How Much Do House Extensions in West London Cost?

Building an extension in the UK can involve substantial variability in cost, largely driven by how big the plan is, the materials used, and how complex the structural demands are. Industry figures often place a single-storey rear extension starting around £1,500 per square metre for a basic setup, while many standard expansions settle in the £1,800–£2,500 per square metre range. Should you introduce advanced steel framing, high-end glazing, or premium finishes, totals can surpass £3,000 per square metre.

In West London, which typically spans boroughs like Hammersmith & Fulham, Ealing, Brent, Hounslow, and parts of Kensington & Chelsea, labour rates generally track moderate to high. Areas such as Ealing or Brentford might have somewhat more moderate fees, while prime zones like Kensington can carry higher labour charges. If your property is a Victorian terrace in Shepherd’s Bush or a 1930s semidetached near Ealing Broadway, you might face additional structural improvements—like underpinning, damp proofing, or loft conversion complexities—boosting your ultimate cost. Large multi-room expansions usually land between £40,000 and £100,000+, whereas smaller side-return or single-storey additions stay more affordable. Multilevel or ultra-luxury expansions involving intricate engineering or bespoke interiors can overshoot typical benchmarks, and if you add a bathroom or kitchen, plan for extra plumbing and wiring costs.

When assembling estimates, confirm if each quote includes foundation digging, structural framing, roofing, fittings, finishing, labour overhead, etc. Also clarify if VAT—commonly 20%—is included in the sum. Though you can find general guidance in renovation magazines or home-improvement websites, precise budgeting comes from gathering multiple quotes from dependable local builders and comparing them meticulously 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.

West London

West London includes a broad assortment of neighbourhoods, from the upscale districts of Kensington and Chelsea to the more suburban enclaves of Ealing, Hounslow, and Brentford. You’ll find Victorian terraces in places like Shepherd’s Bush or Chiswick, 1930s semis around Ealing or Perivale, and modern apartments in areas near White City or around Heathrow’s corridor. Single-storey rear expansions often open up cramped kitchens into spacious, open-plan ground floors. Side-return extensions exploit narrow spaces in older terraced houses, while loft conversions remain popular for pitched-roof homes needing extra bedrooms. Basements appear more commonly in high-value zones like parts of Kensington & Chelsea, though they’re not entirely unheard of in other borough pockets near Hammersmith or Fulham.

Local contractors stress that West London roads, particularly around busy thoroughfares like the A4, A40, or near major transport hubs (Hammersmith Broadway, Ealing Broadway), experience peak traffic, so skip deliveries and large material drop-offs typically avoid rush hours. Labour fees fluctuate across the boroughs: labour might cost more in prime districts near Kensington, yet slightly less in suburban enclaves like Ealing or Hounslow. A well-orchestrated extension project can boost day-to-day comfort and add resale value, especially near strong commuter links such as the Piccadilly line, Central line, Overground, or Crossrail stations. Alerting neighbours early, especially in terraced rows or semidetached homes, helps avoid boundary or party-wall disputes.

Whether you’re designing a simple single-storey enlargement for a 1930s semi in Greenford or a sophisticated multi-storey project in an Edwardian terrace near Hammersmith, moulding your proposal to West London’s architectural variety and each borough’s planning environment is a step toward smoother official approvals.

Getting Started

Before you proceed with the actual build, decide which extension format matches your West London property. A single-storey rear design can unify a small kitchen-diner into a large, open-plan space, while a double storey arrangement gives more ground-floor and upstairs floor area, perhaps adding a new bedroom or an upstairs bathroom. A side extension recovers driveway or alley space if your layout allows it, while a loft or basement might be suitable if outward expansions are constrained—though basements are more frequent in upmarket zones near Kensington or Chelsea, and loft conversions show up in many pitched-roof homes around Ealing or Acton.

