How Much Do House Extensions Cost?

In the UK, extension costs vary widely, shaped by your design’s size, the materials you pick, and overall build complexity. Industry sources suggest a single-storey rear extension often starts near £1,500 per square metre if it’s a basic approach, while more typical expansions land in the £1,800–£2,500 per square metre range. Should you opt for structural steel solutions, bespoke glazing, or premium finishes, costs might exceed £3,000 per square metre.

In Harrow, which lies in northwest London, labour rates tend to be moderate to high, as the borough is well-connected and property values have risen substantially in recent years. If your house is a classic 1930s semi or an Edwardian terrace near Harrow on the Hill, you might face extra structural strengthening or building constraints that increase your total price. Larger expansions can often be in the £40,000–£100,000+ zone, whereas slimmer side-return builds can be cheaper, and elaborate multi-level or luxury designs may overshoot typical brackets if advanced engineering or high-spec interiors are needed. Adding a new kitchen or bathroom always pushes costs higher due to extra plumbing and electrics.

When examining quotes, verify each includes foundation tasks, the structural framework, roofing, fittings, finishing, plus labour overhead. Check whether VAT—commonly 20%—is part of the total or separate. While home-improvement sites or renovation publications give approximate cost references, your most dependable figure arises from acquiring multiple quotes from reliable builders and comparing them item by item.

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.

Harrow

Harrow contains a mix of suburban semis from the 1930s, modern developments, and historic enclaves around Harrow on the Hill, once home to Harrow School. Single-storey expansions are popular for owners wanting a more open and communal kitchen-dining area. Others adopt side-return designs to capitalise on narrow side spaces, while loft conversions add bedrooms or offices in pitched roofs—common in areas near Wealdstone or South Harrow. Basements exist in some prime roads but are not as common as in older, more central parts of London.

Local contractors warn about traffic near Harrow’s busy town centre or along the A409, so scheduling skip deliveries midmorning or midday is advisable. Labour costs hover around mid- to upper-range for outer London, with certain teams adept at preserving period house features. Harrow includes conservation areas, for example around Harrow on the Hill, so planning might get stricter for external changes. The benefit is that a well-designed extension can significantly enhance everyday living and property appeal, particularly if you’re near tube or rail stations. Speaking with neighbours early is wise to address any party-wall boundaries in suburban streets.

Whether you want a single-storey extension to unify your kitchen-lounge or a multi-floor scheme that reconfigures older interiors, shaping your expansion to match Harrow’s suburban ethos and planning environment helps pave a smoother path to approval.

Getting Started

Before building, finalise which extension approach makes sense for your Harrow property. A single-storey rear addition might expand a confined kitchen, while a double storey design yields extra space both below and above. Alternatively, a side extension can reclaim an alley or driveway, and a loft or basement might be relevant if outward expansion is off the table. Basements in Harrow are less widespread than in certain central boroughs, but they do appear if ground space is minimal or property values are high.

Robust planning early on saves time later. Devise a comprehensive budget, factoring in typical labour fees and standard material costs, plus a contingency for structural or finishing upgrades. If your plan entails substantial structural modifications, it’s often prudent to involve an architect or professional designer. Certain builders offer design-and-build, but they’ll usually need rough drawings or partial permission to give a firm quote. Many Harrow homeowners consult an architect first, then gather estimates from construction firms once they have preliminary plans.

Also consider a contractor well in advance, as top local teams often have full calendars. Gathering multiple quotes promptly helps you compare costs, check references thoroughly, and choose a contractor you trust. An experienced builder can also refine your plan—for instance, repositioning a door for improved flow or adding rooflights for a brighter interior.

Project Management

Extensions typically follow one of two project setups:

• Single contract (design-and-build or main contractor)
  You sign one contract with a building company that handles everything—foundations, structural framing, electrics, plumbing, roofing, finishing, and so forth. They subcontract trades, adding about 10–15% for coordination. This centralises scheduling and accountability.
• Multi-trade or split contract
  You personally manage each trade: groundworkers, carpenters, roofers, electricians, plumbers, etc. Although you might avoid main-contractor overhead, you must ensure tasks happen in the correct order. A single delay may stall subsequent tasks, potentially boosting costs or timelines. This path suits owners able to invest plenty of time or with strong construction knowledge.

For major expansions in Harrow, many homeowners lean toward a single-contractor model, freeing them from day-to-day oversight of multiple trades. If you do multi-trade, confirm you can devote consistent weekly time to oversee deliveries and progress.

Do I Need Planning Permission?

Whether planning permission is required hinges on your extension’s size, height, and property location. Under Permitted Development Rights, you can sometimes extend your house without a formal planning application if you remain within particular dimensional bounds. If you surpass these bounds, you generally 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.

Because Harrow has conservation pockets, such as the Harrow on the Hill area, you should ensure local restrictions don’t affect you. Early contact with planning officers clarifies if your plan meets PD or if a formal application is required.

Do I Need Planning Permission For A New Storey?

