How Much Do House Extensions in Merton Cost?

In the United Kingdom, the cost of extending a house can vary enormously based on the design’s scope, chosen materials, and the overall structural complexity. Industry data indicates that a single-storey rear extension might begin at £1,500 per square metre if it’s an uncomplicated concept, while many standard expansions end up in the £1,800–£2,500 per square metre zone. If you incorporate advanced steel frameworks, bespoke glazing solutions, or high-end interior finishes, totals can rise beyond £3,000 per square metre.

In Merton, a south-west London borough known for areas like Wimbledon, Mitcham, and Morden, labour rates typically hover around moderate to high, influenced by strong local demand and easy connections into central London. If your property is an older Victorian or Edwardian house near Wimbledon town or a 1930s semidetached in Morden, it may need additional structural measures—like underpinning or reinforced beams—to accommodate the new extension. Larger or more elaborate projects tend to occupy the £40,000–£100,000+ bracket, while smaller side-return or single-storey expansions might stay cheaper. Multi-floor or ultra-luxury designs can surpass typical cost levels if advanced engineering or premium finishes play a role, and if you add a kitchen or bathroom, expect extra plumbing, wiring, and waterproofing costs.

When requesting quotes from builders, ensure each itemises foundation work, the structural assembly, roofing, fittings, finishing, labour, overhead, and so forth. Also ask whether VAT—commonly 20%—is included in the estimate or separate. While home-improvement publications and renovation websites can give approximate figures, genuine accuracy demands that you gather multiple quotes from trustworthy builders and compare 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.

Merton

Merton is a suburban and diverse borough, incorporating areas like Wimbledon, Colliers Wood, Mitcham, and Morden. Many local homeowners choose single-storey rear expansions to convert smaller kitchens into large, open-plan living spaces. Others add side-return extensions if there’s a gap beside their terraced or semidetached home—this is often seen in older Wimbledon properties. Loft conversions are also common, particularly where pitched roofs allow you to create extra bedrooms or a home office. Occasionally, basements appear in premium roads near Wimbledon Village, although these can be complex due to underground conditions.

Local contractors note that Merton roads, especially around Wimbledon town centre or major transport routes like the A24, can be busy, so skip delivery times should dodge peak traffic. Labour costs typically rate moderate to high, reflecting the proximity to central London and Merton’s appeal as a family-friendly locale. A thoughtfully orchestrated extension can significantly enhance comfort and property value, especially near strong transport links like Wimbledon station or Colliers Wood tube. Alerting neighbours early—particularly if you share fences or walls—can avoid boundary or party-wall quarrels.

Whether you’re aiming to add a single-storey design for a larger kitchen-diner or a more expansive multi-storey arrangement, framing your scheme in line with Merton’s local environment and planning preferences is often vital for faster approvals.

Getting Started

Prior to construction, decide which type of extension best suits your Merton property. A single-storey rear addition might connect an existing kitchen and dining room into a unified open area, while a double storey expansion can enlarge both ground-floor living space and upper-level bedrooms. A side extension reclaims a side alley or driveway, and if outward expansions prove tricky, you might consider a loft or basement, though basements in Merton remain less frequent unless you’re in more expensive roads around Wimbledon Village or Raynes Park.

A robust plan from the outset helps ward off confusion mid-project. Form a detailed budget referencing common trade fees and material expenses, leaving a contingency for unexpected complexities or luxury finishes. If your design calls for large structural adjustments, hiring an architect or experienced designer can be worthwhile. Some builders provide design-and-build services but typically require partial sketches or planning approval to quote accurately. Many Merton homeowners secure an architect first, then approach a curated list of contractors with the preliminary plans.

It’s wise to arrange a contractor before you want to start, as leading teams fill their calendars early. Gathering quotes well in advance allows you to evaluate cost line by line, verify references, and decide on a builder you trust. An experienced contractor can also refine your concept—for instance, suggesting an extra skylight for natural lighting or including a discreet utility cupboard in your open layout.

Project Management

Extensions usually follow one of two main organisational methods:

• Single contract (design-and-build or main contractor)
  You sign one overarching agreement with a building company that handles everything—foundation works, structural frames, electrics, plumbing, roofing, and finishing. They subcontract certain trades, usually adding a 10–15% fee for coordination. This centralises scheduling and oversight, giving you one primary point of contact.
• Multi-trade or split contract
  You yourself hire each individual trade—groundworkers, carpenters, roofers, electricians, plumbers, etc. While you eliminate the main contractor’s markup, you must meticulously arrange each stage. If one trade runs behind, the subsequent tasks can stall, risking cost or timeline overruns. This route suits owners with sufficient time and a degree of construction knowledge.

For bigger or design-heavy expansions in Merton, many choose a main-contractor approach for convenience. If you do multi-trade, confirm you can devote consistent hours to coordinate deliveries, review progress, and ensure tasks proceed in the right order.

Do I Need Planning Permission?

Whether you need planning permission is determined by your extension’s scale, height, and location on the property. Under Permitted Development Rights, you might sometimes extend without lodging a formal planning application, as long as your proposal remains within strict dimensional constraints. If you exceed these, you typically file a householder planning permission request.

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.