Comprehensive planning from the onset helps contain mid-project setbacks. Craft a detailed budget, referencing local labour fees, common material costs, and a buffer for high-end finishing or structural complexities. If your plan calls for major structural revisions—like removing significant bearing walls—an architect or building designer can be extremely helpful. Some contractors do design-and-build, but they usually need partial permissions or sketches to quote reliably. Many households in West London will engage an architect first, then collect bids from preferred builders.

It’s also prudent to choose a contractor well ahead of your hoped-for start date, as the better building teams in areas like Hammersmith or Ealing can be booked up. Gathering quotes early allows you to interpret cost breakdowns, call references, and pick a contractor who matches your vision. An experienced builder can also refine your scheme—for instance, locating skylights for maximum daylight or installing pocket doors to segment a large open area.

Project Management and Contract

Extensions tend to follow one of two management approaches:

1. Single contract (design-and-build or main contractor)
   Under one comprehensive contract, a main contractor manages everything—foundation works, structural frames, electrics, plumbing, roofing, and finishing—subcontracting specialist trades. They typically add 10–15% for coordination. This is often simpler, giving you one main contact.
2. Multi-trade or split contract
   You individually hire groundworkers, carpenters, roofers, plumbers, electricians, etc. Although you avoid a main contractor’s markup, you must handle scheduling. If one trade runs behind, subsequent tasks are stalled, likely adding stress or cost. This arrangement suits owners with enough time and at least moderate construction knowledge.

For more ambitious expansions in West London, single-contractor pathways are common, especially for homeowners lacking availability for daily scheduling. If you do multi-trade, ensure you can consistently monitor the site and keep trades aligned in the correct order.

Do I Need Planning Permission?

Whether you require planning permission for your extension is determined by its size, height, and position relative to property boundaries. Under Permitted Development Rights, you might enlarge a house without submitting a planning application if the project stays under certain dimensional thresholds. If it exceeds them, a householder planning permission must be filed.

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 West London includes varied boroughs with possible conservation areas—like parts of Hammersmith & Fulham, Ealing’s heritage streets, or Kensington’s prime enclaves—some expansions that might typically be PD can require special design approval. Checking your borough’s planning department or a planning consultant clarifies if your extension is PD or must be put through a planning application.

Do I Need Planning Permission For A New Storey?

Loft conversions that modify roof shapes or add dormers typically need planning permission. Still, adding a full new storey can be treated as permitted development if you adhere to rigid standards and file a prior approval form. Otherwise, Householder/Full Planning Permission is used. If your property meets the guidelines, see the bullet points below for 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.

Some extra rules can apply after finishing. In many West London neighbourhoods, preserving uniform roof heights or external lines might matter a great deal, especially in sensitive borough pockets near conservation zones.

Do I Need Planning Permission For A New Basement?

Usually, building a basement requires planning permission, and excavation beneath your house likewise is often subject to permission because of the project’s scope. It’s smart to consult your local borough—like Hammersmith & Fulham, Ealing, or Hounslow—early to identify constraints. Basements are among the most expensive expansions, so even if official planning consent isn’t mandated in particular cases, Building Regulations remain crucial for structural safety, damp protection, and ventilation.

Check local guidelines that exceed PD expansions, especially in prime enclaves near Chelsea, Kensington, or near old conservation pockets in Hammersmith. While basements are more frequent in high-end areas of West London, some suburban roads also adopt them if outward expansions are limited. Expect advanced engineering fees if you’re near the river or in areas with high water tables.

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 must submit a building regulations application. Smaller or straightforward builds might use a Building notice, while bigger developments typically need a Full Plans Application, where you present detailed plans for approval prior to construction. Under a Building Notice, an inspector will review the project as it proceeds.

Keep in mind that electrics, heating, plumbing, or window installations may be undertaken by suitably qualified specialists who belong to competent person schemes (for instance, NICEIC for electrics or FENSA for window replacements). By using these schemes, such professionals can self-certify their work, sidestepping separate building control checks for those elements.

A Full Plans Application often reveals design or safety flaws in the planning stages, whereas a Building Notice could suffice for more modest expansions. Ultimately, your borough’s building control must confirm final compliance with relevant regulations.

