How Much Do House Extensions in Kensington Cost?

Costs to extend a house can vary hugely in the UK, rooted in factors like size, materials, and how complex your structural plan is. Industry estimates frequently put a single-storey rear extension starting at about £1,500 per square metre for a basic format, while many standard expansions fit the £1,800–£2,500 per square metre range. Should you incorporate high-end steel beams, bespoke glazing, or luxury interior finishes, costs can exceed £3,000 per square metre.

In Kensington, officially part of the Royal Borough of Kensington and Chelsea, labour fees tend to be among London’s highest, reflecting prime central location, high property values, and substantial demand for skilled trades. If your home is a grand Georgian terrace near Kensington Palace or a Victorian property in South Kensington, you may face added structural tasks, damp-proofing challenges, or strict heritage obligations, inflating your final costs. Larger or more elaborate builds commonly run from £40,000 to £100,000+, while smaller single-storey or side-return expansions might stay cheaper. Multi-room or multi-floor designs that demand intricate engineering or ultra-upmarket finishes readily surpass typical budgets. Integrating a new or relocated kitchen or bathroom likewise raises expenses for plumbing, wiring, and waterproofing tasks.

When collecting multiple bids from builders, confirm each includes foundation tasks, any structural framing, roofing, fittings, finishing, labour overhead, etc. Also clarify if VAT—commonly 20%—is included or extra. While renovation journals and home-improvement outlets can hint at approximate sums, true precision requires multiple quotes from reputable building firms, each dissected 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.

Kensington

Kensington—one of London’s most affluent districts—lies in the borough of Kensington and Chelsea, hosting grand period residences, stucco-fronted terraces, and modern luxury developments near prime locations like Kensington High Street, Gloucester Road, and the Royal Albert Hall. Many homeowners in Kensington undertake single-storey expansions to fashion bigger kitchen-diners or unify smaller ground-floor rooms, while side-return extensions can exploit narrow external strips in older terraces. Loft conversions appear in some pitched-roof properties, although numerous homes in the area are listed or sit in conservation zones, curtailing typical PD freedoms. Basements are more widespread in high-value streets close to Kensington Gardens or around South Kensington, allowing subterranean gyms, cinema rooms, or extra living quarters.

Local contractors note that Kensington sees heavy traffic near major intersections around the A4, Kensington High Street, and Cromwell Road, so skip drop-offs typically dodge early morning or early evening. Labour charges are among the highest in London, mirroring the prestige and wealth of the borough. A skillfully executed extension can broaden day-to-day comfort, add substantial property resale value, and maintain the area’s distinct architectural appeal, particularly near transport lines like the Circle, District, and Piccadilly lines at Gloucester Road, High Street Kensington, or South Kensington stations. Consulting neighbours—especially in a terrace or if you share a party wall—helps mitigate disputes regarding noise or potential structural changes.

Whether you’re planning a straightforward single-storey design for a Victorian terrace near Earl’s Court or an extensive multi-level scheme near Kensington Gardens, shaping your approach to Kensington’s heritage character and stringent planning standards is vital for achieving a smooth approval process.

Getting Started

Before constructing, identify the best extension approach for your Kensington property. A single-storey rear plan typically merges older ground-floor rooms into an open-plan format, whereas a double storey enlargement grants both ground and upstairs expansions, often yielding new bedrooms or en-suites. A side extension reclaims leftover side land, while a loft or basement may be relevant if outward expansions are constrained—though basements in Kensington are more common in top-tier streets if outward expansions are unfeasible, while loft conversions remain somewhat tricky for listed or conservation properties but still feasible in certain unlisted terraces.

Formulating a detailed budget is crucial, factoring local labour rates, standard materials, plus a margin for hidden structural tasks or higher-grade finishes. If your concept involves major structural revisions—like removing a load-bearing wall in a listed building—employing an architect or building designer is strongly advised. Some contractors do design-and-build, but they typically require partial drawings or permissions for precise quoting. Many Kensington homeowners start by consulting an architect, then gathering estimates from a limited group of high-calibre builders.

