How Much Do House Extensions Cost?

Extension prices in the UK vary greatly according to the size of the property, material selection, and overall build intricacy. Industry data suggests that a single-storey rear extension often commences at roughly £1,500 per square metre for a basic design, with most standard expansions lying between £1,800–£2,500 per square metre. Should you include custom steel frameworks, premium finishes, or complicated layouts, it’s not unusual to exceed £3,000 per square metre.

In Central London, labour fees remain among the highest in the country, though the availability of specialist trades can be beneficial for unique design requests. Nevertheless, older or historically protected buildings around Westminster, Marylebone, or Pimlico may demand reinforced foundations or carefully matched exteriors, pushing your overall costs upward. A significant extension might rest in the £40,000 to £100,000+ bracket, but slender side returns could be less, while multi-storey or high-end projects easily surpass typical estimates if structural elements or luxurious materials are included. Consider that if your plan adds a kitchen or bathroom, you must also budget for plumbing, electrics, and waterproofing.

When collecting bids, see that each contractor’s quote incorporates foundation excavation, structural build, roofing, fittings, finishing tasks, along with labour and administrative overhead. Check if VAT—commonly 20%—is included or extra. Though websites and renovation media can show approximate figures, the most reliable approach is gathering multiple quotes from established builders and evaluating 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.

Central London

Central London encompasses areas such as Westminster, Marylebone, Pimlico, and parts of Camden or Southwark near the city centre. Extensions here frequently revolve around reworking tight terraces or period townhouses. Single-storey rear expansions with broad glass doors can unite interior living zones with small courtyards, letting in more sunlight. Others do side extensions if a narrow passage exists, or loft transformations if outward space is limited. Basements remain a favoured choice for some high-value properties, yet the complexity and engineering costs can soar, especially in listed neighbourhoods or near main roads.

Local contractors stress that traffic congestion and parking restrictions can affect scheduling skip drops or deliveries, so off-peak times generally work best. Labour fees in Central London are high, but experienced trades can handle heritage features if your building is listed. Conservation-area regulations near historical streets, garden squares, or iconic landmarks may impose further hurdles for outward changes. Still, a well-executed extension often garners better day-to-day functionality plus a rise in value, especially if your area has exceptional transport links like the Underground or Crossrail lines.

Adapting a design to blend modern lines with older facades is usually wise, making local planners more receptive and ensuring a cohesive final aesthetic. Many central houses can be joined by party walls, so early neighbour communication is beneficial.

Getting Started

Determine which extension design best fits your home. A single-storey rear extension might enlarge a cramped kitchen-diner, while a double storey layout adds living space across two floors. If you have a side alley, a wrap-around or side build may increase the ground floor. Alternatively, you could choose a loft or basement if outward room is scarce. Basements, though spacious, are more complex and pricey—especially in Central London’s tightly packed streets.

Strong preparation from the outset streamlines the entire operation. Draft a detailed, realistic budget, referencing standard material and labour fees, while leaving reserves for high-end touches or structural complications. If major structural changes are planned, consider hiring an architect or suitably qualified designer. Though some contractors offer design-and-build, they generally need initial sketches or partial planning approval to produce a reliable quote. Many households first engage an architect or design service for conceptual plans, then gather quotes from building firms.

Securing your contractor well ahead is also recommended because top-tier companies remain in demand. Getting estimates early aids in comparing cost breakdowns, verifying references, and appointing a builder who aligns with your vision. Seasoned professionals may further refine your ideas—for instance, relocating doors to improve flow or inserting a roof lantern for extra daylight.

Project Management

In a typical extension, owners choose one of two management methods:

• Single contract (design-and-build or main contractor):
  You sign one contract with a builder who manages the whole job—foundation tasks, carpentry, electrics, plumbing—subcontracting as necessary, adding perhaps 10–15% markup on those trades. This arrangement offers a unified schedule, minimising the need for you to liaise with multiple trades.
• Multi-trade or split contract:
  You coordinate groundworkers, carpenters, electricians, roofers, plumbers, etc. individually. Although you might save on the contractor’s overhead, you take on project-management demands. If an electrician is late, plasterers or decorators might end up idle, inflating your final cost. This route suits those with free time and strong organisational skills.

For major or architecturally intricate builds in Central London—especially if your property is period or has heritage significance—many opt for a single contractor to sidestep daily scheduling headaches. If you do multi-trade, double-check you can invest the time needed to keep each trade on schedule.

Do I Need Planning Permission?

Whether you need planning permission is influenced by your extension’s size, height, and where it sits on the property. Under Permitted Development Rights, a house can be extended without a formal application, assuming you remain within specified dimensions. But if your design goes past these boundaries, you typically submit for a householder planning permission.

