Chelsea Brut, an extension to a 1960s townhouse by architectural practice Pricegore. Photo by Johan Dehlin.
Are you dreaming of more space in your home? Perhaps you're envisioning a larger kitchen, a cosy family room, or even an extra bedroom.
Whatever your motivation, building an extension can be an exciting way to transform your living space and add value to your property. But where do you start?
As an experienced architect, I'm here to guide you through the process one step at a time.
The following guide aligns with the RIBA Plan of Work – an industry-standard system construction professionals use for organising building projects – but sets out tasks in an easy-to-follow format.
Tasks are numbered in sequential order, although some may be carried out simultaneously.
You may wish to adjust the order of tasks to suit the demands of your project. For example, if time is of the essence, you may decide to bring certain items forward to speed up the process.
READ: Building Project Stages: A Homeowner's Guide to the Design and Construction Process
Before we get into the specifics, consider why you might want to extend your home.
Extensions offer a brilliant solution to space constraints without the hassle of moving house. They can provide that much-needed extra room for a growing family, create a home office for remote work, or simply give you the spacious kitchen-diner you've always wanted.
Plus, a well-designed extension can boost your property's value.
Low Energy House by Architecture for London.
This is where your journey begins: define your goals, budget, and timeline while assessing the feasibility of your ideas. Focus on your long-term needs and how the project fits into your broader plans.
Start by carefully evaluating your property and defining your goals for the project. If you live in a built-up area, you may be restricted to a single-storey extension. However, if there is more space between you and your neighbours, a two-storey extension could be an option.
Consider any constraints that may limit the height of your extension, such as limited outside space or planning regulations. If these restrictions apply, digging down to form a basement extension might be your only viable solution.
Alternatively, you may wish to focus on enhancing a specific area of your home, such as extending your kitchen to create additional space and functionality.
Richmond Townhouse by Sam Tisdall Architects.
Extensions can vary widely in cost depending on size, complexity, and finishes. As a rough guide, expect to budget anywhere from £1,500 to £2,000 per square metre for a basic extension and over £3,000 per square metre for high-end finishes.
Don't forget to factor in professional fees, VAT, and a contingency fund of about 10-15% for unexpected costs.
READ: How Much Does it Cost to Build a Home Extension in the UK?
Conduct an online planning search of the surrounding area to understand what might be possible. Local councils provide access to planning applications on their websites and reviewing nearby planning approvals can offer valuable insights.
Burnt House by Hayhurst & Co Architects.
Create a detailed project brief outlining your requirements, budget, and timeline – it’s the roadmap for your project. Appoint your project team and conduct site investigations if needed to ensure a solid foundation for success.
Preparing a brief will help you clarify your wants and needs while engaging with prospective architects. Ideally, it should include:
READ: How to Write an Architectural Brief: A Homeowner’s Guide
Durant Street by Feilden Fowles Architects.
To turn your vision into reality, you’ll need to work with an architect or designer. They will help refine your ideas and create detailed drawings for your extension.
In the UK, anyone can call themselves an architectural designer, but only those registered with the ARB can legally use the title “architect.” Depending on the complexity of your project and your budget, you may choose to work with an architect, an architectural technologist, or another qualified professional.
READ: Do I need an Architect?
When extending your home, you’ll need to consider statutory requirements such as planning permission and building regulations.
Some extensions fall under permitted development rights and won’t require planning approval, but larger projects or those in sensitive areas may need permission. Regardless, all extensions must comply with building regulations to ensure safety and energy efficiency.
Your architect will guide you through this, but understanding the basics early can help prevent delays.
READ: Permitted Development Rights for House Extensions
Inside Out by Mustard Architects.
When planning your extension, it’s important to investigate site constraints, as these may inform the final design. Your architect will assess:
Your architect will develop initial design concepts, which you’ll review and refine. Early cost estimates are prepared as your project starts to take shape through sketches and visualisations. You then refine your chosen concept for planning permission.
Harefield Road East by Gruff Architects.
Before starting any design work, you’ll need a set of existing plans. These will form the basis of all work thereafter, so it’s important that they’re accurate.
Your architect may do this themselves or organise a surveyor to complete it on your behalf.
Work closely with your chosen designer to develop concepts that meet your needs and complement your existing home. This is your chance to get creative and explore different options.
An architect will typically prepare sketch 2D and 3D plans of different solutions. You can then evaluate these against your brief so see which one best meets your requirements.
Flitch House by Oliver Chapman Architects.