Certain areas of Merton—particularly parts of Wimbledon, Morden, or Mitcham—might also feature conservation or special designations that require stricter checks. Early contact with Merton Council’s planning department clarifies if your build meets PD conditions or needs formal consent.

Do I Need Planning Permission For A New Storey?

Loft extensions that modify roof shapes or insert dormers commonly require planning permission. However, you might add a new storey under permitted development if you meet specific criteria and submit a prior approval request to local authorities. Otherwise, a Householder/Full Planning Permission path is needed. If your house meets these rules, the bullet points below cover the relevant 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 guidelines could apply after completion. Particularly, Merton’s suburban roads often have consistent rooflines, so ensuring the new storey melds with neighbouring heights is essential.

Do I Need Planning Permission For A New Basement?

A new basement often requires planning permission, and major excavation below ground is likewise prone to requiring it because of the project’s significant scope. It’s wise to speak with the local planning authority in Merton early on to grasp if any site-specific constraints exist. Basements rank among the bigger undertakings; thus, even if no official planning consent is triggered, Building Regulations remain mandatory to address structure, ventilation, damp, fire safety, and more.

Check whether local rules override standard PD expansions. In more upscale pockets of Merton, such as around Wimbledon Village, basement expansions do appear, but you might need thorough ground surveys, especially where slopes or older foundations exist.

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.

Note that 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.

A Full Plans Application usually catches design or safety issues early, whereas a Building Notice can suffice for simpler expansions. Either way, final sign-off by building control verifies that everything meets code standards.

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 Merton extension qualifies under PD, you must still ensure building regulations compliance for sturdy foundations, correct insulation, damp-proofing, etc. With older or historically interesting houses near Mitcham or Wimbledon, verifying your structural alterations remain safe is crucial.

DIY Contribution

Homeowners occasionally consider handling parts of an extension themselves. Painting, decorating, or landscaping can be reasonable DIY tasks if you possess the time and know-how. Doing your own demolition, to an extent, might also be possible—if you confirm which walls are load-bearing and follow safety protocols.

However, tasks involving electric wiring, gas pipes, or major structural changes almost always require accredited professionals. Errors can jeopardise safety or cost a lot to rectify. Assess your capabilities honestly; overcommitting might disrupt schedules. Builders may have to pause if your portion isn’t finished, hiking final costs. If you do plan a DIY portion, consult an expert first to steer clear of serious oversights.

In Merton, partial DIY tends to focus on interior décor or garden finishing once the contractor finalises main shell and mechanical tasks. Attempting complicated structural or mechanical roles alone can hamper official sign-offs or degrade the project’s quality.

Do I Need An Architect?

If your planned extension is fairly modest, you could rely on a structural engineer or building designer. But for a bigger or more elaborate design, an architect usually brings notable advantages. Skilled architects meld aesthetic harmony, functional layouts, and sensible transitions between the new and existing areas.

They can also clarify if your extension fits PD or if planning consent is likely. Although employing an architect pushes up initial spending, it often prevents mid-project design mishaps that might escalate costs.

Merton hosts a variety of architectural styles—from Edwardian terraces near Colliers Wood to modern housing estates in Morden, plus Victorian semis in Wimbledon. An architect might propose transitional features that unify an older facade with a sleek modern interior, perhaps introducing large folding glass doors or partial glass roofs for greater daylight.

How Long Does It Take To Build An Extension?

Arranging a sensible timetable is essential to remain in command. A smaller single-storey rear build—especially if straightforward, standard materials, and PD-approved—could conclude in a matter of weeks once on-site work begins. Larger expansions, multi-room or multi-storey, typically range from several months up to half a year or longer, reflecting structural complexity and how many trades are needed.

Occasionally, you may decide to move out temporarily if key walls are removed or main utilities shut down. Even if you stay, expect noise, dust, and certain rooms blocked off.

Always insert a margin for potential delays. Inclement weather can slow exterior tasks; inspectors may request minor alterations; or deliveries can get stuck. Setting a realistic timeframe with your contractor minimises stress, and if the job ends earlier than anticipated, it’s a pleasant bonus. Similarly, a financial contingency helps if hidden structural tasks or finishing enhancements arise mid-schedule.

Merton roads near Wimbledon town centre, Colliers Wood, or the A24 corridor often experience traffic, so skip and supply deliveries typically aim for quieter midmorning windows. Shifting design or finishes after building starts can also extend completion.

House Extensions Step By Step

Though an extension is substantial, with adequate planning, trustworthy experts, and an organised budget, it can proceed effectively. Whether you want an enlarged kitchen-living area, additional upstairs bedrooms, or a basement, ensure each phase and typical hurdle is addressed.

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. Sort out any outstanding faults before final payment. Retain building control approvals, warranties, and official records for future property transactions or remortgaging.

After your Merton extension wraps and is signed off, you can delight in a bigger, more practical home environment. Whether you formed a bright open-plan zone or added more upstairs living space, the outcome typically boosts comfort and prospective market interest, particularly near key transport nodes or green areas like Wimbledon Common. A snagging survey identifies finishing imperfections—small cracks, uneven paint lines—so your builder can remedy them. Storing building control certificates and warranties confirms your project followed regulations, soothing any doubts from potential buyers or lenders. With that, you can enjoy your newly expanded Merton house for years to come.