How Much Do House Extensions Cost?

The cost of enlarging a house in the UK varies widely, shaped by the extension’s size, selected materials, and overall complexity. Figures from industry sources show that a single-storey rear extension can start near £1,500 per square metre for a straightforward design, while typical projects often fall into the £1,800–£2,500 per square metre bracket. Once you choose extra steel elements, specialised glazing, or premium finishes, costs may climb above £3,000 per square metre.

Around Barnet, you might find labour fees slightly lower than central London areas, allowing you to afford more upscale fixtures. Yet older houses in the borough occasionally require special foundation measures or reinforcement, raising the total. As a rough overall figure, a bigger extension might lie between £40,000 and £100,000+, while a simpler side-return addition may land below that. Complex multi-storey extensions or highly elaborate layouts can exceed typical ranges if significant structural work or luxurious elements are chosen. Factors like building complexity, local trade rates, and whether a new bathroom or kitchen is part of your plan all influence final costs. If plumbing and electrical tasks are extensive, be prepared for higher totals.

Whenever you compare estimates, check that each covers foundation digging, core structural activities, roofing, fittings, finishing tasks, plus labour and overhead. Also confirm whether VAT (commonly 20%) is included, since it can be a large extra if not already factored in. You may review cost ranges in renovation magazines or websites, but for truly accurate numbers you should compile quotes from experienced contractors and analyse 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.

Barnet

Barnet is a popular North London borough featuring a wide variety of properties, from Victorian terraces to 1930s semis and modern developments. Many owners here decide to build single-storey rear extensions to merge once-separate living and dining areas. Others find that a side extension can transform narrow walkways into new rooms, or a loft conversion yields extra bedroom space while preserving the garden. In certain Barnet neighbourhoods, a basement may be an option, though it requires more significant engineering.

Local contractors highlight that Barnet’s roads can be busy around peak times, so scheduling skip deliveries in the late morning or early afternoon might help avoid major congestion. Labour rates are often lower than central boroughs, making it feasible to allocate more budget for refined fittings or unique design choices. However, if you live near one of Barnet’s conservation zones, anticipate extra steps for approvals to maintain external architectural harmony. Residents who expand an existing home usually see both improved daily living conditions and possible gains in property value, especially if they are near strong public transport links such as the Northern line. Checking early for party-wall matters is also prudent, as many houses in Barnet share boundary walls or close adjacency.

Whether you want a small add-on just to enlarge the kitchen or a more comprehensive plan to restructure multiple floors, it pays to adapt your extension carefully to Barnet’s local context. By doing so, you can enhance your space while preserving the borough’s distinct suburban character.

Getting Started

Before starting, determine exactly which type of extension suits your needs. Some opt for a single-storey rear or side addition, whereas others prefer a double storey extension, and a few choose loft or basement conversions if outward expansion is limited. Basements can be complex, but might be advantageous if you’d like added living area while keeping your ground-level garden intact.

Investing time in upfront planning can streamline the overall process. Begin by establishing a detailed, realistic budget, referencing typical material expenses and labour fees. Keep a contingency for unexpected costs or specialized finishes. If your scheme calls for complex structural modifications, you might need an architect or professional designer. Some building contractors offer design-and-build packages, but usually, they require early sketches or partial approvals to provide accurate quotes. Plenty of homeowners secure an architect first for conceptual plans, then gather bids from multiple contractors.

It is also crucial to look for a capable builder early on, as reputable teams are often booked well in advance. Gathering estimates promptly lets you compare prices, check references, and settle on a contractor that you trust. A skilled builder might highlight potential design improvements or ideas on maximizing natural light.

Project Management

People typically organise an extension in one of two ways:

• Single contract (design-and-build or main-contractor model)
  Here, you sign a single agreement with a main contractor, who oversees everything from structural tasks to plumbing and electrics, subcontracting as needed. They often add a 10–15% markup for subcontracted trades, yet the homeowner enjoys a simplified schedule and accountability.
• Multi-trade or split contract
  In this approach, you handle each trade yourself—groundworkers, carpenters, roofers, electricians, and so forth. Though it might reduce overhead, it demands coordinated project management to keep trades on schedule and materials arriving when needed. If you are short on construction know-how or time, it can become overwhelming.