Choose your preferred design and develop it in preparation for planning permission (if required). This stage, known as the ‘scheme design,’ involves creating CAD drawings that detail the floor layout, elevations, and sections, illustrating both the general appearance and interior arrangement.
The technical aspects of the design – such as construction details, build-ups, and connections – are added later. This approach helps avoid unnecessary work that may arise if the design exceeds the budget or planning constraints, saving time and resources.
When planning an extension, the garden design is often overlooked, even though many extensions feature large expanses of glass that provide generous views and access to the outdoor space.
If your garden is sparsely planted, consider how a new landscaping scheme can complement the extension's design. For a well-established garden, think about how the extension can respond to and enhance the existing planting.
Insulation and heating are often considered too late in the design process. As they may require planning permission, it’s important to address them early. For instance, external wall insulation alters the building’s exterior and typically needs approval. Installing a heat pump or solar panels may also require planning consent.
Note that heat pumps are eligible for government grants, while certain energy efficiency works like insulation upgrades may receive VAT relief.
READ:
Do Heat Pumps Require Planning Permission in the UK?
Do Solar Panels Need Planning Permission? PD & Design Guidance
Before submitting your plans for planning permission, contact builders to request an outline estimate. This will help ensure that your preferred design aligns with your budget.
At this stage, you may also choose to engage a structural engineer to provide an outline structural design. The structural elements often have a significant impact on overall costs, and giving the builder a clear idea of what will be involved will help them calculate more accurate estimates.
Lambeth Road by Archer + Braun in collaboration with Grace Fletcher.
The normal order of events is to appoint a builder after technical design stage when more detail has been added to the design. However, a two-stage tender can work well for extension projects. It involves inviting builders to price at scheme design stage, choosing a preferred builder and then finalising build costs after technical design.
Bringing a good builder on board earlier can help streamline the technical design stage and manage costs.
READ: How to Find and Hire a Builder
If planning permission is required, your architect will prepare and submit the application on your behalf. This process typically takes about 8-10 weeks, so factor this into your timeline. Even if your extension can be built under permitted development, it’s worth applying for a Lawful Development Certificate for peace of mind.
Once the planning stage is complete, detailed technical drawings and material specifications are produced, ensuring compliance with building regulations. Your architect will guide you through the technical aspects to prepare for construction.
Colour Chasing by District Architects.
For most extensions, you'll need the expertise of a structural engineer. They will calculate load-bearing requirements and design any necessary structural elements.
As mentioned earlier, it’s advisable to appoint a structural engineer before finalising the design for planning permission. This helps improve the accuracy of cost estimates, especially for more complex extension projects.
READ: Architect vs Structural Engineer: What's the Difference & Which do I Need?
Once you have planning approval (if needed), it's time to flesh out the details of the project. Your architect will produce detailed technical drawings, specifications, and schedules.
A robust set of documentation includes:
Ensuring that all aspects of the project are fully designed before starting on site will avoid delays and additional costs due to insufficient detail.
Step House by Bureau De Change.
Your detailed plans will need to be submitted for building regulations approval. This ensures your extension meets standards for things like insulation, ventilation, and fire safety. You can choose between two routes:
Full Plans applications usually take longer to process initially but can lead to smoother sailing during the construction phase.
Your architect can advise on the best route for your specific project.
When building an extension, you may need to obtain additional consents.
A Party Wall Agreement is required if your extension affects a shared wall or boundary with neighbouring properties. This involves formally notifying your neighbours at least two months before starting the work. You may also need to appoint a Party Wall Surveyor to manage the process.
A Build Over Licence is necessary if you're building close to or over a public sewer. You will need to submit detailed plans of your proposed works to your local water authority, ensuring that your proposals comply with their technical guidance.
Obtaining these consents early will help prevent delays and legal complications during the construction process.
Vault House by Studio Ben Allen.
A renovation project involves sourcing a wide variety of items, which can make it challenging to decide on everything that needs to be supplied and installed before building work begins.
Start by focusing on the most integral items to the build, such as:
Then, move on to additional items, such as (non-exhaustive list):
Clearly define who is responsible for supplying each item – you or the builder. For high value items like glazing, or items critical to the project’s timeline, ensure that the builder is responsible for both supply and installation. This way, you won’t be liable for delays or issues if something goes wrong.
A schedule of works is a detailed pricing document that complements your drawings and specifications, breaking down all the tasks, materials, and labour required for your project.
It enables builders to calculate more accurate estimates and ensures that all parties are clear on what’s involved. With a completed schedule, it’s easier to compare quotes and avoid surprises.
Without one, you may need to rely on builders to prepare their own pricing documents, making it harder to make accurate price comparisons.
