double storey home extension

Planning Double Storey extension

A double-storey extension allows you to enjoy the maximum space your property can offer you. Did you know that two storeys rear house extension is often considered a more straightforward solution than a single-storey? Here you will discover what you need to know to have a successful extension.

What's in this guide?

  1. What is a double-storey extension
  2. Average cost of 2 storey extension
  3. Average height of 2 story house including roof
  4. Can you do a double-storey extension under permitted development
  5. How long does a double-storey extension take to be build
  6. Planning Time
  7. Types of double-storey extensions

What is a double-storey extension

Fron double storey house extension

A double-storey extension is an additional two storey extension built to extend the size of your property and your living space over two floors; the ground floor and the second floor.

There are several types of two-storey extension, and you can choose any class that best suits your needs. For instance, the extension in a three-bedroom house is different from a five-bedroom house. You can extend to the side, front, or behind the current building. Alternatively, you can have the extension wrapped around your building.

How much does a double-storey extension cost

The double-storey extension costs are dependent on various factors. These factors include;

  • Site access
  • Size in square meters
  • Soil composition
  • Location
  • Roof type and its shape
  • Finishes and dine touches (including flooring, underfloor, furniture, and decorations)
  • Quality of service you intend to employ

The best way to calculate a two-storey extension cost is per square meters. Next, you would have to include the cost of all the professional services and legal agreement required.

Proficiency's professional team of architects, contract managers, project managers and builders will assist you with everything you need to know to build your double-storey extensions from the cost, understanding the best type of extension for your property to how to get planning consent and the construction process.

2 storey rear extension, rear double extension with bi-fold doors

Average cost of 2 storey extension

The average cost per square metre for a double-storey extension add is dependent on the factors above. Nevertheless, you can still figure out a range. The average cost range below will help you to set your budget right while planning your new extension.

  • For a double extension with essential quality, you can expect an average cost of £1,300 to £1,700 per square metre
  • A 2 storey extension with good quality will result in an average cost of £1,700 to £2,000 per square metre
  • If you want a high specification extension, you have to prepare for an average budget of £1,700 to £3,000 per square metre. This type of extension has special rooms like an additional bathroom, kitchen, and library.
modern bedroom of two storey house

Besides the basic budget above, you still have to prepare for some other additional costs. These additional costs include;

  • Survey cost (£500 to £1,500)
  • Architect’s fee (usually 3-7% of the construction cost)
  • Application fees for planning (usually £206 in the UK)
  • Building construction charges by a structural engineer
  • Project management (usually 3-7% of the construction cost)
  • Party wall agreements
  • Building control charges
  • Other fittings and facilities, including shower room, toilets, and other unique rooms

The costs of a house extension can vary depending on your location. You will discover that the cost of building a house like double-storey extension is higher in some parts of the United Kingdom. The expense is higher in places like London compared to other parts. So, it is best to put your location into consideration when creating your budget.

You will also need other consultant fees for;

  • A general survey of your actual building
  • Flood risk assessment to ascertain that your location is free from flood
  • Ecology reports assuring the authority that your construction works will not harm wildlife and forest reserves
  • Tree report to find out if you have to remove or shift a tree
  • Building control fees to help with the certification that you are in compliance with all regulations for the extension
double extension with skylight and bifold doors

Average height of 2 story house including roof

In the UK, the average height of a two-storey house, including a roof, is between 20 to 25 feet. The main reason behind this variation is the difference in the ceiling height.

The ceiling heights vary depending on the preference of the owner. Nevertheless, the ceiling height is usually below 9 feet. Other significant contributors to the height include the foundation, floor, and space for other infrastructures above and below the building. The base of the storey is usually between 8 and 12 feet. The ceiling often covers from 9 feet and below. There is usually 2 feet space for extra infrastructures above and below the two-storey.

Do you need planning permission for a double-storey extension

To extend your property you will need planning permission for a double-storey extension building project. According to the government guidelines, all rear extensions of more than one storey need planning permission. And this has to be in compliance with building regulations and you can seek professional advice.

The guidelines for building a two-storey extension with planning permission include:


luxury open plan living room made from high quality materials

It is vital to ensure that the materials for your two-storey front extension are similar to the existing rooms. This consideration will ensure that it maintains similar quality and standard to your existing house and other building regulations. Also, it should match other neighbouring properties in the area. Overall, the consideration of interior and exterior materials is a test of quality, standard, and uniformity.

Maximum Height

The maximum height of a two-storey extension with permission requirement is restricted to 3 M within the range of the existing roof. Also, the pitch and eaves shouldn’t be higher than the existing property. The height is one of the crucial factors for consideration when determining if your proposed extension plan requires extra permission or not.


Situating the double-storey extension on the side or in front of the available building is not permitted. It is supposed to be behind the existing structure. In most cases, the extension is behind or beside the original house. You only have a front extension on rare cases. In these cases, you have to keep other factors, particularly your neighbour's properties in mind.

