How Much Do House Extensions Cost?

Extending a house in the UK can range considerably in price, determined by the build’s size, materials, and overall intricacy. According to market data, a single-storey rear extension might begin at roughly £1,500 per square metre if you choose a simple configuration, whereas most fall in the £1,800–£2,500 per square metre span. If you incorporate advanced steel elements, bespoke glazing, or high-end finishes, it’s possible for expenses to exceed £3,000 per square metre.

In Bromley, property owners may discover labour rates that are somewhat less than those in the central boroughs, giving them leeway to select more refined materials or custom details. Still, older 1930s or Victorian-era structures in the borough might need special foundation reinforcement, raising the total cost. A bigger extension often settles between £40,000 and £100,000+, while smaller side-return projects might remain cheaper, and multi-storey or ultra-luxurious layouts can exceed typical ranges if there’s substantial structural engineering or premium finishes. Ultimately, cost drivers include design complexity, local labour fees, and the inclusion of kitchens or bathrooms, which require extra plumbing and electrics.

Whenever you contrast builders’ bids, be certain each quote includes foundation work, structural tasks, roofing, fittings, finishing, plus labour and overheads. Also confirm if VAT is part of the total or an added line item. While renovation magazines or online references supply ballpark figures, accurate estimates require multiple quotes from reputable building firms and a line-by-line comparison.

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.

Bromley

Bromley is a large suburban borough in southeast London, recognised for its leafy neighbourhoods, varied architecture, and convenient commuter links to the city. Many households here undertake single-storey rear expansions, merging smaller reception rooms into a bright, multifunctional area. Others choose side extensions if there’s enough driveway or alley space. Loft conversions remain popular too, converting pitched roofs into additional living areas—handy for teenage bedrooms or a dedicated office. Some properties, particularly in hilly regions of the borough like Hayes or Shortlands, might contemplate basements, though it can entail higher costs and more complex engineering.

Local contractors in Bromley warn about scheduling skip deliveries to avoid rush hours near main roads like the A21, so midday can be ideal. Labour charges remain a bit lower than in central boroughs, allowing owners to allocate funds toward better finishes, such as quality roof lanterns or double-glazed folding doors. However, certain conservation areas—like parts of Keston or Farnborough—may impose extra planning scrutiny for external changes. Often, residents who extend their Bromley home gain both functional improvements and a boost in property resale potential if they’re located near strong transport links like Bromley South station. Checking with neighbours early can smooth out any party-wall or shared boundary complications once construction starts.

Whether your vision is a single rear expansion just to expand the kitchen or a multi-level transformation, it’s wise to adapt your plans to Bromley’s local features. Doing so can help you conform to local guidelines while still creating a layout that fits your family’s needs.

Getting Started

Before construction begins, decide which kind of extension suits your property: a single-storey design, a double storey arrangement, or even a side or wrap-around build. Alternatively, you could investigate loft or basement conversions if outward extension is limited. Basements in Bromley may work well for certain homes, but they generally involve specialised work and can be costlier.

Allocating enough time to planning from the outset reduces hurdles. Form a detailed budget after looking at labour rates and standard material prices, ensuring you include a buffer for surprises or premium finishes. If major structural alterations are on the table, hiring an architect or a suitably qualified designer is often beneficial. Although some builders provide design-and-build services, they generally need preliminary drawings or partial approvals to issue accurate estimates. Many homeowners meet with an architect first, then collect quotes from construction firms once they have a clearer plan.

It’s also a good idea to engage a contractor early, since competent teams are often booked for months ahead. Securing bids sooner lets you evaluate cost differences, check references, and choose a builder with confidence. An experienced team can also improve your design if they spot overlooked refinements—like adding a small utility nook or installing an extra skylight to boost natural light.

