only require you to remove debris and clean the affected area before applying new material to patch it up.

Q: Do I need an architect for a flat roof extension?

A: In most cases, you won't need an architect for a flat roof extension. Most experienced builders will be happy to work without an architect. However, if you want to have a very large flat roof extension installed, you may need to hire an architect.

Some companies will recommend an architect for all extension projects. Having an architect draw up the plans can help you to save on costs, and also it ensures that there is a clearly agreed-upon plan for the final project.

Q: How much value can a flat roof extension add to your home?

A: The amount of value that a flat roof extension will add to your home will depend on a combination of the size of the extension and the location of your home.

As a rough guide, you can expect an extension to add around £2000 onto the value of your home for every square metre of additional space that is added.

If you are located in London, you can expect the added value to be around £4000 per square metre. In the majority of cases, the value that is added to your home will be much greater than the cost of the extension itself on a per square metre basis.


Author: Kim J

I have a bachelor's degree in Journalism, which I studied in both the UK and USA. I've been a freelance writer for over eight years and have experience working with many large brands. My experience in writing is over a wide range of different niches, and I've specifically worked in the DIY and trades section for nearly two years. My favourite thing about freelance writing is working wherever I want as I love travelling and discovering new places and cultures.