Extending your house is a great way to add value while gaining the space you and your family need. In this guide, we set out how to plan, design and complete your ideal home extension.
The best approach on any home improvement project is to establish how your house could better meet your needs – so start by creating a wish list of your requirements.
“Think about your current space and what it’s missing,” says Charlie Mills from Oakwrights. “Sketching out floor plans is an exciting task that can inspire you. Look at the new addition as a blank canvas and ignore what is already there, as it may obstruct great ideas.”
In terms of what’s possible, home extensions come in all shapes and sizes. The right style for you will be defined by the property, planning rules and your budget.
A single-storey extension is the most appealing option for many homeowners as it often has minimal impact on the existing house, at the same time as allowing you to greatly increase your interiors.
You can build out to the side, front or rear, but the most suitable location and size for your project will depend on how much external space is available.
If you have enough room and a suitable budget, then a double-height addition could be a value-for-money route to maximising space.
Adding living area over two storeys invites you to redefine the internal layout; so for it to be successful, you’ll need to consider how the floor plan upstairs can be rejigged to suit the extra zones.
“Think about how you can fit a new bedroom in without reducing the size of another room, and how you could accommodate a corridor space,” says Lui Rocca from Welsh Oak Frame.
After you’ve selected the best area of your house to build out from, you can get started on a design brief.
Your new space will impact significantly on the external appearance of your home, so getting the look right is critical.
When it comes to planning the addition, you can take on the job yourself, but many homeowners choose to employ an architect, designer or specialist company to work with them to turn initial ideas into illustrations and formal plans – more on working with a professional in the box opposite.
Here are the key considerations to have in mind that will allow you to come up with the most suitable proposal:
The amount of money you’ve got available to complete the project is an important factor, so make sure you know your limits.
Letting your design team know a realistic budget will help to ensure you get a scheme you can genuinely achieve – but remember that quality and value go hand-in-hand.
“You get what you pay for; it’s tempting to opt for the lowest prices, but this rarely gets you want you want,” says James Upton from Westbury Garden Rooms. “Don’t forget to incorporate groundwork costs into your budget.”
As a general guide, a basic extension could start from as little £1,200 per m2 – but this can easily rise to £2,500+ per m2 if you’re going for a high-spec design. Most people will budget in the region of £1,800-£2,000 per m2. Read more in our in-depth guide to extension costs.
EXPERT VIEW: Extension mistakes to avoid
To ensure your project has the best chance of success, Paul Fitzgerald from Hawksmoor Construction suggests you avoid these common extension mistakes:
How big do you want your extension to be? Many people overestimate their needs.
“The addition should be in character with your existing house, so it’s important to avoid overdevelopment,” explains Ben Lee from Vita Architecture.
In order to work out how much you should extend by, consider how the space will be used as well as how it will interact with the main building and garden.
“Measure the largest room in your house to give yourself a realistic idea of the size you require,” says Lui Rocca.
Whichever extension style you’re planning, it’s important to consider how you want your space to flow.
Will it link with your current home in an open-plan kitchen-diner, for instance, or are you after a more segmented arrangement with rooms designated for different uses? And how will the new space relate to the existing house?
An experienced architect or designer can help you to understand how different types of extension layout can benefit your lifestyle.
Glazed doors, windows and rooflights play a vital role in the flow of natural light, but there are other tricks you can use, too. Glazed internal partitions can increase brightness and allow you to borrow space from adjoining rooms, for instance.
“Extensively glazed extensions afford homeowners the opportunity to add ceiling height, incorporate feature windows and open up spaces,” says James Upton.
Remember, it’s all about what works for your needs: if you want privacy in some zones, solid doors will give you that (and help cut out excess noise transfer, too).
Consider how your household might want to use your extended home in the future – ideas that might work now may not be ideal in five years’ time.
“Architects can propose a design to meet current and prospective needs, balancing the conflicting desires of a family,” explains Nimi Attanayake from Nimtim Architects.
With any alterations to an existing residence, it’s important to understand if you need formal permission from your council’s planning department.
Learn more: Projects You Can Do Without Planning Permission
Your home extension may fall under permitted development (PD) rights, meaning a planning application is not required – but there are circumstances where these might not apply, so always check with your local authority before going ahead with work.
If you’re able to utilise full permitted development rights, then you’ll generally be able to:
Permitted development rights don’t apply to listed buildings or designated regions, such as areas of outstanding natural beauty or conservation areas.
It’s against the law to alter a protected property without the appropriate consents. Any home improvement schemes will probably need to be sympathetic to the original building and require careful planning and well-considered materials.
It is possible to build a two-storey addition under permitted development rules; however, chances are you’ll need formal planning permission.
