How Much Do House Extensions Cost?

Extending a house in the United Kingdom involves an array of potential costs, shaped by your desired design scale, the materials selected, and the complexity of the structural tasks. Industry data indicates a single-storey rear extension can start around £1,500 per square metre for a straightforward, no-frills layout, while many standard expansions end up in the £1,800–£2,500 per square metre bracket. If you incorporate sophisticated steel framing, custom glazing, or top-tier internal finishes, totals may surpass £3,000 per square metre.

In Westminster, one of London’s central boroughs, labour rates tend to skew toward the higher end, reflecting the prime location, substantial heritage, and strong housing demand. If you own a Georgian townhouse in Marylebone, a Victorian terrace in Pimlico, or a modern flat near the river, additional structural demands—like damp control, foundation reinforcement, or major interior reconfiguration—might significantly elevate final costs. Larger or more involved expansion projects typically range from £40,000 to £100,000+, whereas smaller side-return or single-storey builds might be less. Luxurious multi-room or multi-floor additions easily exceed typical cost thresholds if advanced engineering or lavish finishes are sought. Also note that adding or relocating a kitchen or bathroom increases expenditures due to plumbing, wiring, and waterproofing tasks.

When requesting quotes from local contractors, make sure each covers foundation excavation, structural frameworks, roofing, interior fittings, finishing, labour overhead, and so on. Additionally, confirm whether VAT—commonly 20%—is included in each total. Though home-improvement sites and renovation magazines supply rough estimates, accurate sums demand multiple quotes from reputable builders, compared line by line for clarity and thoroughness.

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.

Westminster

Westminster is at the very heart of central London, spanning prestigious areas like St James’s, Marylebone, Pimlico, Victoria, and parts of Regent’s Park. You’ll also find iconic landmarks like the Houses of Parliament and Buckingham Palace within the borough. Residential properties here range from elegant Georgian or Victorian terraced houses near Marylebone to modern apartments overlooking the Thames.

Single-storey rear expansions can transform older ground-floor rooms into open-plan living spaces, while loft conversions might appear in pitched-roof properties around Westminster’s conservation streets. However, large-scale basement digs, though popular in some ultra-prime roads, often face intense scrutiny due to local heritage, potential water table issues, and concern about disturbing neighbouring houses.

Local contractors stress that Westminster roads—particularly near busy zones like Victoria Station or Oxford Street—see continual congestion, so scheduling skip or materials deliveries often avoids commuter peaks. Labour costs in Westminster reflect central London’s premium property market.

Still, a well-executed extension can dramatically improve your day-to-day living areas while heightening property resale prospects, especially if you’re close to major tube lines or popular local spots like Hyde Park or Marylebone High Street. If you share boundaries or party walls, open discussion with neighbours in older terraced streets or mansion blocks helps head off tension over building noise or structural changes.

Whether you’re organising a simple single-storey expansion for a modest Edwardian terrace near Pimlico or planning a more ambitious multi-level overhaul in Marylebone, adapting your design to Westminster’s heritage character and local planning rules is critical for smooth approvals.

Getting Started

Before beginning physical work, decide which extension layout suits your Westminster property. A single-storey rear addition often merges a cramped kitchen-dining zone into a modern open-plan arrangement. A double storey version yields both ground-floor and upstairs improvements—possibly adding a new bedroom or en-suite.

A side extension reclaims narrow leftover land if you have it. If outward expansion isn’t possible, a loft or basement might be an option—though basements, as mentioned, are typically more complex in Westminster due to high land value and strict regulations. Loft conversions remain feasible in pitched-roof houses, but many listed buildings or properties in conservation areas face extra constraints.

Comprehensive planning at the outset keeps mid-project disruptions low. Draw up a precise budget, referencing standard labour rates, average material expenses, and a contingency to handle structural uncertainties or custom finish requests. If your plan involves large-scale structural alterations—like removing key load-bearing walls—an architect or professional designer is commonly recommended.

