How Much Do House Extensions in London Cost?

Throughout London, extension costs vary substantially, determined by how large your build is, the materials you prefer, and the structural demands. Industry sources cite that a single-storey rear extension might start roughly at £1,500 per square metre for a simple concept, whereas typical expansions span £1,800–£2,500 per square metre. Should you choose advanced steel frameworks, high-end glazing, or luxurious finishes, your project can exceed £3,000 per square metre.

In London, labour fees fluctuate, from comparatively lower in certain outer boroughs to extremely high in central areas. If your property lies in an older or listed context, added structural or heritage constraints may raise your overall bill. Large expansions frequently sit in the £40,000 to £100,000+ zone, while smaller rear or side-return projects can be cheaper, and multi-level or premium solutions surpass average cost levels if you incorporate advanced engineering or upscale materials. Introducing a bathroom or kitchen intensifies final spending because of plumbing and wiring tasks.

When evaluating contractor quotes, confirm each covers foundation digging, the structural shell, roofing, fixtures, finishing, labour, overhead, etc. Also clarify if VAT—around 20%—is inside or outside the sum. Though renovation-focused websites and magazines propose general cost references, real precision flows from assembling quotes from multiple reputable companies and dissecting 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.

London

London is a patchwork of distinct boroughs and neighbourhoods, from the historic squares of central Westminster to suburban semis in zones like Ealing or Croydon. Some owners aim to expand ground-floor space for a contemporary open-plan kitchen-dining area, while others opt for side extensions if a side alley is present in their terraced or semi-detached property. Loft conversions are common in pitched-roof homes, especially where outward expansions are limited. Basements appear in prime central districts—like Kensington or Chelsea—but also in some outer borough roads, though they can cost significantly due to water tables or clay soils.

Local contractors often highlight that London traffic congestion—particularly near major roads or central zones—adds logistical challenges. Labour charges can be high, reflecting the city’s busy housing market, with some variance depending on your borough. A well-arranged extension can enhance day-to-day livability and property desirability, particularly near good transport links like the Underground, Overground, or Crossrail. Early and open communication with neighbours is wise, given London’s density and party-wall issues that might arise in terraced or closely spaced housing.

Whether you dream of a single-storey design bridging an outdated kitchen-lounge or a multi-floor project adding upstairs bedrooms, aligning your plan with London’s varied borough-level regulations fosters smoother approvals.

Getting Started

Before you start construction, finalise which extension layout complements your London property. Single-storey rear additions can create a brighter, more inviting kitchen-living area, while a double storey design expands ground-floor and upstairs rooms at once. A side extension can repurpose a side yard. If outward expansions are curtailed, a loft or basement could supply more space—though basements in London require thorough planning and higher costs if you’re near central or historically sensitive zones.

Strong pre-planning avoids confusion mid-way. Outline a detailed budget, referencing typical labour fees, standard materials, plus a contingency for structural complexities or upscale features. If your extension includes notable structural adjustments, engaging an architect or seasoned designer can help. Some building companies provide design-and-build but generally need preliminary sketches or partial planning permissions to quote accurately. Many London homeowners first consult an architect, then gather quotes from contractors once they have rough plans.

It’s also important to lock in a contractor well ahead, as popular teams frequently carry busy schedules. Gathering quotes early allows cost breakdown comparisons, reference checks, and final selection of a builder within your range. An experienced firm might refine your idea—for example, adding rooflights or partitioning an open plan for multi-use spaces.

Project Management

Extensions commonly unfold under one of two management frameworks:

• Single contract (design-and-build or main contractor)
  You sign one agreement with a building enterprise that performs all tasks—foundation prep, structural framing, electrics, plumbing, roofing, finishing—subcontracting trades as needed. They may charge about 10–15% markup. This model merges scheduling, typically easing oversight.
• Multi-trade or split contract
  You independently hire each trade—groundworkers, carpenters, roofers, plumbers, electricians, etc. Although you save the main contractor’s overhead, you accept the project manager role, ensuring trades appear in sequence without delays. If one trade is late, subsequent tasks might cost more or idle. This path suits those with construction knowledge and ample time.

For bigger expansions in London, the single-contractor model is frequently chosen by owners who lack time or experience to juggle trades. If you pick multi-trade, confirm you can handle weekly site checks, deliveries, and scheduling so tasks dovetail properly.

Do I Need Planning Permission?

Whether you need planning permission hinges on your extension’s size, height, and position. Under Permitted Development Rights, you may sometimes enlarge your house without a formal application, if you remain within certain parameters for dimensions. If the design surpasses these, you normally file 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.

Because London comprises many conservation areas and listed buildings—particularly in boroughs like Westminster, Kensington, or Camden—some expansions require a formal planning application even if they appear to fit PD. Checking your local borough’s planning guidelines clarifies your best route.

A New Storey?