With your plans approved, this is traditionally the time to find the right people to bring your extension to life.
Vestry Road by Oliver Leech Architects.
If you’re working with an architect, they can recommend builders they trust.
Always check that your chosen company is properly insured and ask to speak with previous clients. A reputable builder will be happy to provide references.
If you’re tendering the project, make sure to get at least three quotes. Alternatively, if you’re following the two-stage tender process, you’ll already have a builder on board, so skip to Section 24.
Organising a tender involves writing to your chosen builders and sending the drawings, specifications and schedule or works to builders for pricing.
The schedule of works is typically sent as a live document – such as an Excel file – for each builder to complete.
You may get queries asking for clarification on certain items. This is often a good sign as the builder has engaged with the information and is
Comparing tenders can be tricky, so consider retaining the services of your architect to help you understand the differences.
When reviewing tenders, don't just look at the bottom line. Consider the breakdown of costs – has everything be included etc, the timeline proposed, and the contractor's approach to the project. These are all important factors.
Fruit Box by Nimtim Architects.
Once you’ve selected your preferred builder, negotiate a final price for the project.
If their estimate exceeds your budget, consider adjusting the design, reducing the specifications, or phasing the project to spread the cost over time.
Having a completed schedule of works will provide a detailed breakdown of the costs, helping identify areas where savings can be made.
A building contract outlines the terms between you and the builder, covering the build cost, scope of work, payment terms, and project timelines. It should also include a contingency plan for any hiccups – like what happens if the work isn’t finished to the agreed standard or on time.
The contract documents – drawings, schedules and specifications
You can use a standard contract form, such as those from the Joint Contracts Tribunal (JCT), as a starting point. JCT Minor Works is suitable for many types of home extension project. For complex extensions and renovations works consider JCT Intermediate.
READ: A homeowner’s guide to building contracts for renovations and extensions
Your building insurance is unlikely to cover the building works and you’ll need to inform your provider to avoid invalidating your policy.
You will need specialist renovation insurance to cover theft, damage, and weather related issues during construction.
Additionally, ensure your builder has public liability, employer’s liability and contract works insurance.
Finally, it’s time to start the building work! If you’ve followed the previous steps and planned well, the construction phase should be fairly stress-free.
If you're working with an architect, they can oversee the project to ensure everything aligns with the drawings and specifications and manage the build for you. If not, you'll need to rely on the builder to deliver the project successfully.
Terzetto by Conform Architects.
Clear the area where work will take place and consider how you'll live around the construction. You might need to set up a temporary kitchen or arrange alternative accommodation for particularly disruptive phases.
Overseeing the building work is crucial to ensure it aligns with the approved drawings and specifications.
Stay involved throughout the project by making regular site visits and maintaining open communication with your builder. This helps identify and address any issues early.
Monitor both costs and progress closely. If you’re using a recognised building contract it will include a payment schedule linked to key construction milestones.
If something doesn’t seem right, speak up. It's far easier to resolve issues as they arise rather than waiting until the project is completed.
Walled Garden by Nimtim Architects.
Here's a typical order of build for an extension:
Every project is unique, and your extension may follow a slightly different order depending on its specific requirements.
As your extension nears completion, focus on these final steps.
Arrange for final inspections from the building control officer and your architect, if you have retained their services for the build.
Create a snagging list, a comprehensive list of minor issues or defects that need addressing. Common snags include paint touch-ups, adjusting doors or windows, fixing minor plumbing or electrical issues, and addressing uneven surfaces. Work with your builder so they are addressed promptly.
Collect all necessary documents, including:
Finally, once satisfied with the work and all paperwork is in order, make the final payment to your builder.
If you hired a builder under a JCT contract, 2.5-5% of the final payment is withheld over a specified period to fix any defects – typically 6 or 12 months.
Aden Grove by Emil Eve Architects.
Your extension is complete, and it's time to enjoy your extension.
You might find you need to make small adjustments to furniture or storage to optimise the space.
Develop a maintenance plan, including scheduled checks of gutters and any new systems like underfloor heating or air conditioning. Monitor your energy usage to ensure your extension is performing as expected.
By following this guide, you'll be well prepared to tackle your house extension project. Remember, good planning and communication are key to a successful build. With patience, persistence, and attention to detail, you can create a high quality extension that enhances your home and lifestyle.
Best of luck!
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, consult a skilled residential architect featured on Designs in Detail.
About the Author
Aron Coates is an architect with over two decades of experience, working on projects ranging from historic building restoration to contemporary home design.
