How to extend your home: the ultimate guide

From costing and designing extensions to permitted development and building regulations, find out everything you need to know here

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If you're planning an extension but don't know where to start, our guide to extending your home is here to help. Covering everything from costing and designing extensions to permitted development and building regulations, you'll find everything you need to know before you get started. 

How much will an extension cost?

You’ll need to allow around £1,200 per square metre for the building work — and, for higher specification finishes, up to £3,000 per square metre. If you’re adding bi-fold doors, allow £1,200 to £1,800 per metre for these. Bear in mind that small extensions (under 15–20 square metres) have higher costs per square metre, and that clay, peat, lots of nearby trees, or sloping ground will increase foundation costs.

‘As a very rough rule of thumb, costs should be split into 50 per cent for construction and 50 per cent for the interior,’ says Hugo Tugman, founder of Architect Your Home . If you do not want to compromise on the size of what you are building, you may be able to make considerable savings on interior finishes.

It is important that you get quotes from at least four builders as it is almost certain that each of the builders will provide different costs for exactly the same work using the same materials. If you only approach one or two builders, you run the risk of only obtaining high quotes, rather than getting an idea of an average.

Click here to get a comprehensive extension cost calculation using our free extension cost calculator

Planning permission and permitted development for extensions

Planning permission and permitted development are an important stage in the process of building an extension, but can be tricky to navigate if you're not in the know.

How to extend without planning permission

‘Homeowners are sometimes surprised at how much can be built on to a house under permitted development rights,’ says Alan Cronshaw of Acronym Architecture & Design. Even in conservation areas you can build rear extensions as long as they meet the size criteria and are in matching materials.

If you are planning to build an extension under permitted development rights, study the criteria carefully and apply for a certificate of lawful development from your local authority. This only costs £86 and you will then have the paperwork in place to prove that your extension did not require planning permission.

industrial style glazed kitchen extensions

Making a planning application

The cost of a planning application is £172, an expenditure that you don’t want to have to make several times. Make sure you have gone through your plans thoroughly with an architect or builder that is familiar with the local planning council and their preferences.

When building a more ambitious extension you will need planning permission if:

  • Your extension covers half the area of land surrounding your home
  • If you are extending towards a road
  • You are increasing the overall height of the building
  • You are extending more than six meters from the rear of a semi-detached house*
  • You are extending more than eight meters from the rear of a detached house*
  • Your single storey extension is taller than four meters
  • Your single storey extension is to the side of the property and more than half the width of your house
  • You are using materials that differ from the original style of the house
  • You plan on building a balcony or raised veranda

For more detail on when you will need planning permission, visit the Planning Portal

*For a period of six years, between 30 May 2013 and 30 May 2019 – householders will be able to build larger single-storey rear extensions under permitted development. Take a look at The Planning Portal for more information.

glass and wood extension attached to a home by IQ glass

(Image: © IQ Glass)

What to do if a planning application is rejected

‘First, it’s important to try to understand exactly why the application was rejected,’ says Hugo Tugman. ‘The proposals may be largely acceptable, but simply contain a detail that the local authority can’t approve, in which case resubmitting a new application that has been amended accordingly should be enough to get the permission that you require.

‘It may, however, be that what you are proposing is fundamentally outside the planning policy or guidance the planners are working with. If that is the case, you really need to understand what these policies are and redesign so that your scheme falls within these parameters.

‘The third possibility is that you feel your scheme was within the policy guidance, but the planning department has made an unreasonable interpretation of the rules and refused it. In this final case it may well be worth going to an appeal, where an inspector (not local authority) makes an independent assessment of whether policy has been applied correctly and reasonably.’

Changing your extension design

Changing your extension design after your planning application has been accepted will require a retrospective amendment, or a new application all together. The key to avoiding this situation is to study your plans in detail and have a physical or digital model created to help with visualization.

country traditional style extensions


‘Occasionally, situational developments may mean that changes are unavoidable and retrospective amendments sound like a good idea. However, in recent years planning departments have been less ready to go down this path.’ says Hugo.

‘If the changes are very slight, it is indeed possible to apply for a "non-material amendment", but changes that may affect anything significant, such as the overall height or positioning of upstairs windows, would be “material”, and the planners would have to advertise all over again, so they generally push you down the route of a new application.’

