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Cost of a two storey extension

Adding a two-storey extension is not only a practical solution to improve floor space but an exciting process too. Whether you want to increase the size of current rooms, add new rooms, increase the value of your home, or alter the layout of existing rooms, there are many great options.

Instead of adding two single floor extensions separately, choosing to extend two storeys at the same time means you will save money in the long run plus reduce project time and any disruption.

Looking for other extension types? Then view our in-depth house extension cost guide.

What’s the average cost of a two-storey extension?

Type of extensionCost + VAT
(Range low - high)
Average cost
Standard quality two-storey extension per sqm£1,300 - £1,750£1,500
Good quality two-storey extension per sqm£1,500 - £2,000£1,750
Excellent quality two-storey extension per sqm£1,750 - £2,500£2,100
Extension planning drawings cost£500 - £1500£950
Foundations based on 60m2 double storey (30m2 per floor)£8,000
Framing based on 60m2 double storey (30m2 per floor)£36,000
Roofing based on 60m2 double storey (30m2 per floor)£9,000
Surveyors fees£500 - £2,000£1,250
Party wall agreement£500 - £1000£750

To find the cost of two-storey extensions we spoke to the online estimators at My Build Estimate – a professional estimating company monitored by the Royal Institute of Chartered Surveyors (RICS).

This guide features the estimated two-storey extension cost examples they provided.

The average per sqm price of a two-storey extension is £1,783.33.

The average timescale of a two-storey extension is 8 – 14 weeks.

For the prices in this guide, costs are based on standard construction, foundations, framing and roofing as per the table on the right.

How do two-storey extensions compare to single extensions?

If you are hoping to eventually extend both the bottom and second floor of your home, it is a wise decision to tackle both at the same time. As you are already paying for a roof and the foundations of your new extension this cost will not change when choosing to extend two storeys instead of one.

Then there is the time difference; double storey extensions do not take double the time of single ones. It is also worth noting that the overall cost of a two-storey extension vs a one-storey extension is an average increase of 50% – 70%, a large reduction compared to completing both floor extensions separately.

Types of two-storey extensions

Where you locate the extension on your home will play a key role in how much you get out of it. Choose badly and you could be faced with obstructed views, reduced privacy, reduction of light through your and your neighbours’ windows and reduced garden access.

Location of extensionCost + VAT
(Range low - high)
Average cost
Two-storey side extension cost (5m x 4m)£52,000 - £80,000£66,000
Two-storey front extension cost (5m x 4m)£52,000 - £80,000£66,000
Two-storey wraparound extension cost (5m x 4m)£52,000 - £80,000£66,000

The following are the most common locations for two-storey extensions:

  • Rear of home: These extensions, while the most popular, can drastically reduce your garden space so are more viable for homes with large gardens.
  • Front of home: Less common than rear extensions, front extensions completely alter the look of your home and must comply with strict regulations.
  • Side of home: A good option for detached or semi-detached homes, side extensions don’t affect garden size or the front of your home.
  • Wraparound: There is great extra space potential when working with wraparound extensions (typically made up of side and rear extensions) however they also can be very costly and time consuming.

How much does a two-storey side extension cost?

As described above, two-storey side extensions are less popular than rear extensions but perfect for detached or semi-detached homes. They have little impact on garden space and home appearance.

On average a two-storey side extension costs £66,000 for a 5m x 4m extension.

Two storey interior worksFactors affecting the cost

Architect and surveyor fees: Whilst not a legal requirement, an architect can optimise the construction of your extension and can cost £800 – £2,500. Surveyors can ensure that your planned project is safe to build and they cost on average £1,250.

Size: The larger your new extension, the higher the cost will be due to the extra materials needed and time taken.

Quality: The quality of materials used and work done will affect the overall two-storey extension price.

Finish: Depending on whether you require your new extension to be plastered or painted this will incur an additional cost.

Groundworks: Foundations and groundworks are essential and vary from project to project. Therefore, depending on your current situation this price can vary.

Scaffolding on two storey extensionScaffolding: This is an essential cost when building a two-storey extension to ensure safety and a high quality job.

Insurance: It is important to inform your home insurance company of any structural changes to your home, which may affect your premium cost. If you do not have an up-to-date policy you may not be covered for any accidents or damage.

Location: The location of your home in the country plus the location of your new extension on your house will alter the cost of your two-storey extension.

Drainage: Bathrooms and kitchens must have the correct drainage added to ensure waste is properly dealt with. Due to the extra work this will increase the overall cost.

Electrics: Essential for any extension, even just for small power points, electrics must be fitted and inspected by a qualified electrician at an added cost.

Function of extensionCost + VAT
(Range low - high)
Average cost
Bathroom cost (additional cost)£2,500 - £11,000£6,750
Kitchen cost (additional cost)£5,000 - £25,000£15,000

Purpose of the room: Be mindful that adding a new kitchen or bathroom is more costly than a bedroom due to electrics, plumbing and the finish of the room. The addition of a kitchen or bathroom may add £7,000 – £10,000 to the job.

Windows and doors: Choosing to add windows and doors to your new extension can increase light and access to any new space. This may increase the overall cost of your two-storey extension.

Stairs: Adding stairs to a new extension can be very important to allow ease of access. This can also increase the extension price.

Openings to outdoors: Any doors that allow access to outside the home must be secure and properly insulated which will likely affect the cost.

Do I need planning permission?

You must ensure that any new extension meets legal and structural regulations. These may include the following:

  • Planning permission: This may be necessary and you should check with your local authority. This usually costs in the area of £200.
  • Party wall agreement: Before adding an extension it may be essential to draw up a party wall agreement which costs roughly £750 per neighbour.
  • Surveys: These are critical to ensure your extension is built in a safe and structurally sound way. Your tradesperson will be able to help decide on the right surveys for you. For example, if you have a tree survey, this will cost approximately £200.

Two storey extension in progressHow long will a double storey extension take?

While two-storey extensions do not require double the time necessary for single storey extensions, the time taken will be significant and largely depends on the above factors. Time is fundamentally focused in two stages:

  • Preparing and planning: Architect designs, any drawings, obtaining planning permission, structural planning etc. – Can be from 2 weeks to several months depending on any legalities and issues that arise.
  • Construction: 8 – 14 weeks dependent on the size of the extension and any of the other important factors listed above.

Can I build a two-storey extension myself?

Building an extension is not an easy and quick job. It is very difficult to work with heavy materials especially at the heights required for this job.

Aside from the arduous construction process, not finishing the job to a high standard opens you up to endless difficulties down the line. These include water damage from moisture seeping into the home, electrical and plumbing issues, danger to yourself and others living in the house from collapse and unstable foundations, not to mention the legalities and regulations that you may have broken. This can lead to your insurance provider not covering your home and you may face legal issues. As such we would strongly recommend hiring a professional.

To find a local, reputable, tradesperson in your area and get a personalised cost for a two-storey extension use our free search feature.

Key takeaways

  • Extensions can be added to the rear, side or front of your home or can even wraparound the side and back of the house.
  • There are a large number of factors such as size and location that affect your two-storey extension price.
  • Adding a two-storey extension does not cost twice the amount of a single storey extension.
  • There are several important legal regulations when extending your home that must be adhered to.
  • It is dangerous and extremely difficult to add a two-storey extension yourself.

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