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 Central London, numerous conservation areas and listed streets mean stricter external guidelines. Always confirm with the local council early on whether your plan is valid under PD or if you need to file an application.

Do I Need Planning Permission For A New Storey?

Loft extensions that alter roof lines or add dormers typically need planning permission. Yet building an entirely new storey can be considered permitted development, so long as you meet tight criteria and apply for prior approval with the local authority. Otherwise, you’d go through Householder/Full Planning Permission. If your house matches these criteria, see the bullet points below for crucial restrictions:

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.

Further rules may apply once the build is complete. Always verify these points with planning officials in Central London, where uniform facades and consistent rooflines can be critical in certain districts.

Do I Need Planning Permission For A New Basement?

Building a new basement typically necessitates planning permission, and large-scale excavation nearly always calls for it due to the complexity of the works. It’s best to consult your local authority early, clarifying the conditions. Basement development is also extensive; even without explicit planning approval, you must fulfil Building Regulations regarding the structure, exit routes, ventilation, fire protection, and more.

Check for local rules potentially affecting your property. Occasionally, an extension allowed under PD might be blocked by existing planning constraints or local designations. Investigate local council advice or hire a planning consultant for total clarity.

Central London dwellings sometimes add basements if outward or upward expansion is impractical, but the expense can be considerable, particularly near historically protected terraces or main thoroughfares.

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 commonly identifies design or structural issues pre-build, whereas a Building Notice may be suitable for modest expansions. Either route concludes with building control signing off your completed extension.

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 don’t require a formal planning application under PD rules, you still have to demonstrate that your new extension satisfies building regulations. Older terraced blocks in Central London can pose unique challenges, like ensuring load-bearing walls can handle an extra storey or that party walls remain stable throughout the build.

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.

In Central London, partial DIY might be confined to cosmetic steps, as many older buildings require careful structural or mechanical work that novices shouldn’t attempt. This approach ensures that the main contractor’s schedule isn’t derailed if DIY tasks lag behind.

Do I Need An Architect?

If your extension is relatively simple, a structural engineer or building designer might suffice. Yet for larger or more intricate designs, an architect often adds tremendous value. Skilled architects weigh aesthetics and functionality—room arrangement, natural brightness, external harmony—while integrating the new build with your existing home and outdoor space.

They can also help ascertain whether you remain within permitted development boundaries or need planning permission. Although an architect’s fee raises front-end costs, their expertise usually prevents expensive errors and can yield a better final layout.

Central London sites commonly involve period terraces or distinctive facades, so bridging old and new styles demands creativity. Architects may propose hidden steel supports, rooflights, or glass expansions that marry your property’s historic frontage with a more open interior.

How Long Does It Take To Build An Extension?

Determining a timeframe is key for a controlled build. A smaller single-storey addition—like a modest rear extension—might wrap up in just weeks if the design is uncomplicated, it fits permitted development, and standard materials are used. More substantial expansions, multi-room or multi-storey, could extend from several months to half a year or more, depending on the design’s complexity and the trades required.

At times, you might need to move out temporarily if work disrupts essential walls or systems. Even if you remain, dust, noise, and limited access to some spaces will be part of life.

Always insert a buffer. Bad weather might halt exterior construction, building inspectors may order tweaks, or deliveries can be late. Working with your contractor to form a feasible schedule reduces friction, and completing early is an unexpected bonus. Likewise, a financial contingency is recommended for unforeseen demands or design modifications.

Central London often involves hectic streets with parking limits or Congestion Charges, so skip drop-offs usually happen in non-rush hour windows. If you decide to shift finishes or structural aspects mid-project, that also pushes back your final date.

House Extensions Step By Step

While extending your home can be substantial, with determined planning, qualified pros, and a careful budget, you can accomplish it. Whether you need a bigger social hub, extra bedrooms, or a loft, it’s essential to grasp each stage and anticipate potential hurdles.

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. Address any final flaws before completing payment. Retain building control certificates, warranties, and other documents, which may help if you refinance or sell later.

Once your extension is approved and finished, you’ll enjoy a more accommodating and tailored interior. Whether you added a sunlit rear living area or crafted an extra floor, it generally raises both livability and potential resale worth. Central London homes, set near major cultural and transport nodes, can benefit greatly if expansions respect their historical context while bringing in modern practicality. Carrying out a snagging survey catches small issues—like hairline cracks or uneven finishes—so the contractor can rectify them. Keeping building approvals and warranties proves your build adhered to local rules. With everything finalised, you can relish the expanded living space in Central London, adapted to your daily routine and lifestyle aspirations.

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