Some building companies deliver design-and-build services but typically want initial sketches or partial planning consents for accurate quotes. Numerous Westminster homeowners begin with an architect, then approach contractors for quotations once they have a preliminary design.

It’s also prudent to finalise your builder in good time, as skilled teams around high-demand areas—like Marylebone, Mayfair, or Victoria—can be booked months ahead. Gathering quotes early helps you compare cost breakdowns thoroughly, confirm references, and choose a builder aligned with your requirements.

A knowledgeable builder might refine your layout—for instance, factoring in a discreet utility corner or suggesting additional rooflights for a bright, airy feel in your extension.

Project Management

You typically organise an extension via one of two frameworks:

1. Single contract (design-and-build or main contractor)
   You sign a single agreement with a building firm that handles or coordinates all tasks—foundation digging, framing, electrics, plumbing, roofing, finishing. They subcontract trades, normally applying a 10–15% fee for coordination. This approach simplifies scheduling, offering a single contact for queries.
2. Multi-trade or split contract
   You personally hire groundworkers, carpenters, roofers, electricians, plumbers, decorators, etc. Though you skip a main contractor’s markup, you also handle day-to-day scheduling. If one trade runs late, subsequent tasks may wait, likely causing cost escalation or frustration. This method suits owners who can devote the time and have enough building know-how.

For larger expansions in Westminster, single-contractor pathways are popular, particularly among those who can’t manage multiple trades themselves. If you do prefer multi-trade, ensure you can regularly visit the site, schedule deliveries, and keep each trade’s timeline aligned.

Do I Need Planning Permission?

Whether you need planning permission stems from your extension’s size, height, and its position relative to the original house and boundaries. Under Permitted Development (PD) Rights, some home extensions can bypass formal planning if they remain within strict dimension thresholds. If you exceed those, you must submit a householder planning permission application.

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.

However, Westminster is home to numerous conservation areas (like areas near Regent’s Park, Belgravia—even though part of Belgravia is also in Kensington & Chelsea—and St James’s), as well as many listed buildings. This reality may override typical PD allowances or add constraints. Reviewing Westminster City Council’s planning rules or consulting a planning adviser clarifies if your expansion is PD-friendly or requires a formal application.

Do I Need Planning Permission For A New Storey?

Loft conversions that adjust the roofline or add dormers typically need planning permission. Yet if you fulfil specific requirements and file for prior approval, adding a new storey might be allowed under permitted development. Otherwise, Householder/Full Planning Permission is the path. If your property meets the conditions, below are the bullet points listing essential 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.

Further stipulations might apply post-build. In older Westminster streets, matching local roof shapes or heights often matters significantly, so meticulous planning is recommended if you’re thinking of an extra storey.

Do I Need Planning Permission For A New Basement?

A new basement generally demands planning permission, and excavating below ground typically likewise triggers permission given the extensive scope. It’s wise to speak with Westminster City Council’s planning officials early, since many roads host listed buildings or stand in highly sensitive heritage areas. Basements can be among the costliest expansions, so even if a particular scenario might not always prompt official planning scrutiny, Building Regulations remain mandatory for ensuring structural safety, damp protection, and escape routes.

Check local regulations surpassing PD expansions—some premium roads in Belgravia or Marylebone might have strong local opposition to basement excavations. While basements are more prevalent in wealthier sections of Westminster, you’ll still face advanced engineering fees and potential party-wall issues if outward expansion is not feasible.

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 file a building regulations application. Smaller or straightforward expansions could rely on a Building notice, whereas complex or larger builds typically require a Full Plans Application, which involves submitting detailed plans for pre-approval. A Building Notice means an inspector checks on compliance as work proceeds.

Keep in mind that tasks like installing new electrics, windows, plumbing, or heating equipment may be carried out by qualified specialists under competent person schemes (for instance, NICEIC for electrics or FENSA for windows). Professionals registered with these schemes can self-certify their work, instead of scheduling individual building control visits for those items.