It’s also wise to finalise your contractor choice well before your anticipated start date, as top-end firms in Kensington can be fully booked, particularly around times of heightened building activity or seasonal surges. Collecting quotes early ensures thorough cost comparisons, detailed reference checks, and alignment on a builder whose work and style suit your aims. A competent builder may refine your scheme—for example, adding large glass panels for daylight or introducing a hidden utility area behind your new open-plan kitchen-lounge.

Project Management

Extensions typically follow one of two principal organisational setups:

1. Single contract (design-and-build or main contractor)
   Under one overarching agreement, a building firm undertakes all tasks—foundation works, structural frames, electrics, plumbing, roofing, finishing—subcontracting specific trades. They charge 10–15% for coordination. This route unifies scheduling and gives you a single contact if issues arise.
2. Multi-trade or split contract
   You personally hire each trade—groundworkers, carpenters, roofers, electricians, plumbers, etc. Although you bypass the main contractor’s markup, you must handle each scheduling step. If one trade’s job runs behind, subsequent tasks might stall, raising costs or tension. This approach fits owners with time and a decent building knowledge.

For more complex expansions in Kensington, single-contractor pathways are commonplace, especially among owners who can’t manage daily scheduling. If you pick multi-trade, confirm you can dedicate the necessary hours each week to oversee progress, handle deliveries, and coordinate trades.

Do I Need Planning Permission?

Whether you need planning permission hinges on how large and tall your extension is and how close it is to property edges. Under Permitted Development Rights, you may extend a house without an official planning application if you abide by dimension constraints. If not, you lodge 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.

But Kensington is inundated with conservation areas—like Kensington Palace and Courtfield, plus many listed buildings. Such realities frequently override or limit PD expansions. Consulting the Royal Borough of Kensington and Chelsea’s planning department or a planning consultant clarifies if your build is PD or demands a formal planning application.

Do I Need Planning Permission For A New Storey?

Loft conversions changing roof outlines or introducing dormers usually need planning permission. Yet adding a new storey might qualify under permitted development if you meet strict criteria and file a prior approval. Otherwise, Householder/Full Planning Permission applies. If your home aligns with these conditions, the bullet points below detail 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.

Additional stipulations often arise, particularly if you’re near listed properties in Kensington or within a terrace with uniform rooflines.

Do I Need Planning Permission For A New Basement?

A new basement typically needs planning permission, and large-scale excavation under your dwelling likewise is likely to trigger it due to the project’s magnitude. Engaging the borough council early is vital if you live near prime heritage areas or in roads with clay soil or potential flood risk. Basements are among the priciest expansions, so even if formal planning isn’t always mandatory, you must still satisfy Building Regulations for structural stability, damp proofing, ventilation, and so on.

Some premium roads in Kensington—like those near Kensington Palace or South Kensington—embrace basements for extra living quarters, yet advanced engineering fees and party-wall negotiations can escalate costs.

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.

A building regulations application is necessary. Smaller or simpler builds might use a Building notice, whereas more extensive expansions commonly use a Full Plans Application, involving detailed plans for pre-approval. Under a Building Notice, an inspector validates the work on site as it unfolds.

Note that tasks such as installing new electrics, windows, water pipes, or heating systems can be managed by trades registered in competent person schemes (e.g. NICEIC for electrics, FENSA for window work), allowing them to self-certify compliance without separate building control checks.

A Full Plans Application typically detects any design or safety flaws before building starts, whereas a Building Notice can suit simpler expansions. In every scenario, building control must grant final clearance that your extension meets code.

What’s The Difference Between Planning Permission And Building Regulations Application?

Whereas planning permission looks at whether the extensions meets local and national planning standards, building control ensures structural safety, code adherence, and overall regulatory compliance.

