House extensions: The ultimate guide

Typically you will need planning permission for an extension, if:

• The extension will cover more than half the area of land your home sits within.
• It will be higher than the highest part of the existing roof, or higher at the eaves than the existing eaves.
• The height of the eaves will exceed three metres and the extension will come to within two metres of the boundary.
• It is to be built forward of the main elevation of the house, or will front a highway.
• A veranda, balcony or raised platform is included.
• There is to be a new microwave antenna, such as a TV aerial on the extension.
• The extension will have a chimney, flue or soil and vent pipe.
• Work will include alterations to the roof of the existing house.

In the case of single-storey extensions, permitted development only applies, if:

• The new structure extends beyond the rear wall of the original house by less than four metres for a detached house, or more than three metres for any other house OR, in the case of extensions not in a designated area (such as a Conservation Area) or a Site of Special Scientific Interest and, subject to Prior Approval, will be less than eight metres if detached or six metres for any other house.
• Does not exceed four metres in height.
• Materials used are similar to those on the existing house.

In the case of two-storey extensions, permitted development only applies, if:

• The extension does not extend beyond the rear wall of the original house by more than three metres, or will lie within seven metres of any boundaries opposite the rear wall of the house.
• The roof pitch matches that of the existing house as much as possible and practical.
• Any windows on the upper floor on a side elevation are obscure-glazed and non-opening (unless the openable section is more than 1.7m above floor level.
• Materials used are similar to those on the existing house.

It is hard to put an exact timescale on an extension project. Variables such as location, size, style, site conditions, and your build route will impact the length of your project, as well as what you’ll use the extension for. As a rough guide though, the average single storey extension tends to take between 3-6 months.

When it comes to the size of your extension, there are no hard and fast rules, although there are a few items to note:

• Planning permission will definitely be required if your extension covers more than half the garden (including previously built extensions and any outbuildings).

• There are usually planning restrictions that will prevent you from building an extension higher than the existing house.
• In the case of side extensions, roofs and walls may be required to be set back from the house a little (often around 15cm).

While you obviously want your new extension to meet your space requirements, bear in mind that potential buyers can be put off by an extension that takes up most of the garden, and you could be left craving more outdoor space.

Speak to your designer about what you want from your new extension: be that a kitchen diner or a two-storey structure that offers a guest bedroom. From there you’ll be able to work out how big it needs to be without it becoming a huge eyesore. A good designer will be able to come up with a solution in proportion to the existing house and its plot, which still meets all your requirements.

While some people choose to act as their own project manager on extension projects, it is important to consider whether you have the time and knowledge to do this successfully. This route may well save you money but it will also require you to be on site regularly, to arrange labour and materials, and to swiftly deal with all the issues that arise. If you work full-time or have never carried out any home improvements before, it may be best to give the job to an experienced professional.

Many builders will happily take on the project management role: co-ordinating all trades, overseeing work, calling in materials, and liaising with you on a regular basis. Project management can also be carried out by a design and build company, or package build company. 

While hiring a project manager may cost more this is a move that can save you money in the long run, as your builder will be able to schedule in the relevant trades when needed and get the job done more efficiently.

For the new extension to feel like part of your home internally, it is important to consider how the two spaces will flow into one another.

For a kitchen/diner extension aim for the junction between the old and new spaces to be as open as possible. That way they should feel more like one single space rather than an old one with a new one bolted on.

For a new opening to be created, it will need to be spanned by a joist, usually made from steel. This joist will support the walls and floors above, where the old wall has been cut into for the extension. The size of the joist and any necessary supports should be part of your structural engineer’s calculations. 

In the majority of cases, it is possible to hide the joist within the ceiling (or a false ceiling can be created) in order to create a seamless appearance. Where this isn’t possible, some people choose to turn the new joist into an architectural feature.

Don’t assume that your existing home insurance and your builder’s public liability cover will offer sufficient cover for your house extension. This won’t always be the case, particularly if you take on some elements of the work yourself.

Specific extension insurance should cover your existing home, as well as the new extension work, materials, equipment hire, and your liability to others. It should also cover you for any damage to your existing home caused by the extension.

