Getting started

Before you begin, decide precisely what kind of extension you need. Common types are single-storey extensions and double storey extensions.

Popular options among these are also read, side and wrap-around extensions. However, you can also convert lofts or basements if that suits your property. You may also consider a basement excavation, although that is typically more involved.

Spend time planning thoroughly. Early preparation helps the work progress more smoothly and reduces delays. Start with a detailed, realistic budget.

Research the approximate costs of materials and trades, then set a financial framework you can stick to. Bear in mind that some design features or specialised finishes can push your spend higher than expected, so it’s wise to include a little contingency.

For greater extensions, or those involving significant structural change, you often need an architect or a suitably qualified designer. Some UK builders offer a design-and-build service, meaning they handle both the drawings and the construction.

However, it’s usually difficult to get a firm quote from any builder until you have at least some preliminary drawings, specifications, or planning approval in place. Most homeowners start by engaging an architect or design service to draft the proposal and then gather contractor bids.

It’s also essential to start looking for a contractor in good time. Skilled builders are frequently booked up well in advance.

Getting quotes early helps you compare costs and ensures you can hire your preferred firm. Additionally, a contractor with the right experience may suggest improvements to your plans that you hadn’t considered.

When selecting builders, diligence is key. You can use reputable accreditors like TrustMark, Federation of Master Builders and similar to verify contractors.

How much does house extensions cost?

UK extension costs vary widely depending on size, materials, and complexity. Industry data suggests that a single-storey rear extension starts from around £1,500 per square metre for a simpler build, while a more typical range is £1,800–£2,500 per square metre.

Projects with a higher specification—custom glazing, complex steelwork, or high-end finishes—can easily push well above £3,000 per square metre.

The average cost for a larger extension can therefore settle anywhere between £40,000 and £100,000+, though smaller side-return extensions might cost less, and multi-storey additions can exceed those ranges if the design is elaborate.

The variation is considerable, often hinging on factors like structural complexity, ground conditions, local labour rates, and the standard of finishes. Bathroom or kitchen extension costs will also be higher than extensions that don’t require the same work with plumbing, waterproofing and wires. 

When evaluating quotes, confirm that each one includes all expected work—from foundations and structural tasks to roofing, fittings, and finishing touches. Labour, materials, and administrative overheads should be specified. Also ask whether VAT is included, as this can be a significant extra for homeowners if you aren’t aware of it in advance (typically 20% in most cases).

You can also search for cost benchmarks. Many home-improvement magazines and websites publish average extension costs for different UK regions. For a more accurate figure, gather multiple quotes from qualified contractors and compare them carefully, line by line.

Project management

In general, you have two main routes when organising an extension project: a single contract (often called a design-and-build or main-contractor approach) or a multi-trade contract (sometimes referred to as split contracts).

With a single contract, you sign just one agreement with a builder that oversees all aspects of the work, including finding electricians, plumbers, and any other subcontractors. The main contractor is usually a building company that carries out the core carpentry or structural work in-house and subcontracts anything else.

They take responsibility for project coordination, schedules, and quality control.

If you opt for a split contract, you manage the project yourself. You deal directly with all the individual trades—groundworkers, bricklayers, roofers, electricians, plumbers, and more. This arrangement can, in theory, reduce costs, but it demands substantial project management skill.

You will be responsible for sourcing materials, coordinating delivery, and making sure tasks happen in the right order without delays. If you have limited construction knowledge or time, it can become overwhelming.

A single-contractor model can sometimes prove more cost-efficient overall. Established teams work together smoothly, and project timelines are easier to manage. If you choose to coordinate the job yourself, be certain you can commit the necessary time to keep everything running on schedule.

In many UK homeowner surveys, the majority prefer hiring a main contractor for large extensions, especially if they lack deep construction or project management experience.

Do I need planning permission?

Whether you need planning permission depends on your proposed extension’s size, height, and location on the property. Under Permitted Development Rights you are allowed to extend a house without needing to apply for planning permission if specific limitations and conditions are met. However, if your project goes beyond these allowances, an application for householder planning permission will likely be needed.

Under are the general rules that apply to all extensions: 

• Only half the area of land around the "original house" can be covered by extensions or other buildings.
• Extensions cannot be higher than the highest part of the existing roof; or higher at the eaves than the existing eaves.
• Where the extension comes within two metres of the boundary the height at the eaves cannot exceed three metres.
• Extension cannot be built forward of the ‘principal elevation’ or, where it fronts a highway, the ‘side elevation’.
• The work cannot include:
• verandas, balconies or raised platforms.
• a microwave antenna (e.g. TV aerial or satellite dish).
• a chimney, flue or soil and vent pipe.
• any alteration to the roof of the existing house.
• On Article 2(3) designated land the work cannot include cladding of the exterior.
• The materials used in any exterior work must be of a similar appearance to those on the exterior of the existing house.

Source: Planning Portal. 

Different types of extensions

There are also different requirements for different extensions. Under is a list of the different types of house extensions: 

• Side extensions
• Single storey extensions
• Extensions more than one storey

They each have their own specific limits concerning such as rear extensions requirements, height, roofing and windows. For extensions of more than one storey, there is for example a requirement that roof pitch must match the existing house as far as practicable.

Do I need planning permission for a new storey?

Loft extensions involving dormer windows or raising the roof often require planning permission. Adding a new storey is considered permitted development, meaning planning permission is not required, if strict criteria are met. This is also contingent on an application for the local authority’s prior approval.

