How Much Do House Extensions Cost?

The cost of extending a house in the UK depends on size, materials, and how complicated the design is. Industry figures show that a single-storey rear extension might start near £1,500 per square metre for a basic arrangement, whereas many fall into the £1,800–£2,500 per square metre zone. If you choose elaborate steel beams, custom glazing, or premium-level finishes, outlays can top £3,000 per square metre.

In Brentford, owners often find labour fees below super-central boroughs but still on the higher side for west London, offering a chance to spend more on premium interior features. Meanwhile, older terrace homes or listed buildings here may need structural tweaks that raise your total. Larger expansions usually run between £40,000 and £100,000+, although a narrower or simpler addition might cost less, and a multi-storey scheme with complex steel or engineering can exceed these figures. Main factors include the scale of structural changes, local labour rates, and whether your extension incorporates a kitchen or bathroom with added plumbing, electrics, and waterproofing.

While comparing builders’ quotes, ensure each covers foundation work, structural builds, roofing, all fittings, and finishing tasks. Also check if VAT is included to dodge last-minute cost shocks. Although home-improvement publications and renovation websites can supply approximate data, accurate budgeting derives from gathering multiple quotes from seasoned contractors and scrutinising them thoroughly.

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.

Brentford

Brentford is a west London district near the River Brent and the Thames, known for a blend of period terraces, modern builds, and ongoing redevelopment along the riverfront. Many residents opt for single-storey rear additions with expansive glazing to maximise daylight, linking indoors and outdoors. Others create side expansions if there’s a narrow path, enhancing ground-floor space. In taller terraces, loft conversions are also popular, adding upper-floor bedrooms. Basements, although feasible, can be costlier and often less common due to water table concerns near rivers.

Local contractors emphasise careful scheduling to avoid heavy congestion near major roads like the A4 or Great West Road. Labour expenses in Brentford generally come below the highest London rates, enabling owners to invest more in design elements or top-quality kitchen fittings. Meanwhile, older properties near heritage sites or near the river may impose stricter planning demands. The reward for a successful extension is often better daily living comfort plus heightened resale interest, especially if you remain near new transport links (like the Elizabeth line) or scenic riverside walks.

Whether you are planning a compact extension only for a larger kitchen-diner or a complete multi-storey rework, it pays to take Brentford’s location and style into account so that your changes meet local regulations and street aesthetics.

Getting Started

Before building, decide which extension layout fits your property: a single-storey rear addition, a double storey design, or a side or wrap-around extension. Alternatively, a loft or basement might be best if outward space is minimal. Basements can produce valuable extra rooms but require thorough structural work, especially near watercourses like the Brent or close to the Thames.

Devoting ample time to up-front planning will generally spare you difficulties mid-build. Lay out a detailed budget by researching typical material rates and labour costs, then keep some contingency for top-tier finishes or structural surprises. If your idea includes substantial structural modifications, think about hiring an architect or suitably qualified designer. Though some builders offer design-and-build, they typically need preliminary sketches or partial planning approvals before delivering fixed quotes. Many Brentford residents start with an architect to map out floorplans, then approach builders for formal bids.

It is also wise to locate a capable contractor early, as top-rated teams can be booked. By obtaining quotes rapidly, you can weigh price differences, call references, and finalise your builder. Skilled companies often propose useful tweaks to your vision—like adding a glass roof section for more illumination or separating a small utility space within a larger kitchen.

Project Management

Organising an extension can go in two main ways:

• Single contract (design-and-build or main contractor):
  You sign one deal with a principal building firm that manages all tasks, from structure to electrics. They subcontract specialties at about a 10–15% markup, but you get streamlined scheduling and a single point of accountability.
• Multi-trade or split contract:
  You personally coordinate each trade—groundworkers, bricklayers, roofers, plumbers, electricians, and so on. Although you avoid the main contractor’s markup, you must orchestrate deliveries and confirm that each phase finishes before the next can begin. If any trade lags behind, you might see ballooning costs or idle time.

For large or intricate projects in Brentford, many owners pick a single-contractor setup, especially if they lack time or experience to manage multiple trades themselves. If you have strong planning skills and reliable local trades, you could do multi-trade, but be prepared to invest hours each week in scheduling.

Do I Need Planning Permission?

Whether planning permission is necessary depends on your extension’s scale, height, and property placement. Under Permitted Development Rights, you might be able to extend a house without seeking a formal application, provided your plan meets specific size and positioning criteria. If, however, the proposal overshoots those boundaries, you will typically need to 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.

In Brentford, near heritage spots or on river-adjacent land, you might find extra local constraints. Always consult planning officials to confirm if your scheme fits Permitted Development or if you need a householder application.

Do I Need Planning Permission For A New Storey?