For large expansions, many Barnet homeowners prefer a main contractor setup, especially if they lack the ability or free hours to juggle every trade. If you decide to manage the trades personally, confirm you can devote enough time to organise deliveries, ensuring no trade must wait on another.

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.

In Barnet, certain conservation areas—like those around Monken Hadley or near Hadley Common—may have extra conditions that limit how far you can extend or alter your facade. Always confirm with the council to see if your plan fits Permitted Development or requires a householder 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. Always review these guidelines with local authorities, especially in Barnet’s uniform terrace rows, to ensure a newly added storey does not break visual continuity.

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.

In Barnet, basements are not as prevalent as in certain central boroughs, yet some homeowners do create partial below-ground levels to expand living space if outward or upward extension is unfeasible.

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 often reveals issues before the build starts, whereas a Building Notice can be acceptable for simpler expansions—though adjustments may be requested on site if inspectors spot something unexpected.

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.

Even if you do not need planning permission under Permitted Development, building regulations remain mandatory. They confirm that structural work, damp-proofing, insulation, and so on comply with modern standards—crucial in Barnet, where many older properties may need extra care to ensure stability.

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.

Barnet owners occasionally handle aesthetic tasks once the main structure is completed, leaving the complex or potentially hazardous elements to experienced trades. This partial DIY strategy ensures the build remains on track while still letting you customise finishes.

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.

Barnet spans an array of housing eras—from Edwardian terraces near High Barnet to 1930s semis in other districts—and an architect can help blend these older styles with modern additions, potentially adding well-placed windows or pitched roofs that complement local character.

How Long Does It Take To Build An Extension?

Establishing a timeline is vital for staying in control. A smaller extension—like a single-storey rear addition—may conclude in mere weeks once on-site work starts, especially if it’s within permitted development, has simple design features, and uses readily available materials. Larger efforts, such as multi-room rear extensions or multi-level builds, can span several months up to half a year or beyond, shaped by complexity and the number of trades involved.

In certain cases, you may need to move out temporarily if significant structural modifications disrupt utilities or involve demolishing key walls. Even if you remain, you should anticipate dust, noise, and partial room closures.

Adding buffer time is wise, as setbacks often occur. Poor weather might stall exterior building; inspectors may request small changes; or material deliveries can arrive late. Working out a practical schedule with your builder lessens tension, and finishing ahead of time is always a bonus. Meanwhile, ensuring you have a financial contingency can address any mid-project changes or unexpected structural demands.

Barnet roads, especially near major intersections or high streets, can complicate deliveries and skip exchanges. Builders commonly plan them during quieter periods—perhaps mid-morning or early afternoon—to reduce disturbances. Should you alter materials or final layouts mid-build, note that your end date may shift further.

House Extensions Step By Step

An extension is a substantial project, yet with well-structured plans, skilled trades, and a thoughtful budget, there’s no reason it can’t be successful. Whether you aim for a larger kitchen-diner, additional bedrooms, or a new loft level, understanding each step and the usual stumbling blocks is crucial:

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 remaining issues should be corrected before final payment. Retain critical documents, including compliance certificates and warranties, if you ever sell or refinance.

Once your extension is completed and passes its final checks, you will enjoy a bigger, more versatile home. Whether it’s a bright rear living space or a brand-new upper floor, a well-executed extension adds both comfort and potential value to your property. Barnet families often see these upgrades as a way to stay within the borough’s suburban appeal while personalising layouts to modern needs. If at some point you decide to sell, extra living area is typically a strong selling point in a borough prized for decent commutes and greenery.

Conducting a snagging survey helps identify small flaws—like slightly misaligned paintwork or hairline cracks—so the builder can rectify them before you settle the last invoice. Keeping track of building control approvals, warranties, and completion certificates enables you to reassure mortgage lenders or prospective buyers that you complied with all regulations. With that in place, your Barnet extension can bring new functionality and style to your home, tailored to your specific goals.