Land Coverage

inside of 2-storey home with a view to a garden

The footprint of a double-storey extension should not exceed 50% of the remaining land around the house. So, it is vital to keep the land area and space management in mind when designing your house extension in London. The width of your house extension should not be more than 50% of the existing construction. The length can vary depending on the location and construction. The maximum length of a semi-detached property or terraced building is usually 3m. But it allows up to 4m for a detached building.


The height of your actual building and extension are determinants for this consideration. The height of your 2 storey extension should not exceed the eaves of your existing building according to building regulations. Also, if the distance of the existing structure to your boundary is less than 2m, you cannot have a two storey. You can only have a single storey extension under permitted development.

Terraces and Balconies

Permitted development rights do not authorise terraces and balconies for two-storey extensions. You have to seek special planning permission if you want to integrate these features into your extension.

Can you do a double-storey extension under permitted development

Rear garden of a property

Doing a double-storey extension can stay under the category of permitted development. This category means you do not require any special permission before you build your double-storey extensions. All you need is to meet the requirements and stay within the permitted development rights.

One of the primary considerations is that it should not exceed certain meters within the boundaries of your original house. You can also refer to other guidelines above to build a double-storey extension under permitted development. Any exception to these guidelines requires special permission.


The foundation depth for double-storey extension is usually 0.5 deep and 2 feet wide. The measurement of the depth of footings is essential for determining the building strength and coverage of the double-storey extension.

two storey front extension

How long does a double-storey extension take to be build

On average, a double-storey extension will take between 14 to 16 weeks for complete construction but it will depend on the design of your project.

If you want a highly sophisticated double extension, it can take up to 18 to 24 weeks before completion. The duration of the build is divided into different stages. Each stage requires a period range to ensure quality and appropriate standard. The major stages for this extension construction include;

  • Planning time
  • Preparation time
  • Construction and finishing time

Planning Time

Planning time includes the time to visualize your construction and perfect your design. This period can take from 8 weeks to 12 weeks. The extent of the planning time is often dependent on your commitment and seriousness with the project. It would be quicker if you already havedouble storey rear extension ideas in mind. But you might also have to consult with the local planning authority.

Preparation Time

The preparation time is another vital period that can make or mar your project. It is the time where you make all party wall agreement, building registrations, structural designs, building regulations approval, and technical drawings. The agreement part of time usually takes the most variable. It can take anywhere from 2 weeks to months, depending on the pace and difficulty of the negotiations. Other parts, including the technical design 4 weeks to 8 weeks.

Construction and Finishing Time

double storey kitchen extension

The construction and finishing time of the extension project is usually very sensitive. It varies depending on the nature of your extension, quality of services, and availability of materials. This period often takes from 10 t0 16 weeks. Does that sound a lot? let's do a quick breakdown below.

  • Foundations and evacuations (app 2 weeks)
  • Columns and structural beams (app 1 week)
  • External walls (app 2 weeks)
  • Suspended timber floor and ground floor slab (app 1 week)
  • Roof (app 2 weeks)
  • Plastering and dry-lining (app 2 weeks)
  • Plumbing and Electronics (app 1 week)
  • Painting and other decorations (app 1 week)
  • Finishing touches, windows, skirting, flooring, doors, and other joineries (app 2 weeks)
detached house double storey extension,two storey extension drawings

Types of double-storey extensions

There are four different types of double-storey home extensions.

  • Wrap-around two-storey extension
  • Side return two-storey extension
  • Two-storey rear extension
  • Front two-storey extension

Wrap-around two-storey extension

multi storey wrap around extension

Wrap around extension is a combination of both side and rear house extensions. It almost wraps itself around the existing house; hence, its name. It is suitable as an extension for detached or semi-detached houses. The main advantage of this type of house extension is that it helps you to maximize space. Also, it is a good way of improving the existing size of your current house.

Side return two-storey extension

The side return double-storey extension uses the smallest possible space. You can utilize a narrow margin on either side of your house without interfering with the outstanding natural beauty of your garden.

This type is one of the best fits for ground floor extension. This option will not compromise your garden space, and it is suitable for detached and semi-detached houses. However, if there is a nearby property without overlooking neighbouring windows, it might restrict the extent of your options.

Two-storey rear extension

rear extension of an house

The rear wall extension is situated behind your existing building. It is a good option because it also allows a building extension for a terraced house. This type of extension is suitable for both detached and semi-detached buildings.

Front two-storey extensions

The front 2 storey extension is located in front of an existing building. But it is the least common type because it can change the entire appearance of the building. But you have to be careful not to make significant alterations. It is particularly suitable for a detached house with a front-house garden.

In summary, executing a double-storey extension is not a difficult task if you know the right path to follow. Start by understanding the need for an extension and how the extension appears. Next, ensure that your construction plan and design is in compliance with the specifications from authorities.

A 2 storey extension project is not time-consuming. The time required is usually within the range of 14 weeks to 24 weeks. The variation is dependent on a number of factors, including the type of extension you intend to create.

Building a room

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Clara Annesley

Clara Annesley is an interior design and construction content writer. She holds a BSc (Hons) in Architectural Design Technology from the University of West London. Clara specialises in residential construction for topics like health & safety, architectural design and writing cost guides for renovation and remodelling projects.