How Much Do House Extensions Cost?

Across the UK, the cost of extending a house can vary dramatically, shaped by the build’s size, choice of materials, and overall complexity. According to industry figures, a single-storey rear extension might begin at roughly £1,500 per square metre for a basic concept, whereas a more prevalent range lies between £1,800–£2,500 per square metre. Should you incorporate bespoke glazing, complex steel elements, or luxurious finishes, outlays can climb well over £3,000 per square metre.

In many around London boroughs, homeowners find that labour rates fall somewhere below the steep charges in central districts, allowing them to invest in better-quality fittings or glass doors. However, older buildings can require additional foundation work, especially if the soil is clay-based or the structure is fragile, which can raise costs. Consequently, a larger extension might land anywhere between £40,000 and £100,000+, though a smaller side-return expansion could remain cheaper, while multi-storey designs can exceed typical ranges if the layout is exceptionally involved. Factors like structural detail, local labour fees, and the finishes you choose will all influence the final price. If your plan includes a kitchen or bathroom, expect extra plumbing, waterproofing, and electric installations that can substantially add to expenses.

Whenever you compare contractor quotes, confirm that each includes foundation tasks, structural work, roofing, fittings, and finishing. Labour, materials, and administrative expenses should be spelled out clearly. Also, check whether VAT is accounted for, as it is commonly around 20% and can be an unwelcome surprise if not included in the initial figure. Many renovation magazines or websites provide general cost references for the UK, but for precise estimates you should request several bids from established builders and contrast them line by line.

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.

Around London

Around London, house extensions commonly offer a solid middle ground between suburban comfort and fairly accessible connections to the city centre. Some owners of interwar or mid-century semis build a single-storey rear addition, merging smaller rooms into a bigger social hub. Others create side extensions if there is an underused strip of land alongside the house. For properties where outward expansion is challenging, a loft conversion may unlock more bedrooms or a home office above the main living areas. A basement can also be an option, though it typically demands more structural resources.

Contractors in these boroughs frequently mention scheduling or parking constraints, so booking skip drops and deliveries away from peak traffic hours helps. Labour charges might be moderate compared to central zones, potentially giving you budget flexibility for design extras like vaulted ceilings or modern glazing. Meanwhile, conservation areas or historically designated streets can impose tighter constraints on outward appearances, meaning you might need planning approval for even modest changes. With careful planning, an enlarged living area around London often boosts everyday usability and enhances resale potential, particularly if transport links remain convenient.

Getting Started

Before commencing, decide precisely what kind of extension you want. Single-storey or double storey expansions, side or wrap-around builds, and even loft or basement conversions can be viable options, depending on your property’s layout. Basements, though increasingly popular, are typically more complex if you cannot expand outward or upward.

Devoting enough time to the initial planning stage can mitigate delays as work proceeds. Draw up a detailed and realistic budget, looking into typical material costs and fees for each trade. Reserve some contingency for more luxurious finishes or emerging structural concerns. If your plan is on a bigger scale or demands a lot of structural work, you might need an architect or suitably accredited designer. While some building companies provide design-and-build, they tend to require preliminary drawings or partial planning approvals to give an accurate quote. Many homeowners hire an architect or design service for initial sketches, then request formal bids from builders.

It also pays to secure a contractor early. Skilled professionals often have full schedules, so researching and obtaining quotes promptly helps you compare offerings, consult references, and pick a suitable builder. An experienced team might notice improvements you had not initially considered—like adding more windows, repositioning doorways, or opening space for built-in storage.

Project Management

When organising your extension, there are two common methods:

• Single contract (design-and-build or main-contractor approach)
  You sign just one contract with a building company that oversees everything—carpentry, structural work, and subcontracting trades like plumbing or electrics. They may charge a 10–15% markup on these subcontracted jobs, but you gain a unified schedule and a single point of contact.
• Multi-trade or split contract
  You become the project manager, dealing separately with groundworkers, roofers, electricians, plumbers, and more. Although this might reduce overhead, it demands considerable coordination. If one trade falls behind, subsequent tasks could be delayed, requiring close oversight to maintain momentum.

For big or structurally demanding extensions, many choose a main contractor, especially if they lack construction expertise or time. If you do split the project yourself, ensure you can provide the attention needed to manage deliveries and confirm each phase completes on schedule.

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 criteria 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.

Some around London boroughs add further local constraints, especially in older or more scenic areas. Always confirm with your borough’s planning department if your design meets these conditions or requires a formal application.

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. Confirm with local planning officers to avoid misinterpretations, particularly if your property is located in a terrace row around London, where uniform rooflines matter.

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.

For many around London homeowners, basements can unlock additional living area if outward expansion is not feasible, though these projects tend to be more expensive and require extra care regarding structural support or party-wall matters.

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.

A Full Plans Application frequently identifies design issues before you start building, while a Building Notice can be acceptable for simpler expansions. Either process requires final approval to ensure correct structural work, fire safety, insulation, and so forth.

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.

Even if you don’t need a formal application (under Permitted Development), you must still follow building regulations to confirm safe foundations, adequate ventilation, and so on. Around London, older dwellings might require extra checks for damp or load-bearing capacity, so building control remains indispensable.

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.

Around London, partial DIY should come after the main shell is complete, leaving intricate structural and mechanical tasks to specialists. This approach can manage risk while letting you personalise finishing details

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 or permitted development 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.

Around London, merging modern additions with older suburban or Victorian exteriors might require suggestions on materials, roof lines, or large openings for light. An architect can help unify the design so that the new space feels cohesive.

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.

Around London, busy roads or constrained parking can complicate deliveries and skip removals, so many builders schedule them at off-peak hours—perhaps mid-morning—to avoid rush-hour stalls. Any adjustment in materials or design mid-project can also shift your end date further.

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.
8. 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 unresolved concerns should be addressed before final payment. Keep your documents (compliance certificates, warranties) handy if you later decide to sell or refinance.

Once the extension is finished and officially signed off, you can enjoy the benefits of a more spacious, functional property. Whether it’s a sunlit kitchen-diner or an entire second storey, a well-executed extension adds both comfort and value to your home. Around London, such improvements might be particularly appealing should you ever opt to sell, as buyers tend to prioritise generous living areas and easy commuting. By conducting a snagging survey, you can pinpoint small defects—like slight cracks or uneven plaster—so the builder can correct them before final payment. Storing any building control approvals, warranties, and completion certificates means you can prove the work was done properly if you change mortgages or move on in the future.

With that, your around London extension can give you the extra room you need, designed to suit your lifestyle and preferences, while adhering to all necessary regulations and guidelines.