Extending a house: the ultimate guide for your house extension

Extending a house can be stressful but find out everything you need to know, from costing and designing extensions to permitted development and building regulations, to smooth the process

Extending a house: Modern house with wooden cladding

Planning a house extension, from getting the required paperwork in place to agreeing the contract with your chosen builder to ensuring the schedule runs smoothly is vital to ensure your project stays on budget and is completed in time. Follow this expert extension planning advice to get yours right.

Applying for planning permission for extending a house

Gaining planning permission is an important stage in the process of planning an extension, but can be tricky to navigate if you're not in the know. Use our expert guide to planning permission to find out more, and make sure you have gone through your plans thoroughly with an architect or builder who is familiar with the local planning authority and their preferences. 

As a general guide, 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 metres from the rear of a semi-detached house;*
  • You are extending more than eight metres from the rear of a detached house;*
  • Your single storey extension is taller than four metres;
  • 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;
  • Your house is listed.

*Until 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.

Planning an extension: industrial style glazed kitchen extensions

Other planning considerations for house extensions

What else might affect your planning application that you hadn't considered? Here are just a few things to bear in mind when you're planning an extension.

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.)

Discover how to apply for planning permission in our guide. 

Planning an extension: glass and wood extension attached to a home by IQ glass

(Image: © IQ Glass)

Considering access issues for your house extension

Organising the access to your house extension is an important early step because it could limit your design options. So, if access to the rear of your home is restricted, perhaps because you live in a terraced house, you will need to talk to your builders and neighbours early on in the process to ensure you won't be causing issues for later on in the build. Having materials craned over a house is possible but it's extremely expensive and you need to get the agreement of the council to close the road temporarily.

What to do if your extension's planning application is rejected

If you're planning an extension and your application is refused here's what will happen: your planning refusal notice will outline whether or not you can appeal and specify a time period in which an appeal must be submitted. An independent planning inspector, who will take into account national and local policy, as well as the impact of the proposal, will then consider the appeal.

Making changes to your extension's design after planning permission is given

Sometimes changes to an extension's design are unavoidable and any that are slight should be given the okay with few issues. However, any significant changes made to your extension's design after the planning application has been approved will require a retrospective amendment, or a new application all together. Find out more in our planning permission guide.

Planning an extension: country traditional style extensions


Planning an extension under permitted development rights

‘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.

Side return extensions are a good example. In many cases, two-storey side extensions may not even be a problem, but it’s a good idea to discuss your proposals with the planners before spending money on drawing up detailed plans. Even in conservation areas you can build rear extensions as long as they meet the size criteria and are in matching materials.

Building regulations for extending a house

This is important to factor into your extension planning routine. 

All home extensions need to comply with the building regulations. 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.

‘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:

  • 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.

Planning an extension? Comply with fire regulations

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.

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.

Planning an extension: living room extension to converted barn with wood-burning stove and grey walls

(Image: © Brent Darby)

Hiring an architect or designer to plan an extension

It's always best to pick a designer with a style or track record of projects that match up to what you're trying to achieve. Ideally, a local firm is a better choice than one that's distant, since they will be more familiar with the local vernacular and the local council's foibles. 

‘Organisations such as the Federation of Master Builders, Home Improvements Guarantee, Which? Local and Checkatrade offer builders who have been assessed in some way,’ says Kate Faulkner. ‘Make sure they have insurance and a warranty service that survives something happening to them or their business, and check they are happy to sign a contract.’

'It is good to have a brief for your designer, but this can be loose,’ says Alan Cronshaw of Acronym Architecture & Design. ‘If you want an open-plan layout, a better relationship between house and garden or simply more space, your designer will advise you on how to achieve this, perhaps with options. If you want expensive items, such as a certain brand of kitchen, or bi-fold or large sliding doors, let the designer know so the added cost can be factored into your budget.’

Bear in mind that some designers will charge for an initial visit. Check their fee structures and flexibility before you appoint, too.

You can find architectural technologists on ciat.org.uk and architects via architects-register.org.uk or RIBA. Find out more in our guide to working with architects.

Planning an extension: find the best builder

We all know that recommendation from friends or neighbours locally, as well as via trade bodies such as the Federation of Master Builders, is the best way to find a good builder. Questions to ask when looking for referrals? Was the project completed to schedule and as expected? Was it finished on budget? Were there any unforeseen problems and how were they dealt with?

Ask to see examples of previous work and talk to past clients as well. Compare at least three written quotations and ask  for clarification on any detail so you’re comparing like for like. Finally, make sure you’re comfortable – you’ll be working closely for months.

‘Decide if you want to use a larger, faster but more expensive firm that may be more flexible, or a smaller firm that will be less expensive,’ says Marta de Sousa of property developers Lux Reality.

‘Make sure that any contractors you use have warranties in place, such as Masterbond. Draw up a watertight contract using a template, such as a JCT homeowner contract, and make sure to state in it that payments be made following the completion of specific parts of work and not at different stages, as it is usually very hard to define when you are halfway or a quarter of the way through an extension project.’

Use our guide to finding reliable contractors to choose the best firm for your extension project.

Planning an extension: cube_exterior

The annexe is such an improvement in size, shape and practicality compared to their old leaky garage. The Red cedar cladding is from Benchmark timber, the glass porch is from Optigen Engineering

Hiring a project manager for your extension

A project manager, often the architect or lead builder, will oversee the project to ensure it runs smoothly, on budget and to schedule. Or, you may wish to save money by running the extension project yourself. Use our guide to how to project manage an extension to find out what you need to know about the process. And, either way, checking our week-by-week extension planner will give you an understanding of the extension process and help yours be built to schedule.

Extending a house: design considerations >>