Loft conversions changing roof lines or inserting dormers frequently need planning permission. However, you might add a new storey under permitted development if rigorous conditions are satisfied and you file a prior approval with the borough planning office. Otherwise, you proceed via Householder/Full Planning Permission. If your house meets the requirements, the bullet points below define the constraints:

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.

Subsequent conditions may apply upon completion. Within London’s dense terraces or uniform rows, ensuring roofline consistency can be essential.

A New Basement?

Typically, adding a basement needs planning permission, and excavation beneath a property is similarly likely to demand it because of the project’s scale. It’s prudent to consult your local borough’s planning authority from the onset to gauge relevant rules. Basements constitute a large undertaking, so even if no direct planning application is mandated, Building Regulations remain, covering structural stability, damp, ventilation, fire protection, etc.

Check local constraints that might block PD expansions. Certain central boroughs or historically significant areas in London can impose restrictions on basement digs. Consulting your council’s resources or a planning consultant confirms feasibility.

Homeowners in prime, high-value spots—like central Westminster or Kensington—frequently adopt basements when outward expansions aren’t possible, though costs can rise dramatically due to intricate engineering, party-wall considerations, and high-end finishes.

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 flags design or safety flaws early, while a Building Notice can suffice for modest expansions. Both methods culminate in building control approval once the extension meets current code standards.

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.

Even if your London extension falls under PD, building regulations remain a must for secure foundations, correct insulation, damp-proofing, and so on. In older terraces or semis throughout the city, verifying new loads or altered walls remain safe is paramount.

DIY Contribution

Homeowners sometimes consider performing parts of an extension themselves. Painting, decorating, or landscaping can be a worthwhile DIY approach if you have enough knowledge. Doing your own demolition, with caution, might be possible—if you’re sure which walls are load-bearing and you adhere to health and safety.

But tasks such as electrical rewiring, gas connections, or critical structural changes should rest with qualified pros. Errors in these areas are hazardous or expensive to fix. Being realistic about your skills is key; overshooting them can cause hold-ups. Builders could have to wait if your DIY element isn’t finished, driving up total expense. If you plan to do some parts, talk with a professional first to reduce missteps.

In London, partial DIY is most common once the main shell is up, letting owners do cosmetic final touches. Trying advanced structural or mechanical tasks alone can jeopardise official inspection points or degrade build quality.

Do I Need An Architect?

If your extension is small or straightforward, a structural engineer or a design professional might do. For a bigger or more complex build, though, an architect typically offers major advantages. Experienced architects handle functionality, day-to-day flow, and design coherence, ensuring your new area integrates well with the property’s structure.

They can also advise whether your plan is PD-eligible or if you’ll need planning permission. Although an architect’s fee raises initial costs, it commonly saves money by avoiding design flaws that emerge halfway through.

London includes a huge assortment of housing, from Victorian and Georgian terraces in the inner boroughs to 1930s semis in the outskirts, plus modern developments. An architect can ensure your old facade merges gracefully with a contemporary expansion, perhaps using partial glass roofs, big bifold doors, or interior partitions for multi-use spaces.

How Long Does It Take To Build An Extension?

Planning a feasible schedule is crucial. A smaller single-storey extension—if straightforward, standard materials, and PD-permitted—might be done within weeks once actual building commences. Larger expansions, multi-room or multi-storey, regularly last from a few months to half a year or more, depending on how structurally complicated and how many trades you involve.

At times, a temporary move might be necessary if key walls are demolished or major utilities are disrupted. Even if you remain on-site, dust, construction noise, and certain room closures will be part of life.

Always add slack time for delays. Weather can impede outside building, inspectors may require design changes, or supply orders might be late. Fixing a realistic timeline with your contractor mitigates stress, and finishing ahead of plan is a bonus. A financial contingency is likewise encouraged to absorb unforeseen structural issues or mid-project design shifts.

London experiences notorious traffic congestion, especially in central or high-density boroughs, so scheduling skip or supply deliveries outside rush hours frequently works better. Adjusting your finishes or structure after the build starts might also move completion further away.

House Extensions Step By Step

Though an extension is a sizable challenge, with detailed planning, competent professionals, and a realistic budget, it can come to fruition smoothly. Whether you crave a wider living-dining zone, extra upstairs rooms, or a basement addition, it helps to watch each step and avoid typical stumbling blocks.

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. Fix any remaining shortcomings before paying the last invoice. Retain your building control approvals, warranties, and compliance documents for mortgage or resale.

When your extension in London reaches final sign-off, you’ll enjoy a more spacious, functional home. Whether you fashioned a sunlit rear lounge or created an extra storey, the improvement usually boosts everyday comfort and adds property market interest, particularly if you’re near good public transport. A snagging survey spots small faults—hairline cracks, incomplete paint lines—so you can get them remedied. Storing building control certificates and warranties confirms the work was done lawfully, reassuring lenders or prospective buyers. Then you can appreciate your newly extended London home for many years to come.