Home Extension Guide: How to Extend a House

Extending your home could add value and significantly improve your quality of life. Jennifer Grimble sets out where to start and the key things to consider
Jen Grimble
by Jen Grimble
2nd August 2022

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.

Extension Cost Guide

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.

Designing a Home Extension

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.

Extension by Oakwrights

Do you want your extension to complement or contrast your current home? Incorporating an opposing material, such as oak, makes for a wow-factor finish, as this Oakwrights project proves. Photo: Lydia Harper

Here are the key considerations to have in mind that will allow you to come up with the most suitable proposal:

Budgets & Extension Costs

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:

  • Avoid confusion when requesting extension quotes by supplying prospective contractors with detailed architectural drawings or quantity surveyors’ plans to work from. Then you can compare like-for-like quotes and ensure nothing has been missed out. If the details shared with each contractor vary or change, then the quotes and prices likely will too. And remember, cheapest is very rarely the best choice!
  • Research your contractors before you agree to work with them. Check for membership to the Federation of Master Builders (FMB) where trades must pass a vetting and independent inspection process before joining. Ask to visit other extension projects to ensure their work is up to a high standard, too.
  • Be wary of contractors who are not forthcoming with key information or are slow in responding to simple requests. A trustworthy contractor will always be open to communication with you. You should reciprocate by communicating openly and regularly with your builder, too.
  • One of the biggest mistakes to avoid is trying to project manage yourself if you’re inexperienced. Instead, use a main contractor to manage all the tradespeople involved. Don’t try to cut corners or save money by project managing things yourself if you don’t have the knowledge and time. This will expose you to unnecessary financial and liability risk, not to mention additional stress.
  • Finally, be wary of paying too much money upfront. Sign a contract for staged payments with your builder and aim to put down no more than a 10% deposit at the outset of the extension project.

Paul Fitzgerald is Managing Director of Hawksmoor Construction, which is an FMB member company specialising in quality high-end homes both in London and across the home counties.

Extension Size: How Big to Go

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.

Extension Layout & Flow

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.

side return extension

A low budget to extend this Edwardian mid terrace in London didn’t mean the homeowners had to skimp on style. Nimtim Architects used inexpensive materials to extend and radically alter the ground floor for a more family-friendly layout [Credit: Megan Taylor]

Natural Light in Home Extensions

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).

Future-Proofing your Extension

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.

Extension Planning & Permitted Development

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:

  • Build on up to 50% of your home’s existing land (within the confines of other PD limits).
  • Extend up to the highest part of the roof.
  • Build out by 4m or 3m on an detached/attached dwelling (currently 8m or 6m retrospectively in England).
  • Create a side extension on a single-storey with a height of 4m and a width up to half that of the original house.
  • Put windows on upper floors of side elevations as long as they’re obscure-glazed and non-opening.

Listed Building Consent

Permitted development rights don’t apply to listed buildings or designated regions, such as areas of outstanding natural beauty or conservation areas.

Learn more: Understanding the Planning Changes for Extensions in England

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.

Multi-Storey Extensions

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.

Party Wall Act

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.

Building Regulations for Extensions

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

  1. Consider glazing to square footage ratios. If you’re extending from an exterior wall, you can’t just move the same number of windows out to that new wall and expect to gain the same amount of light, because you’ve added square footage. You should aim for about 25% window to square footage. The problem with extensions is there’s only so much space on the exterior wall, which is why rooflights are ideal in this scenario.
  2. Rooflight size. Installing one big rooflight to make a statement is tempting. Howver, it can have the adverse effect of letting in too much light and reducing thermal performance by substituting higher performing insulation with lower performing glazing. Instead, consider several smaller rooflights to reduce how easily a room warms up in direct sunlight and provide an even distribution of light. Linked rooflights also look striking.
  3. Rooflight placement. Installing a rooflight to create a spotlight effect above a specific feature can look very cool. But consider the placement. A spotlight above a kitchen island is a wonderful idea in theory, but you might find you can’t see what you’re doing on particularly sunny days due to glare.

Visit the Rooflight Company to learn more about how to maximise light in your extension.

Building Your House 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:

Get a Contract with Your Builder

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).

Access to Site

Logistics can be complicated when you’re dealing with an existing building – especially if you’re extending to the rear of a terraced house.

home extension with glazing

Consider how windows and doors will play a role in your extension. Yellow Cloud Studio used extensive glazing in this project to fill the property with natural illumination and allow for clear views out to the garden

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.

Project Managing an Extension

Whatever type of extension you’re doing, someone will need to oversee the works. The role of project manager generally involves:

  • Organising who’s coming onto site and when.
  • Keeping on top of budgeting and what’s being spent.
  • Scheduling and checking materials deliveries.
  • Ensuring work is progressing to the quality and schedule that you’re expecting.
  • Responsibility for health and safety on site.

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.

Who can Design Your New Home Extension?

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.

Find a specialist designer for your home extension

Architects & Designers

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.

Specialist Extension Companies

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.

Insuring Your Extension Project

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).

If not, it’s sensible to purchase a separate extension insurance package to protect your house and possessions for the duration of the build. Well-known providers include Protek and Self-Build Zone.

Main image: Fraser Marr

This article was first published in February 2021, and was last updated on 2nd August 2022

Leave a Reply

You may be interested in

Our sponsors