Extension design considerations

It's important to ensure that your extension suits your needs during the planning process, as it can be costly to make changes further down the line. There are many options to consider, including:

Planning a side extension

In many cases, two-storey side extensions should not be a problem, but it’s a good idea to discuss your proposals with the planners before spending money on drawing up detailed plans, especially if you are extending over two storeys. They will consider:

The extension's footprint

There are few limits on the size of your extension’s floor area unless it’s likely to cover more than half the garden (including any existing extensions and outbuildings).

The height of your extension

You’re not normally allowed to build higher than the existing house. However, side-extension roofs and walls often need to be set back slightly from it (perhaps by 10–15cm).

Overlooking neighbours

Features such as upper-floor balconies can be contentious if they overlook the neighbours. For windows, you can use obscure frosted glass, install them at a high level, or fit skylights.

Overshadowing neighbours

Building a two-storey, or higher, extension too far out from the back of the house into your garden may overshadow the neighbours, which will limit the permissible size.

Highways and your extension

If your proposed extension could interfere with visibility for motorists, it will also limit how far out you can build.

‘Another factor to bear in mind when building within three metres (or in some cases, six metres) of neighbouring buildings is the Party Wall etc Act, which requires you to formally notify the adjoining owners two months in advance of the proposed project.’ Says Ian Rock. (See planningportal.gov.uk.)

Matching your extension with your existing home

‘There are no hard-and-fast rules on what materials will be accepted by the planning office. It depends on the building, the area, local planning policy, and you — the homeowner,’ says Hugo Tugman.

‘It used to be that planners generally wanted extensions to be in keeping with the original building, which led to a rash of pastiche additions to older buildings, but these days there is more and more of a prevailing view that allowing an original building to be itself.

kitchen extensions exterior

This single storey, innovative extension is clad in a blue render, and cost less than £100k,

‘Contrast does not have to mean shiny modern or hi-tech. It is generally a good idea when extending an old building for the extension to play a quieter role to that of the original building, and it is quite possible to produce a relatively contemporary design that is modest and calm in its expression.’

This said, it's important to consider the transition between an existing building and a new extension. Renovation expert Michael Holmes suggests the following:

Maximise the opening

The wider and taller the opening that links the two spaces, the more they will feel like a single room.

All new openings will need to be spanned by joists, usually steel, to support the walls and floor above. The joist size and its supports should be calculated by a structural engineer (you can find one via the Institute of Structural Engineers at istructe.org). The smaller and less visible these elements, the more seamless the flow between old and new. In most instances it is possible to conceal the joist within the ceiling void, especially if you’re removing only a non-load-bearing partition wall.

Create a continuous ceiling level

The ceiling height between old and new spaces should, ideally, be the same. If they’re different, however, the higher ceiling can often be brought down by adding new battens and plasterboarding over the top.

There is no minimum ceiling height under the Building Regulations, other than above staircases, but 2.3-2.4m is standard. If this is not a practical solution, then it is best to have a smaller opening with a boxed bulkhead to conceal the step-in ceiling levels.

Make sure the flooring is laid at the same elevation

When setting out floor levels for an extension, it is important to work backwards from the finished floor level in the existing property to ensure they will be identical once they’re linked.

When you’re remodelling, rather than extending, any differences in floor level can be overcome by building up – often using a quick-setting silicone floor screed. If the same level can’t be easily achieved, it is best to create a full step, (H)19-22cm, rather than a small difference that could end up being a trip hazard for children.

Use matching finishes

Old and new spaces can be linked seamlessly by using the same flooring material throughout. This principle also applies to the same architectural detailing such as windows, doors, skirting, architraves and coving; and the same décor, including colour schemes, flooring, curtains and furniture.

How to create an open-plan living space

Open-plan or, more frequently, semi-open-plan living, is very popular, as it suits most people’s modern lifestyle and enhances the sense and use of space within the home,’ says Hugo Tugman. ‘I do generally encourage people to go this way; however, there are a number of things to bear in mind when working out how best to use the space.