Otherwise, an application for Householder/Full Planning Permission will be needed. If your house meets the requirements, the limits on the proposed development is as follows: 

• Number of additional storeys:
  
  • One storey can be added to a single storey house
  • Two storeys can be added if the house has more than one storey.
• Height increases:
  
  • The house cannot exceed 18 metres in total height
  • Each added storey cannot add more than 3.5 metres to the total height
  • If not detached (e.g. terrace or semi) the total height cannot be more than 3.5 metres higher than the next highest building that the house is attached to, adjoins, or is in the same row as.
• The additional storeys must be constructed on the principal part* of the house
• The additional storeys must not exceed 3 metres in height or the height of any existing storey in the principal part of the house (measured internally from floor to ceiling)
• Engineering operations must only include works within the existing curtilage of the house to strengthen existing walls and foundations
• The materials used must be of a similar appearance to those used in the construction of the exterior of the current house
• Windows must not be placed in any wall or roof slope forming a side elevation of the house.

There are also requirements following completion of development. 

Do I need planning permission for a new basement?

Creation of a new basement will usually need planning permission, and excavating to create a basement is also likely to require planning permission due to the major works required. It’s a good idea to discuss your project with your local planning authority to understand requirements before applying.

Basement excavation is similarly complex. Even if the project does not require explicit planning approval, you still must comply with the Building Regulations to ensure the extension is structurally sound and meets standards for escape routes, ventilation, damp proofing, fire safety, and more.

Always check any and all possible regulations that may apply in your area and for your specific home. In some cases, an extension that would normally fall under Permitted Development can be restricted by previous and local planning conditions. Check local council guidance or consult a planning expert to be certain. 

Remember building regulations application

Regardless of the size and shape of your extension, it must meet building regulations requirements. This is also regardless of whether you need planning permission or not. 

You will need to submit a building regulations application. For smaller projects you can submit a Building notice but for larger extensions you’ll probably need a Full Plans Application. In this case full details of the proposed work will be submitted for approval before the work begins. With a Building Notice an inspector will inspect and approve the work as it is performed. 

Note that building work involving electrical, heating, plumbing, or window installations can be certified by registered professionals through Competent Person Schemes (such as NICEIC for electrical work or FENSA for windows), allowing them to self-certify compliance without requiring separate building control inspections.

What’s the difference between planning permission and building regulations application?

Whereas planning permission assesses whether the extensions meets local and national requirements, building control concerns structural safety and regulations compliance. 

DIY Contribution

Homeowners sometimes consider tackling parts of an extension themselves. Painting, decorating, or landscaping can be good DIY tasks if you have the skills and time. Doing your own demolition, to an extent, can also be feasible—provided you understand which walls are load-bearing and follow safety precautions.

However, tasks such as electrical installation, gas connections, and structural changes almost always require qualified professionals. Mistakes in these areas can be costly and dangerous. Being realistic about your abilities is crucial; if you overcommit, you risk causing delays.

Builders may need to pause until you finish a particular job, which can add to your final bill. If you’re determined to do some work yourself, consider hiring a professional for a consultation, as their input can help you avoid pitfalls.

Do I need an architect?

If your design is straightforward, you might rely on a structural engineer or a professional building designer rather than a fully chartered architect. Yet for many extension projects, especially bigger or more complex ones, engaging an architect can be hugely beneficial.

Good architects consider not only aesthetics but also practicalities such as natural light, flow between rooms, and how the extension integrates with your existing house and garden.

They can also guide you through planning permission, permitted development applications and building regulations applications. While an architect does mean extra upfront costs, their expertise often pays for itself by creating a better-functioning layout and avoiding design errors that could cause problems later.

How long does it take to build an extension?

Creating a timeline helps you stay in control. A modest extension—like a small single-storey addition—might be completed in a matter of weeks once work begins, especially if it’s within permitted development, the design is simple, and materials are readily available.

Larger projects, such as a multi-room rear extension or a double-storey extension, can run for several months to half a year or more, depending on complexity and the number of trades involved.

In some scenarios, you may need to move out temporarily if major structural work is involved—especially for loft or multi-storey builds that disrupt essential services or require partial demolition of existing walls. Even if you can remain in your home, prepare for noise, dust, and limited access to certain rooms.

It’s wise to add some buffer time to your schedule. Delays are common. Rain or snow might slow exterior work; building inspectors might request minor alterations; material deliveries can be late.

Setting a realistic timeline with your contractor helps reduce stress, and finishing sooner than expected is always a pleasant surprise. Similarly, including contingency in your budget is recommended to cover unforeseen issues or changes to your plans.

House extensions step-by-step

Adding an extension can be a substantial undertaking, but with careful planning, the right team, and a sensible approach to budgeting, there’s no reason your project can’t succeed.. Whether you’re planning to expand for a new kitchen-diner, an extra bedroom, or a loft room, the key is to understand the steps involved and the potential pitfalls.

1. Start by confirming what you want to achieve—more natural light, a larger social area, or additional bedrooms. 
2. Then decide how best to organise the project. 
3. Check if you need planning permission and what kind of building regulations application you need to send in. 
4. Gather quotes from at least three builders if possible, ensuring each has a solid track record and good references. 
5. Check that their quotes are like-for-like in terms of scope. 
6. Factor in the cost of planning applications, building control fees, and professional services such as architectural drawings or structural engineering. 
7. Then develop a realistic schedule, communicate frequently with your chosen contractor, and keep a close eye on both time and budget.

Finally, as the build nears completion, consider a thorough inspection—often called a snagging survey—conducted alongside your main contractor or an independent professional. Any outstanding issues should be addressed before final payment. Retain all relevant documents, such as compliance certificates and warranties, since you may need them in future if you sell or refinance your home.

Once the extension is finished and officially signed off, you can enjoy the benefits of a more spacious, functional property. Whether it’s a light-filled kitchen extension or an additional storey, an extension done right adds both comfort and value to your home.