How to build an extension – everything you need to know

A well designed addition has the potential to transform your home. We share all the know-how you need to plan a successful project

As well as boosting your home’s floor space, an extension can help pave the way to a reconfigured layout that complements your lifestyle. A cleverly-conceived project also has the potential to add wow factor and increase your home’s resale value. Here’s everything you need to consider before getting started, including tips on design, budgeting and planning.

1. Create your brief

kitchen with white wall and counter and stools on wooden floor

(Image credit: TBC)

First you need to formulate a comprehensive design brief. ‘The best way to start is by identifying your needs,’ says Jelena Cousins, creative director at Cousins & Cousins (020 7482 4009, ‘For example, do you need a bigger kitchen or living space? A separate bedroom or a laundry room? Writing this down will help to identify what your priorities are.’ Once you’ve highlighted the key functions you want from your extension, the design process can begin. Remember to plan the extension around your unique requirements – don’t fall into the trap of working around an idea you’ve seen on television if it doesn’t reflect your day-to-day needs.

2. Engage an architect

garden room with glass door sofa and wooden dining table

(Image credit: Future PLC/David Giles)

While it isn’t essential to bring a professional on board, a designer’s vision can be hugely beneficial. Their insight on positioning, orientation, structural systems and building materials can help in creating a stunning addition. Having an expert involved can also help when construction begins – particularly if you encounter any unforeseen issues.
Once your design concept is in the bag, it’s worth touching base with neighbours. ‘Keep them informed throughout the process,’ says Jelena. ‘If you need planning consent for the works, their support could be crucial for the scheme’s success.’

3. Manage the budget

garden room with brick wall white and grey chairs

(Image credit: Future PLC/David Giles)

Keep a close eye on your budget throughout the project, from the design phase right up until completion, to avoid any nasty surprises. Start by allocating an available project fund for your architect to work with. If you let them know what your key goals are – a bespoke kitchen, for example – they’ll be able to tweak other aspects of the scheme to bring it in on budget. As a guide, you should set aside between £1,100 and £1,300 per sq m for small, single-storey schemes, and between £1,300 and £1,500 per sq m for two-storey extensions. Allow at least £2,500 per sq m for large scale projects or those featuring high-spec building materials. It’s also a sensible idea to ringfence a 10% contingency fund to help minimise financial pitfalls, too.

Interested in building a garden room? READ: Installing a garden room – everything you need to know

4. Apply for planning permission

room with white wall and wooden ladder

(Image credit: Future PLC/David Giles)

While many extension schemes can be achieved under Permitted Development rights, anything that’s pushing the boundaries in terms of design is likely to require formal consent from your local authority; likewise if you live in a designated zone, such as a Conservation Area or Area of Outstanding Natural Beauty. Working with an architect or professional planning consultant can increase your chances of success at this stage. They are aware of local planning policy and will be best placed to create a design based on a thorough understanding of the guidelines. It can also help to find relevant examples of similar extensions on your street, as these can serve as a precedent for your project.

5. Choose a builder

garden room with white wall white counter stool and plants

(Image credit: Future PLC/Richard Gadsby)

Once you have the green light to go ahead with the build, you can put your project out to tender. Try to source quotes from at least three contractors – going by word of mouth recommendations is often a smart way to find a reliable firm. Alternatively, if you’re working with an architect, he or she might be able to recommend a company they have worked with before. Always ask to see examples of past projects or find out if you can talk to previous clients. Then the build can begin. Depending on the complexity of the project and the size of the structure, construction will usually take between three and six months. Use this time wisely by sourcing products for the internal fit-out and bear in mind that kitchen suites and bespoke joinery often come with a significant lead time.

Want more planning and project advice? READ: Loft conversions – how to plan and cost your dream space

Are you planning to build an extension? Why move when you can improve? There are lots of reasons to extend, whether you want to create more space, add value to your home or just love the area in which you live.

Deputy Editor

Jennifer is the Deputy Editor (Digital) for Homes & Gardens online. Prior to her current position, she completed various short courses a KLC Design School, and wrote across sister brands Ideal Home, LivingEtc, 25 Beautiful Homes, Country Homes & Interiors, and Style at Home.