‘It’s usually a good idea to identify different zones within your layout — such as the kitchen, dining and living areas. The look and feel of your space can be fine-tuned by the extent to which the boundaries of these are defined or blurred, and there are lots of devices, such as continuing through or changing the floor finish, for example, which can control this degree of separation.

‘While walls can block up an open space, they can be helpful when it comes to positioning furniture, storage and radiators, for example. One of the reasons that underfloor heating works so well in an open-plan space is because often there are not enough suitable walls on which to position radiators.’

Choosing bi-fold doors for your extension

‘There are a range of different options available for bi-fold doors, from two-door models through to large eight-door configurations, set-ups for bay arrangements and entire 90-degree corner sections,’ explains Neil Ginger, CEO at Origin. ‘The price of bi-fold doors varies but, as a guide, a bespoke, aluminium design would cost from £1,200 per door leaf.

‘The space allowance for the doors to open outside depends entirely on their width. Bi-fold doors can be as narrow as 40cm, protruding less than half a metre outwards, while you will need to allow just over a metre of space for doors with a width of 1.2 metres.

‘Doors that open inwards are ideal for projects where space outside is limited — on a balcony, for example. In the majority of cases, it is recommended that outward-opening doors are chosen to prevent any rainwater from coming into the home when the doors are opened after it’s been raining.’

Bi-fold doors can also be installed internally, as the low threshold of the design can create a seamless transition from one room to another.’

Building regulations for extensions

‘All home extensions need to comply with the building regulations,’ says Ian Rock. ‘Most obviously, this relates to structural stability — including foundations, window and door openings, lintels, beams and roof structures. Therefore, your design will normally need to incorporate a structural engineer’s calculations, submitted together with drawings as part of your building regulations application.

‘When it comes to submitting your application, you can either do this via local authority building control, or an independent firm of approved inspectors. Either way, there are two ways of making an application — either “full plans”, or the short-cut method known as a building notice.

For a major project such as an extension, it makes sense to get your design approved with the former before you start work, otherwise you could run into trouble if your project doesn’t comply with the regulations.

industrial style kitchen extensions in london

‘When work is due to start, it is essential to liaise regularly with building control, as they will need to carry out site inspections at key stages, commencing with start on site and excavation of foundations. Finally, once your new extension is built, don’t forget to obtain proof of compliance in the form of a completion certificate — this is a key document when you come to sell.’

What to submit to make sure you comply with building regulations

For all applications:

  • Completed application form;
  • estimate of costs;
  • the appropriate fee;
  • two copies of detailed drawings at a scale of 1:100 minimum;
  • two copies of a site plan showing the proposal, site boundaries and sewer positions;
  • two copies of any plans and specification to accompany drawings including structural design and calculations;
  • four copies of plans for buildings covered by fire safety legislation, showing fire resistance, fire detection, alarms, emergency lighting, means of escape and signage.

*If online applications are accepted, only a single copy of each plan is required.

Complying with fire regulations for a new extension

‘Most extensions should naturally comply with fire regulations thanks to the inert qualities of building materials such as plasterboard, bricks and concrete blocks, which can normally resist the spread of fire for at least 30 minutes,’ says Ian.

‘However, where you have any exposed major structural components such as timber posts and steel beams, they will normally need to be protected, for example with skimmed plasterboard lining. Also, where holes are cut in ceilings for recessed lighting, they may need to be fitted with fire hoods. Extensions built with modern timber-frame wall panels are lined internally with inert plasterboard and also incorporate integral cavity barriers to slow the passage of smoke and fire.

‘If your design includes an integral garage, then the walls and ceilings need to resist fire — which most materials should manage, although special pink- coloured plasterboard (fireboard) is the ideal cladding for ceilings and stud walls. Ceilings to integral garages must be plastered, and any doors from the house must be fire doors with a suitable step down into the garage (normally 10cm).

‘Requirements become a lot more demanding for extensions of three storeys or more. Considered as part of the newly enlarged house, this might involve fitting special fire doors to all new and existing rooms as well as ensuring there is a safe escape corridor (usually via the landing and stairs) down to a main exit door, with the stairs protected with a fireproof lining.

‘If your extension is two storeys or higher, it is best to assume that you need to fit a mains-operated smoke alarm to the upstairs landing(s) in the newly extended house.’

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