A Full Plans Application often highlights design or safety flaws before construction starts, while a Building Notice might suffice for smaller or simpler expansions. Ultimately, building control must confirm the final extension meets code standards for safety and function.

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

Whereas planning permission looks at whether the extensions meets local and national planning policies, building control focuses on structural soundness and compliance with building regulations.

Even if your extension in Westminster is valid under PD or obtains planning consent, you must still show building regulations compliance—for example, that your foundations are solid, your insulation meets current standards, or your wiring is safe.

DIY Contribution

Homeowners sometimes consider taking on parts of an extension themselves. Painting, decorating, or landscaping can be reasonable DIY endeavours if you have the requisite skill and time. Doing your own demolition, in part, may also be feasible—if you identify structural walls and observe safety measures thoroughly.

But installing gas lines, rewiring electrics, or changing major structural elements nearly always calls for certified professionals. Errors may be hazardous or cost a fortune to rectify. Realistically gauge your capabilities; overcommitting can derail timelines. Builders might halt if your DIY tasks aren’t completed, inflating final expenses. If partial DIY appeals, consult an expert early to sidestep severe oversights.

In Westminster, partial DIY typically arises once the major shell is done—like painting or reconfiguring small outdoor spaces. Doing advanced structural or mechanical tasks alone jeopardises official checks and can degrade the ultimate build quality.

Do I Need An Architect?

If your extension is straightforward, you might manage with a structural engineer or building designer. However, for a more complex or substantial layout, an architect usually adds substantial value. Skilled architects balance practicality, aesthetic quality, and seamless transitions between the old property and new addition.

They can also clarify whether your plan falls under PD or if planning permission is mandatory. While employing an architect raises upfront spending, it typically prevents design flaws or structural conflicts that balloon mid-project costs.

Westminster spans historically famed locales: Marylebone, Pimlico, Mayfair, and more. An architect might propose partial glass roofs or imaginative internal layouts to unify a period terrace facade with a modern extension, reflecting the borough’s mix of heritage and contemporary living.

How Long Does It Take To Build An Extension?

Laying out a practical timeline is crucial. A smaller single-storey rear extension—especially if it’s a basic design, standard materials, and PD-friendly—could complete in several weeks once groundwork begins. Bigger expansions, multi-room or multi-level, can extend from a few months to half a year or longer, contingent on the structural engineering needed and the variety of trades involved.

Sometimes, homeowners consider short-term relocation if essential supporting walls are dismantled or if key utilities are cut. Even if you stay, be prepared for noise, dust, and limited access to certain rooms during the build.

It’s sensible to plan for delays. Inclement weather could slow exterior tasks; inspectors may demand small design tweaks; deliveries can run late. Cooperating with your builder to form a realistic schedule eases stress, and completing early is always pleasant. Similarly, keep a financial contingency for hidden structural demands or finishing upgrades discovered mid-project.

Westminster roads near Victoria Station, Oxford Street, or Marylebone Road can face constant congestion, so skip drop-offs usually avoid rush hours. Changing finishes or layout details mid-construction can also postpone your final completion date.

Step By Step

Though an extension is a large-scale undertaking, with careful planning, capable trades, and a well-defined budget, you can carry it out successfully. Whether you aim to expand for a spacious family lounge, a new storey, or a basement, it pays to consider each phase and watch for frequent challenges.

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 lingering flaws before final payment. File away building control certificates, warranties, and associated compliance documents in case you sell or remortgage your home.

Once your Westminster extension is completed and passes all necessary checks, you’ll have a more spacious and functional property. Whether you formed a bright open-plan living zone or constructed an additional level for bedrooms, this investment generally enhances comfort and property value—particularly in a borough famous for its central location and distinctive London charm. A snagging survey spots tiny cracks, paint alignment, or uneven surfaces, so your builder can correct them. Storing building control approvals and warranties confirms adherence to regulations, reassuring potential buyers or lenders if you choose to move on. Ultimately, your newly enlarged Westminster home can accommodate your lifestyle for years to come.