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What is the cost of a flat roof extension?

The construction industry employs an average for flat roof extension projects which can act as a useful estimate when you are working out your budget. The usual cost of a flat roof extension is £1,900 per square metre. So measure out your intended shape, say 4 x 6 metres, calculate the area which is 24 square metres and then multiply by £1,900 = £45,600. This is a conservative figure and based on a countrywide average; in London and the south-east, it can be nearly £2,000 and rising per square metre.

Flat roof extension costs depend entirely on whether you are opting for a single storey or full height extension, the square footage you are looking to gain, the type of roof you choose for a single storey build and how you will be fitting out the interior space.

Last Updated: October 2022

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Extending your home has never been more popular as an alternative to moving house. The mediocre property market followed by the protracted uncertainty over Brexit has done nothing to improve house prices or the dynamic when it comes to buying and selling houses. And there are always costs attached to moving - estate agents charges, Solicitors fees and stamp duty – which are not recoverable even if you do end up with a bigger and better home at the end of it.

So, many people look to extend their homes as a more economically viable alternative to moving. It avoids all the cost and uncertainty plus the hassle. And if you like your home, why start again in a new house? Just make your existing one bigger. This article covers:

  • What is a flat roof extension?
  • Why are homeowners choosing the flat roof option?
  • How are the costs of a flat roof extension broken down?
  • What is the process of installing a flat roof extension?
  • Frequently asked Questions

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What is a flat roof extension?

A flat roof extension is a single-storey extension built on either the side or rear of the property. Flat roof extensions are ideal where there is not the space, budget or demand for a full-height extension and where it would be difficult to install a pitched roof. This is usually because of the design of the existing house - a pitched roof would interfere with the storeys above – the proximity of neighbouring properties or, the extension is simply not big enough or is the wrong shape to warrant a pitched roof either structurally or aesthetically.

Why are homeowners choosing the flat roof option?

There are several factors which make a flat roof extension a popular option for homeowners:-

  • If you are looking for extra space, it is a much easier route than the upheaval and cost of moving
  • A flat roof extension can make a kitchen larger, increase living space, add a bedroom, home office or even another bathroom where outside space is limited and would not support a pitched roof extension
  • A flat roof extension is generally cheaper than a pitched roof on a new single storey build as there are less labour and materials
  • A flat roof offers greater flexibility than a pitched roof in terms of design and styling, for instance, flat roof lanterns which are multi-panelled installations designed to flood the interior space with as much light as possible plus opening up sky views to the occupants.  These are a real feature
  • Open plan living space downstairs is becoming really popular and a bigger family area by taking down internal walls and adding an extension can also link well with the garden providing connected indoor-outdoor living
  • A flat roof extension can be used to run along the rear of a larger property effectively opening up all of the ground floor rooms on that side of the house so you can extend the kitchen and the living rooms.  This colonial verandah style design or wraparound extension can look very stylish and doesn’t use a huge amount of area so prevents excessive incursion into the garden but offers huge potential to the ground floor rooms
  • Opening up a side or rear wall of the property can allow natural light to flood in and transform dark or gloomy interiors.  And fitting roof lights specifically designed for flat roofs can increase the daylight even more

How are the costs broken down?

The bulk of the money will be spent on the build itself but there are other costs to factor in which will include:-

  • Architect’s fees - you will need to employ an architect to survey the site and draw up plans which comply with current building regulations and which are then submitted to the local council for planning permission approval if required.  On a straightforward design with no hitches at the planning permission stage, most architects can quote you a set fee which will be based on the size of the project – work on a figure of between 3% and 7% of the construction cost.  You may not need planning permission but you will need someone professional to advise you on whether or not it is necessary so you don’t inadvertently fall foul of the planning regulations. As you will need to employ an architect anyway for the drawings and structural compliance, they would usually take care of the planning permission element as well
  • Party Wall Agreements – you may need to comply with the 1996 Party Wall Act if your new building is close to a boundary wall or within a certain meterage of a neighbouring property.  Usually, your architect or surveyor will deal with this as part of their professional service to you
  • Planning permission fees – these are a flat charge of around £200 for a single-storey extension intended for residential use
  • Tree report – this may be required as part of the application for planning permission if you have trees in the vicinity of the build or your extension will affect trees or requires them to be removed.  This will attract an additional fee
  • Flood risk assessment – required for some planning applications for properties sited on flood plains or in flood zones
  • Ecology report – if your build is going to affect any habitats or species
  • Interior fit-out – depending on how many rooms you create or extend and what you want those rooms to be.  For interior styling for living areas, this is the usual cost of redecorating a room – paint or wallpaper – curtains or blinds and carpets or tiling.  But if you have extended your kitchen and are now looking to refit a new kitchen then expect to spend between £5,000 and £20,000
  • Central heating – this will need to be extended to heat the new area or you might want to consider other alternatives such as underfloor heating.  If you have installed a long extension or wraparound then you may need to upgrade your boiler to cope with the additional heating demands

Use our roof cost calculator to understand more about how much you need to budget for your roofing project.

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What is the process of installing a flat roof extension?

There is a natural sequence and order to the process as follows:-

  • Choose a building firm based on their expertise and quotation which will be’ in principle’ at this stage.  Discuss and agree on the scope of the works
  • Instruct an architect or surveyor to draw the plans
  • Agree a more precise costing with the builder based on the plans and a detailed specification of the works which should be in writing
  • Obtain the necessary planning consents and agreements such as Party Wall agreements
  • Construction of the extension, building inspectors may visit at pre-determined stages during the build to ensure the structure complies with building regulations before signing off the project
  • Interior design and decoration which could be extensive such as the re-fit of a new kitchen or simply involve decoration

Flat Roof Frequently Asked Questions

Which is best?  Flat roof or pitched roof?

The answer to this depends on many variables which include the style of the original house, where the extension is going to be located,  the size and style of the extension and how it sits in relation to neighbouring or adjacent properties. Deciding on the type of roof could affect the style of extension you opt for so always be open-minded and consider all the different options before reaching a final decision

How long does a flat roof extension take?

It will take several weeks to go through all the documentation with the architect and builder so the drawings and obtaining planning permission.  Once you have the green light to start, a good average period to work to would be 3 months to complete the job. This could be shorter, for instance, flat roofs are quicker to install than pitched roofs.  But equally, you could encounter unexpected hitches or delays which might extend that period beyond 12 weeks

Do you need planning permission?

Not always if your flat roof extension does not extend more than three metres beyond the rear wall of the house.  Most small extensions can avoid the need for planning permission which of course saves time and money. But always be certain if you don’t need permission that this is confirmed either by your local planning department or an independent professional.

What materials can you use for a flat roof?

There is a huge range of modern materials such as torch-on felt, asphalt, rubberised roofings and liquid roofs – these new-generation products look very stylish and are far more weatherproof and durable than old-style flat roof coverings. You should bear in mind that different materials will impact the new roof cost.


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