Contact your existing insurance provider to talk through any additional cover you might require. Once your extension is completed, your homes and contents insurance will need to be updated to reflect the additional space and the new value of your home.

FMB Insurance can advise you on the right type of insurance cover for your extension.

The height of your extension and the placement of any windows or raised platforms will, to a certain degree, be dictated by planning restrictions — permitted development will not apply in all cases and an application will be required if your extension fails to meet certain requirements (see Planning Permission for an Extension).

To avoid any issues with neighbours consider how features such as balconies will affect adjoining properties. Think about how these features can be positioned so they won’t overlook anyone else, if you are including them in your build.

Windows that are likely to overlook neighbours should be fitted with frosted glass or window film. Alternatively, talk to your designer about alternative ways to bring in natural light, such as high level windows, rooflights, or roof lanterns.

You may also need a Party Wall Agreement with your neighbour(s) if your extension will be built on or at the boundary of your two properties, or if work will affect an existing party wall or structure. You will also need to inform your neighbours if the extension will require you to dig below and near the foundations of their property. 

You may need to consider how you will get services, such as water and electricity to your extension, depending on how you wish to use the space. A kitchen extension will require electricity, water, waste and, in some cases, a gas connection. You will need to make early decisions on the layout of your kitchen, and plan where appliances will be. If you are installing a bathroom, cloakroom or utility then you will need to look at where the new pipework for the plumbing will be located. In two-storey extensions that include a bathroom, aim to keep the pipework, and especially the soil pipe, in the same position as the plumbing beneath. That helps keep things simple and can help to keep costs down.

Make sure your builder is aware of your plumbing and wiring requirements from the start. Moreover, if you are project managing, then keep your builder fully informed so they know which trades (such as electricians and plumbers) will be on site and when.

Kitchen extensions and new bathrooms inevitably add to the demands placed on your hot water system. Speak to your builder and a qualified plumber to calculate the heating requirements of your new space, what size radiators will be needed, and whether your existing boiler will need to be upgraded, along with the hot water cylinder. You can find a plumber in your area using our Find a Builder tool.

If your extension is likely to be built over a sewer, you will need to get in touch with your local water board. You may need to reconsider the size of the extension or, if a shared sewer is within three metres of your extension, you may need a Build Over Agreement with your local water authority.

This can add to costs so speak to your house designer and builder about possible solutions. You can find out more at Water UK.

If you are planning a rear or side extension and live in a terraced or semi-detached house, it is important to consider access — in other words, how will building materials and larger items, such as diggers, be brought round to where they are needed?

Have a conversation with your builder about the options, and keep your neighbours in the loop so you come to a mutually beneficial arrangement. 

Remember to inform suppliers of equipment and material of any access issues — in some cases very large or bulky items may need to be craned over the house and into place. 

Adding extra space can mean additional heating and lighting. Not only does this add to household bills, but also to the environmental impact of your home, so it really does pay to consider constructing your extension in the most energy efficient way that is practicable. Some people choose to install as much as possible, with solar panels, ground source heat pumps, green roofs and rainwater harvesting, but there are many smaller ways you can make your extension eco-friendly.

There are also certain energy efficiency requirements stipulated in the Building Regulations that concern minimum insulation levels in walls, floors (and sometimes lofts) glazing, low energy lighting, and the need for thermostatic valves on new radiators.

Designers often refer to a ‘fabric first’ approach when it comes to modern construction. That means a building is constructed to be as energy efficient as possible without adding eco-friendly ‘extras’ such as solar panels and the like.

Along with double or triple glazing and high levels of insulation, it’s also important to consider the orientation of glazing and how this can maximise solar gain and minimise heat loss.

Energy efficiency decisions need to be taken at the design stage and your builder should be made well aware of any extra measures you require, so they can be factored into the schedule of works.

A TPO is a Tree Protection Order. TPOs exist when a local planning authority has chosen to protect certain trees, or groups of trees. That means the affected trees cannot be cut down, removed, damaged or cut back without the authority’s written permission.

You will need to apply for permission to remove a tree protected by a TPO if it’s standing in the way of your proposed extension. If permission is declined then you will have to rethink your plans. Check with your local authority before tampering with any tree, although you should have been made aware of any existing TPOs when you purchased the property.