Loft extensions that involve dormers or raising the roof usually call for planning permission. Nonetheless, introducing a new storey can be classified as permitted development, meaning no planning permission is required if rigid criteria are met and a prior approval is filed with your local authority. Otherwise, you would submit a Householder/Full Planning Permission request. If your property aligns with these rules, the bullet points below clarify 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 conditions can apply after the extension is finished. Clarifying with Brentford’s local authority can prevent conflicts, particularly if your row of terraces exhibits uniform height lines.

Do I Need Planning Permission For A New Basement?

Usually, constructing a new basement mandates planning permission, and excavation is likely to do so due to the scale of the work. It’s recommended to speak with your local planning authority to clarify specifics before starting. Basement works are also inherently extensive, and even if official permission is not strictly needed, you must still adhere to Building Regulations to ensure structural safety, proper ventilation, escape routes, and overall fire protection.

Always investigate any rules that might pertain to your location and property. Sometimes, an extension that would ordinarily be permitted can be disallowed by prior or local planning conditions. Look at local council resources or engage a planning consultant for surety.

In Brentford, basements aren’t as frequently chosen because of potential water table issues near the rivers, although a few owners do explore going underground if outward or upward development is restricted.

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 might catch design or structural shortcomings before building starts, while a Building Notice can be fine for simpler expansions. In either scenario, building control’s final approval is necessary to verify that key stability, fire safety, and damp-proofing standards are met.

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 Brentford extension qualifies under Permitted Development, you still have to abide by building regulations to validate safe foundations, adequate insulation, and so on. Many older west London terraces may require closer checks to handle extra weight or to address damp risk.

DIY Contribution

Homeowners sometimes consider tackling parts of an extension themselves. Painting, decorating, or landscaping can be good DIY tasks if you have the skills and time. Doing your own demolition, to an extent, can also be feasible—provided you understand which walls are load-bearing and follow safety precautions.

However, tasks such as electrical installation, gas connections, and structural changes almost always require qualified professionals. Mistakes in these areas can be costly and dangerous. Being realistic about your abilities is crucial; if you overcommit, you risk causing delays. Builders may need to pause until you finish a particular job, which can add to your final bill. If you’re determined to do some work yourself, consider hiring a professional for a consultation, as their input can help you avoid pitfalls.

In Brentford, partial DIY typically focuses on finishing touches, once the structural skeleton and mechanical/electrical aspects are wrapped up. This strategy helps preserve the project timeline and safety.

Do I Need An Architect?

If your extension is modest or straightforward, a structural engineer or professional building designer could suffice. Yet for many larger or more elaborate jobs, an architect can offer real advantages. They consider both aesthetic factors—like natural light and room flow—and practical elements such as how the new space integrates with your existing layout and garden.

They can also guide you through the planning permission process or confirm if you fit within permitted development. Although employing an architect adds initial costs, their detailed oversight commonly prevents design slip-ups that can be expensive to fix.

Brentford showcases a mix of older terraces, riverside apartments, and modern developments, so bridging your property’s style with a fresh extension may need an architect’s nuanced perspective. They might suggest using complementary materials or creating large openings that capture light and views of the river.

How Long Does It Take To Build An Extension?

Establishing a time plan is vital for staying organised. A modest single-storey project—a small rear addition, for instance—might be completed in a handful of weeks if the layout is simple, resources are readily available, and it falls under permitted development. Larger expansions, such as multi-room or multi-level designs, can last from several months up to half a year or beyond, depending on intricacy and the volume of trades involved.

At times, a temporary move might be required if major structural tasks sever vital services or remove critical walls. Even if you stay in place, expect dust, noise, and partial restrictions on using certain rooms.

Always schedule a buffer. Delays may arise from inclement weather, inspector requests for revisions, or tardy deliveries. Working out a realistic timeframe with your builder helps lower stress, and completing earlier than projected is always pleasing. Likewise, a financial contingency for unplanned modifications or extra structural work is recommended.

Brentford is prone to traffic along major thoroughfares like the A4, so booking skip drop-offs in mid-morning often avoids commuting rushes. Introducing special materials or design changes once the build is underway might also expand your timeline further.

House Extensions Step By Step

An extension is a big project, but with systematic planning, professional craftsmanship, and a decent budget, it is entirely feasible. Whether you seek an expansive kitchen-living area, additional bedrooms, or a loft, learn the steps and note potential 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. Resolve any imperfections before paying the final invoice. Retain building control certificates, warranties, and other crucial documents for future property transactions or mortgage adjustments.

After final approvals, you can savour a more expansive layout. Whether you installed a luminous rear living space or constructed a new upper floor, the extension commonly increases both liveability and property appeal. Brentford dwellings, set near waterfront views and improving transport links, can see higher market interest if they feature well-designed expansions. Conducting a snagging survey identifies minor flaws—like small cracks or uneven surfaces—so the contractor can correct them. Storing approvals and warranties proves your extension met all official standards, reassuring mortgage lenders or prospective buyers. With that, your newly enlarged Brentford home will be ready for comfortable modern living.