Loft conversions that add dormers or change roof height commonly need planning permission. However, you can add a new storey under permitted development if you comply with specific conditions and file a prior approval request with your local authority. Otherwise, Householder/Full Planning Permission is the norm. If your house meets the necessary criteria, see the bullet points below for the core limits:

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 might apply upon completion. In Harrow’s suburban roads, maintaining a consistent roofline is often key if your extension stands among similarly designed houses.

Do I Need Planning Permission For A New Basement?

Generally, adding a basement requires planning permission, and digging beneath your property typically demands planning permission because of its extensive nature. It’s prudent to talk with the local planning authority from the beginning, clarifying relevant guidelines. Basement works are substantial, so even without explicit planning approval, compliance with Building Regulations is critical for structural integrity, escape routes, ventilation, damp-proofing, etc.

Check if local rules in Harrow override standard PD expansions. Certain roads, particularly near Harrow on the Hill’s historic area, might have narrower acceptance for underground expansions. Browsing council resources or getting a planning expert’s advice can confirm feasibility.

A handful of owners in prime Harrow streets attempt basements if outward expansions are restricted, but expect a higher price tag due to ground surveys, party-wall pacts, and complex foundation tasks.

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.

A Full Plans Application highlights potential design or safety flaws early, while a Building Notice is sometimes enough for a smaller expansion. Both approaches conclude with building control giving final approval to certify code compliance.

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 Harrow extension meets PD parameters, you still have to secure building regulations approval to guarantee safe foundations, stable structural changes, and other essentials. In older 1930s or Edwardian houses, verifying load-bearing walls can support new loads is vital.

DIY Contribution

Homeowners sometimes contemplate handling parts of an extension themselves. Painting, decorating, or landscaping can be good DIY activities if you have confidence and time. Doing your own demolition, to an extent, might also be possible—provided you identify structural walls and observe safe methods.

However, tasks such as installing gas lines, electrics, or vital structural components almost always need qualified pros. Mistakes can lead to hazards or costly reworks. Being candid about your competencies is key; overshooting them can cause delays. Builders could halt their workflow if they’re waiting for you to finish a certain task, increasing the final tab. If you plan some DIY, think about consulting a professional to avoid serious mishaps.

In Harrow, partial DIY often comes into play at the last stage—for example, painting walls or landscaping the garden—once the main structural phase is complete. Attempting advanced mechanical or engineering tasks alone can derail official inspection timeframes.

Do I Need An Architect?

If you have a fairly basic plan, you might use a structural engineer or building designer. Nonetheless, for bigger or more intricate designs, employing an architect can be highly advantageous. A proficient architect balances practical features—like interior flow and natural light—and aesthetic considerations to deliver a unified extension.

They can also clarify whether your proposal fits PD or if a planning application is needed. Though paying an architect’s fee up front increases early costs, it often avoids design flaws that may lead to expensive mid-project revamps.

Harrow accommodates a variety of property styles, from 1930s semis in North Harrow to Edwardian terraces near Harrow Weald. An architect may help unify older exteriors with modern expansions, perhaps recommending large windows, folding glass doors, or partial skylights to transform the living experience.

How Long Does It Take To Build An Extension?

Planning a timeline is crucial to remain in command. A smaller single-storey project—especially if basic, standard materials, and allowed under PD—could wrap up in weeks after initial groundworks. Larger expansions, multi-level or multi-room, might last several months to half a year or more, depending on structure complexity and how many trades are needed.

Sometimes, you might consider moving out briefly if crucial walls are torn down or if basic services must be disconnected. Even if you stay, dust, noise, and partial room closures typically occur.

Incorporate a buffer in your schedule. Stormy or cold weather may delay outdoor works; inspectors may demand tweaks; deliveries can run behind. Sticking to a pragmatic timetable with your builder lowers stress. Finishing earlier than expected is always nice. Similarly, a contingency in your budget manages unforeseen structural tasks or changed finishing choices mid-flow.

Harrow sees moderate congestion on key roads near the town centre or along major routes like the A409, so skip drop-offs typically avoid peak travel times. If you alter design aspects or materials once construction starts, anticipate a later final date.

House Extensions Step By Step

Though an extension is a big undertaking, with solid planning, expert contractors, and a methodical budget, success is well within reach. Whether you want an open-concept family space, more upstairs rooms, or even a modest basement, keep track of each stage and look out for typical pitfalls.

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 issues before making the last payment. Keep building control approvals, warranties, and other official documentation safe for prospective sales or refinancing.

When your extension in Harrow receives its final sign-off, you gain a more spacious, practical home. Whether you extended the ground floor for a bigger kitchen-diner or inserted new bedrooms upstairs, the end result generally boosts both daily comfort and property value. A snagging survey identifies any superficial flaws—hairline cracks, imperfect paint lines—so they’re rectified before the contractor wraps up. Storing compliance certificates and warranties proves your project satisfied all regulations, reassuring possible buyers or mortgage lenders. In the end, your newly expanded Harrow property is ready to accommodate your evolving needs for many years ahead.