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Double Storey Extensions: Here’s Why They’re the Cost-Effective Option

a large double storey extension to a modern property
(Image credit: French + Tye for Bradley Van Der Straeten Architects)

While a double storey extension may seem like a much larger project to undertake than a single storey one, they're often a more budget-efficient way to add extra space to your home and cheaper than moving house when it comes to additional rooms for a growing family. 

(MORE: Get a quote for a new extension)

Of course, a double storey extension will also transform the look of your home in a much more meaningful way from the outside, so it's important to design the right extension for your property. 

From the planning and building regulation implications to how much it will cost to build a two storey extension, here's everything you need to know. 

How Much Does a Double Storey Extension Cost?

A double storey extension gives you double the living space of a single storey one, but the cost of the foundations and the roof are pretty much comparable for both build types, meaning the money spent per metre square is less with a double storey addition.

Before you start planning your extension design, set your budget, with the help of this list:

  • If you’re extending on tight a budget, expect to pay £1,320 to £1,620/m² for basic quality
  • For a good quality double storey extension, you’ll pay between £1,620 and £1,860m²
  • For a high spec extension, calculate spending between £1,860 and £2,100/m²

interior of a double storey extension with large glazing

Bradley Van Der Straeten Architects created created a two storey box to sit on the back of the existing building that is clearly separated by a canyon of glazing, bringing natural light to the ground floor.  (Image credit: French + Tye)

On top of the build costs, you’ll need to factor in the following:

  • Architects’ fees (around 3-7% of construction costs)
  • Project Management (around 3-7% of build cost)
  • Planning application fees (currently £206 in England for a double storey extension)
  • Survey costs (between £500-1,500)
  • Structural engineer (£500-1,000)
  • Building control charges (up to £900 for a 80-100m² extension)
  • Party Wall Agreements (£700-1,000 per neighbour if needed)
  • Interior fit out costs

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Do I Need Planning Permission for a Double Storey Extension?

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The Planning Hub is a new online resource that will help you understand how to get to grips with complex planning rules. Join today for access to easy-to-read guides which will provide you with key information to help you secure planning permission.

While it can be possible to build a double storey extension under Permitted Development rights, it is very likely that you will need planning permission.

In any case, it’s always worth checking with your local planning authority, but the basic rules are that planning permission will be required if the extension:

  • covers more than half the area of land surrounding your home
  • extends towards a road
  • increases the overall height of the building
  • extends more than 6m from the rear of an attached house
  • extends more than 8m from the rear of a detached house
  • is taller than 4m
  • is more than half the width of your house
  • uses different materials to those of the original house
  • includes a balcony or raised veranda

If your proposed extension affects a party wall, then make sure you give 'Notice' to your neighbours before starting work under the Party Wall Act

Does a Double Storey Extension Need to Comply with Building Regulations?

Yes. Any extension project will need to comply with the Building Regulations. You will need to submit an application for the work to your local authority’s building control department.

Some contractors are building control certified, meaning they can carry out work without the need for involving the building control department.

double storey extension to a cottage

These homeowners have doubled the size of their run-down cottage with a complimentary contemporary-style extension, designed by CaSA Architects. (Image credit: Simon Maxwell)

How Big Can my Double Storey Extension be?

Planning rules usually require extensions to be sympathetic to the existing house, particularly in Conservation Areas, so the height of a double storey addition’s ridge and eaves can not be taller than the existing roof. If your existing house is on the small side, this can present a challenge when building an extension tall enough to include two full (2.4m) storeys.

There are ways around this, such as designing upstairs rooms with lower ceiling heights or integrating them into a pitched roof space. You could also have the extension dug down into the ground, resulting in a basement level or split-level design which can work well on a sloping site.

double storey extension to a granite farmhouse

This contemporary double storey extension to a granite farmhouse in rural Aberdeenshire, designed by David Wilson of Room Architects, is clad in larch and features expansive glazing from Velfac. (Image credit: Nigel Rigden)

Designing the extension with a series of small, intersecting traditional pitched roofs or with an area of flat roofing hidden behind a more traditional pitched roof can give you much-needed ceiling height, too.

Planning rules limit how far a double storey extension can project, and how close it can be to your property’s boundary, so as to prevent a loss of light to neighbouring properties.

It must project no further back than a line set at 45 degrees horizontally from the centre of neighbouring windows — the so-called ‘sight lines’.

Designing a Double Storey Extension

As with a single storey extension, you need consider your needs carefully before you start. If the ground floor is going to be home to your new kitchen, do you need a separate utility room or cloakroom? Do you need more storage space that is best factored in at the design stage?

When it comes to the second storey, if you’re taking a two-bedroom house to a three-bedroom house, you can realistically do without another bathroom; but if you’re taking a three bedroom house up to a four or five-bedroom house with your new extension, it will be more than worthwhile making space for an additional bathroom, shower room or at least a wet room, too.

Consider how to blend the extension with the existing home to ensure a cohesive interior space. You can do this by matching floor and ceiling levels between the new and existing space, as well as choosing the same decorative elements, such as mouldings, door furniture and colour schemes throughout.

You may also have to reconfigure the layout of the upstairs hallway to make access to the new rooms feel natural. The key is not to compromise existing rooms too much to create the new spaces.

Oak frame double storey extension to a bungalow

This oak frame (the frame was provided by English Oak Buildings), two storey extension has transformed a tired looking bungalow on a generous plot. (Image credit: English Oak Buildings)
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The position, size and shape of doors and windows to capture the best of the daylight and the views of the garden can mean that even when building an extension on a tight budget, you can achieve a stunning space. 

Choosing bi-fold doors is also a great way to bring the outside in. Blending indoor and outdoor spaces can make both spaces feel bigger and more cohesive. 

As well as bringing in light with bi-fold doors, continuous flooring creates the illusion of one space — think stone floors that run from the kitchen out onto a patio. Finally, consider interior and exterior materials – from wall treatments to planting – because the more harmonious they are, the more successful your extension will be overall.

If you are building a complementary extension, choosing materials that match those used on the original house can make your extension look like it’s always been there. But if it’s not possible to match the materials, it may be better to choose a striking, contrasting design which can work just as well, and is sometimes more acceptable to planning departments.

Where Should I Build a Double Storey Extension?

For a terraced house or semi-detached property with limited space, a rear extension is the best (and often only) place to site a double storey extension. In this case, be careful not to reduce your garden’s size too much. A large, family home needs a garden to match.

If you have room at the side of your house, a side extension can be a better option as it will allow you to retain the majority of your garden space.

If this is the case, careful consideration needs to be given to the design of the extension, since the frontage at least will be visible from the street. Not only do you want it to be sympathetic to the original house, the planning department may make it a condition. An experienced architect will be able to advise what you can and can’t do.

An experienced homes journalist and editor, Lucy is Editor-in-Chief of Homebuilding.co.uk's sister website, RealHomes.com.