To be allowed under PD, the eaves and ridge height of the addition must be no taller than that of the existing building. The roof pitch should also match.
If you’re extending to the side, remember that the addition will be partly visible from the road, so your design should be sensitive.
If you’re planning to work on walls that you share with neighbouring properties, or building close to an adjacent boundary, you’ll need to make sure you comply with the Party Wall Act. This legislation aims to prevent and resolve disputes.
This means letting your neighbours know of your intention to extend, and how the work will be done. A party wall surveyor may need to resolve any disagreements and set parameters to protect neighbours’ property.
Even if planning consent is not needed, the work must still comply with Building Regulations.
You can either send your local council a full plan submission (best for high-value schemes), which the authority will then check against the current guidelines, or a building notice of your intent to start. The work will be inspected at key stages.
EXPERT VIEW: The Benefits of Channeling Natural Light from Above
Peter Daniel, product innovation director at the Rooflight Company, gives his top three tips for maximising natural light in an extension
Visit the Rooflight Company to learn more about how to maximise light in your extension.
Before work starts, you’ll need to find a suitable contractor. Put your plans out to tender by contacting several firms for quotes ahead of selecting the right team for the job:
Agree pricing in advance and draw up a contract so that everyone knows what is expected of them and when – this is your security blanket should disagreements arise.
Your contractor should also provide you with details of what is covered within your defects liability period, along with a timescale for snagging (post-completion fixes).
Logistics can be complicated when you’re dealing with an existing building – especially if you’re extending to the rear of a terraced house.
Your contractors will need to get to and from the building zone, as well as park vehicles and machinery near your house. You may need to provide parking on your land, especially if there is only limited off-street parking nearby – or this zone may need to be dedicated to materials deliveries.
If you’re working upstairs, materials will be carried through your home, so talk to your contractor about minimising mess by sealing off rooms, using protective sheets and providing welfare facilities.
Whatever type of extension you’re doing, someone will need to oversee the works. The role of project manager generally involves:
There’s a lot to stay on top of, so if you don’t have enough experience or time to commit to the job, it’s worth considering bringing in a professional or using a general contractor.
If you’re planning to work with a professional, seek recommendations from neighbours and friends and browse completed projects online to find the designer that best suits your style.
Take into account finishing materials as this may impact on whether you need to work with a specialist.
For instance, if you want an oak frame structure, then you’re best to choose a firm that’s experienced with this construction method.
Deciding to come up with your own extension plans without help from a professional might seem like a good cost-cutting exercise, but don’t underestimate the skills and knowledge a designer can offer.
There’s flexibility in terms of how much you want to involve them – for instance, they can just draw up the plans or apply to planning on your behalf, or source contractors and project manage, if that’s something they offer.
“Create a brief your designer can work with, so they can efficiently balance your requirements,” says Nimi Attanayake. “Collaborate with your architect and contribute your ideas.” Visit the Royal Institute of British Architect’s (RIBA) website for a list of chartered professionals in your area.
Design and build firms can offer a hassle-free route to an extension. They can provide design, project management and builders, so as much or as little as you like is taken care of.
“One advantage of such firms is that costs are provided in advance, so you never make decisions without knowing a price tag,” explains Lui Rocca.
QUICK GUIDE How Build Route Affects Extension Costs
Build Route A: Main contractor – standard option
The figures in our benchmark cost table are based on a main contractor route, where a general building firm manages the project to completion on your behalf, using a standard contract. You can bring prices down by taking on more of the responsibility yourself. Here are typical indicative savings for other popular build routes:
Build Route B: Builder plus subcontractors – potential saving 10%
You could potentially reduce build costs by circa 10% by hiring a main contractor to complete the structure to watertight stage. At this point you take over from the main contractor as a project manager and the remaining work is undertaken by subcontractors (individual trades), whom you manage through to project completion.
Build Route C: Self project managed – potential saving 20%
By project managing the entire scheme yourself, including the main structural phase, you could knock up to 20% off total build costs. This route doesn’t involve undertaking any construction works yourself, but rather fully managing the subcontractors on a DIY basis. So you are both client and building contractor, hiring trades and supplying plant, machinery, tools and most of the materials. You will need to be confident that you can keep the works on schedule to meet your budget.
Build Route D: DIY – potential saving 25%
Undertaking a large proportion of the build on a DIY basis could enable you to reduce project costs by as much as a quarter. This route assumes you’ll use trades for the key structural and infrastructure works, but will carry out much of the second fix tasks, landscaping, general labouring, decorating, tiling etc yourself. You will also be project manager, buying most of the materials and supplying all tools, plant, scaffold etc.
When you undertake a big home improvement scheme, you should always inform your insurance provider of your intentions. Ask whether your current policy will cover any possible damage during the extension works (whether to the new or existing structures).