Residential Architect

How to Build a House Extension - Full Guide

A House Extension can give a wealth of additional space, but how much does it cost? How can you build it and who do you need? Find out in our full guide

Designing your 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 architecture company to work with them to turn initial ideas into fully working designs, illustrations and formal plans. A good professional can make the most of your site while keeping costs low and managing the stresses of construction.

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

House Extension Wood

1. Budget

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 Architect 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.

As a general guide, a basic extension could start from around £1,200 per m2 – but 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 and use our dedicated cost calculator for an approximate cost.

Building budget architect

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2. Extension size

How big do you want your extension to be? Many people overestimate their needs.

It is important that the extension goes well with your existing house, often larger doesn't necessarily mean better, particularly if the space has been designed well. The size of the extension needs to be balanced against what will be accepted as permitted development or in a planning application.

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. 

A typical extension will often be around 20 - 24m2.

Single storey extension

3. Layout, Light & Future-Proofing

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?

These are all things an Architect will pick up from the very beginning in order to determine what to prioritize and make the space specifically to how you live.

Doors, windows and rooflights play a vital role in the flow of natural light. Glazed internal partitions can increase brightness and allow you to borrow space from adjoining rooms, while solid doors aid privacy and create boundaries.

Roof lights and glazed bifold or sliding doors are very popular options to achieve lots of light, but it is important to match this with the plans for the interior.

Consider how your household might use your home in the future – what might work now may not in five years’ time.

Architects can propose a design to meet current and prospective needs, balancing the conflicting desires of a family.

Kitchen extension architect

4. 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.

Projects You Can Do Without Planning Permission

Your 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
- Put windows on upper floors of side elevations as long as they’re obscure-glazed and non-opening.

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

It’s against the law to amend such properties without the appropriate consents. Any improvement schemes will probably need to be sympathetic to the original building and require careful planning and well-considered materials

Construction architect drawings

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.

Double storey house extension

5. Building Regulations & Construction

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.

Constructing 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 sign a JCT minor works or JCT IC 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).

The small JCT contracts make sure to include for anything that needs to be included before the project starts and provides solutions to any problems that arise during the build.

House construction drawing

6. Building On-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. The contractor 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 management

Someone will be required to oversee the works. This role of project manager involves organising who’s coming onto site and when, keeping on top of what’s being spent, when materials deliveries are scheduled, ensuring work is progressing to the quality that you’re expecting, plus keeping an eye on
health and safety on site. This service can normally be shared between the Architect and contractor in order to produce health and safety materal, check progress, administer the contract and more.

it’s worth considering an Architect for the contract administration, meaning they will take on much of these tasks. A general contractor will often be able to provide everything else while conducting the work.

House architect

What Professionals Are Needed?

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.


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. Often the time and money spent on a good architect is made back five-fold due to the careful planning and management. The completion date and quality of the finished product is also likely to be far higher.

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.

Structural Engineer & Approved Inspector

A Structural engineer and Approved/LA inspector must be appointed on a extension project. This is to ensure compliance at building regulations stage.

These consultants will often communicate with the Architect on the project in order to confirm the design complies and is specified with building regulations compliant products.

Architect Project manager

7. Insuring Your Extension

When you undertake a big home improvement scheme, you should always inform your insurance provider of your intentions.

Your current policy may already cover any possible damage caused during extension works, but if not, you may need to buy a separate package to protect your house and possessions for the duration of the build. Providers include Protek and Self-Build Zone.

House extension cost

Still have questions? A no commitments consultation with us can be a great place to start.

residential architect

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