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

Whereas planning permission determines if the extensions meets local and national planning guidelines, building control ensures that the extension complies with structural and regulatory codes.

Even if your West London extension meets PD or secures planning consent, building regulations remain mandatory—for instance, verifying foundation depth, suitable insulation, or safe wiring. In older period houses around Hammersmith, Ealing, or Brentford, ensuring new loads don’t strain existing walls is crucial for a stable build.

DIY Contribution

Homeowners occasionally wish to handle certain extension tasks themselves. Painting, decorating, or landscaping can be sensible DIY tasks if you have ample experience. Doing your own demolition, in a partial sense, may also be possible—if you identify which walls are load-bearing and conform to safe demolition practices.

However, tasks like rewiring electrics, installing gas lines, or altering main structural components generally require accredited professionals. Mistakes might endanger occupant safety or generate big rework costs. Assess your skills honestly; tackling too much can delay progress. Builders might hold off if your DIY portion lags, increasing final expenses. If you seek partial DIY, talking with an expert first helps minimise pitfalls.

In West London, partial DIY usually emerges once the structural shell is complete—like painting or re-landscaping the garden. Attempting advanced mechanical or structural tasks alone can undermine official checks or degrade build quality.

Do I Need An Architect?

If your extension is modest or straightforward, you might rely on a structural engineer or building designer. Yet for a bigger, more elaborate plan, an architect frequently provides notable value. They balance your property’s style, day-to-day functionality, and the practical aspects of merging new rooms with existing ones.

They can also clarify if you can proceed under PD or must apply for planning permission. Though using an architect raises initial fees, it commonly prevents design miscalculations or structural oversights that surface mid-project.

West London includes a wide variety of housing, from Victorian terraces in Hammersmith or Acton to 1930s semis near Ealing or Hounslow, and prime heritage architecture near Kensington. An architect might suggest partial glass roofs or folding glass panels to fuse classic exteriors with modern open-plan living, reflecting the diversity of the region’s residential architecture.

How Long Does It Take To Build An Extension?

Setting a realistic timeline for your extension is essential to keep things under control. A smaller single-storey rear extension—especially if simple and using standard materials—might be completed in a handful of weeks once groundwork and framing begin, if it’s PD-eligible. Larger expansions, multi-room or multi-floor, typically last from a few months to half a year or more, governed by the complexity of the structure and how many separate trades are required.

Sometimes, a short-term move is necessary if you remove load-bearing walls or if essential utilities must be paused. Even if you stay, you’ll handle dust, noise, and restricted room usage while the build’s underway.

Always pad your schedule for possible holdups. Poor weather can slow external tasks, inspectors might ask for minor layout changes, or supply deliveries can arrive late. Working out a realistic timetable with your contractor reduces stress, and finishing earlier than planned is a bonus. Similarly, a budget buffer helps if unexpected structural demands or finishing tweaks surface mid-project.

West London sees heavy traffic on major routes—like the A40, A4, or near major roundabouts in Brentford—so skip deliveries typically slot into midmorning windows. Changing design elements or finishes once building commences may shift your final completion date further out.

House Extensions Step By Step

Though planning an extension is large in scope, with thorough preparation, reliable trades, and a carefully structured budget, it’s feasible. Whether you want a bigger family living area, extra upper-floor space, or a basement, consider each phase and guard against usual 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. Rectify any outstanding details before releasing final payment. Keep building control certificates, warranties, and official documentation for future mortgage or sale.

Once your West London extension is finished and passes its final checks, you’ll enjoy a more expansive and comfortable living environment. Whether you unlocked a cramped ground floor or added a new level, the change generally enhances daily life and property value—particularly near handy transport lines or in sought-after pockets of Ealing, Hammersmith, or Kensington. A snagging survey ensures minor cracks, paint defects, or uneven finishes are remedied. Holding onto building control approvals and warranties confirms compliance with rules, easing potential buyers’ or lenders’ concerns if you sell or refinance later. Ultimately, your newly expanded West London home can serve your evolving household for years to come.