Even if your Kensington extension passes PD or obtains official permission, you must still align with building regulations—for instance, guaranteeing robust foundations, safe electrics, and appropriate insulation. In older Georgian, Victorian, or stucco-fronted homes across Kensington, verifying that your new structure won’t compromise existing walls is crucial.

DIY Contribution

Homeowners sometimes want to do certain aspects of an extension themselves. Painting, interior decorating, or landscaping might be feasible DIY tasks if you have relevant skills. Doing your own demolition, partially, may also be possible—if you confirm which walls are load-bearing and adhere to safety protocols.

However, tasks involving gas lines, electrical rewiring, or major structural changes typically need qualified professionals. Mistakes can endanger occupant safety or incur costly fixes. Evaluate your abilities honestly; overcommitting might slow the entire project. Builders could have to wait if your DIY piece runs behind, inflating total expenses. If partial DIY is appealing, get a professional’s input early to avoid large pitfalls.

In Kensington, partial DIY normally appears after the main structural build is done—like painting interiors or refining gardens. Attempting complicated mechanical or structural tasks personally risks failing official checks or diluting final build quality.

Do I Need An Architect?

If your extension is modest or has a relatively simple plan, you might manage with a structural engineer or building designer. But for larger, more intricate projects, an architect often offers substantial advantages. Capable architects incorporate space efficiency, design cohesion, and synergy with any heritage features your home has.

They can also advise if your proposal qualifies for PD or if planning permission is likely. Though an architect’s fee can raise initial outlays, it frequently minimises bigger mistakes that arise if a plan isn’t well thought out.

Kensington includes high-value neighbourhoods around Kensington High Street, Gloucester Road, and Earls Court, featuring period townhouses and stucco terraces. An architect could suggest partial glass roofs or folding glass walls to unify a period facade with modern open-plan living, resonating with the borough’s balance of heritage and contemporary design demands.

How Long Does It Take To Build An Extension?

Enforcing a pragmatic timeline is vital for a controlled build. A smaller single-storey addition—especially if it’s simple, uses standard materials, and meets PD—might conclude in a few weeks once ground and frame work begin. Larger expansions, multi-floor or multi-room, commonly run several months to half a year or longer, contingent on how extensive the structural changes are and how many distinct trades must coordinate.

Sometimes, a brief relocation might be required if you remove key walls or sever core utilities. Even if you stay, you’ll face noise, dust, and restricted room access for some period.

It’s smart to incorporate a buffer for unforeseen hitches. Unfavourable weather might delay exterior tasks, inspectors may ask for small plan modifications, or supply deliveries can be late. Collaborating with your contractor on a realistic schedule helps limit stress, and finishing faster than expected is always a pleasant outcome. A budget contingency is also recommended if hidden structural demands or finishing changes emerge mid-project.

Kensington roads near main arteries like the A4 (Cromwell Road), Kensington High Street, or Earl’s Court can encounter heavy traffic, so midmorning skip drop-offs usually bypass rush hour. Changing finishes or structural elements after the build kicks off can push your final completion date out further.

House Extensions Step By Step

While an extension is large in scope, with thorough planning, dependable trades, and a methodical budget, you can accomplish it. Whether you’re seeking a bigger kitchen-dining zone, a new upper floor, or a basement, following each phase and spotting typical pitfalls is key.

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. Rectify any minor or cosmetic issues before final payment. Keep building control certificates, warranties, and other compliance documents for mortgage or potential sale in the future.

Once your Kensington extension is finalised and approved by official inspectors, you’ll have a more expansive, functional home. Whether you constructed a bright new open-plan living area or added another floor for bedrooms or an office, the changes frequently improve both comfort and property value—especially near popular thoroughfares or iconic spots like Kensington Gardens. A snagging survey helps iron out finishing defects, such as faint cracks or uneven paint. Keeping building control sign-offs and warranties shows you adhered to regulations, reassuring prospective buyers or lenders. Ultimately, your newly extended Kensington property will suit your evolving lifestyle for years to come.