Project Management

Most people opt for one of two methods to organise their extension:

• Single contract (design-and-build or main contractor)
  You sign a contract with one building company handling the entire process—foundation work, carpentry, electrics, plumbing, roofing—and subcontracting as necessary. They usually charge around 10–15% markup for overseeing subcontractors. This structure simplifies scheduling, giving you one main contact for any issues.
• Multi-trade or split contract
  You personally manage each trade: groundworks, bricklayers, electricians, plumbers, roofers, and so on. While in theory you might save the contractor’s markup, you also take on project management. If one element delays, subsequent tasks might be postponed, potentially increasing final expenses or stress. This approach is best if you have ample free time or construction knowledge.

For substantial expansions in Bromley, families often pick the single-contractor model, especially if they can’t oversee daily scheduling. If you do multi-trade, confirm you can dedicate enough hours each week to keep trades coordinated.

Do I Need Planning Permission?

Whether planning permission is required hinges on your planned extension’s scale, height, and location on the property. Under Permitted Development Rights, it may be possible to add onto your home without a formal planning application, as long as you adhere to certain size and placement conditions. If, however, your proposal surpasses these thresholds, you will likely need to 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.

In Bromley, living within a conservation area or close to listed buildings might restrict your external alterations. Confirm early with local planning officers whether your concept qualifies as Permitted Development or needs an official application.

Do I Need Planning Permission For A New Storey?

Loft extensions involving dormer windows or raising the roof typically require planning permission. However, you can sometimes add a new storey under permitted development, meaning no planning permission is needed if you meet specific conditions and submit a prior approval application to the council. Otherwise, a Householder/Full Planning Permission request is mandatory. If your house complies with the relevant criteria, the bullet points below detail these limitations:

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.

Once built, additional rules can apply upon completion. Even if your blueprint seems to match each requirement, verifying details with Bromley’s planning authority can stop confusion in roads of uniform design or where rooflines are carefully regulated.

Do I Need Planning Permission For A New Basement?

Creating a new basement typically calls for planning permission, and excavating underground is likely to do so because of the large-scale works involved. It is wise to consult your local planning authority at the outset to clarify any conditions. Basement excavations are also inherently complex. Even if official planning consent is not explicitly needed, Building Regulations still apply for structural soundness, escape routes, ventilation, fire protection, and more.

Always check any local rules that might affect your extension and your specific property. In some instances, even an extension normally allowed under Permitted Development can be negated by prior or local planning constraints. Consult your council’s guidance or a planning specialist to be sure.

In Bromley, basements remain relatively uncommon, but for households short on outward or upward space, going underground might be the only option. The cost tends to be higher due to engineering demands.

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 commonly highlights design or stability concerns pre-construction, whereas a Building Notice can serve for simpler expansions, but you should be prepared for on-site changes if inspectors spot issues.

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 no planning application is mandatory under PD, you must still prove that your structural work and basic home safety aspects conform to building regulations. Bromley, with many older semi-detached and period houses, can warrant extra checks for foundation strength, moisture prevention, or fire safety compliance.

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.

Bromley owners frequently prefer doing simpler finishing steps—like painting or basic landscaping—once the main shell and core systems are complete, limiting the chance of halting key trades.

Do I Need An Architect?

If your project is fairly straightforward, a structural engineer or a professional building designer might be sufficient. Yet for many larger or more complicated extensions, an architect can offer major advantages. Skilled architects factor in aesthetics as well as practicalities—such as internal flow, natural light, and how any new space meshes with existing rooms and the garden.

They can also help you navigate planning permission or confirm if your design remains under permitted development. Although hiring an architect adds to initial costs, their expertise often prevents missteps and gives you a more coherent final layout.

Bromley contains diverse housing styles, including 1930s estates near Petts Wood or Victorian terraces near Bromley North. An architect’s viewpoint can unify an older facade with a brand-new extension, perhaps recommending complementary materials or strategic window placements.

How Long Does It Take To Build An Extension?

A realistic timeline helps you keep the project under control. A smaller extension—a single-storey rear build with basic design—might conclude in just weeks once construction commences, particularly if it’s within permitted development and relies on commonly sourced materials. Larger expansions, multi-room or multi-storey, may last from several months to half a year or longer, depending on complexity and the quantity of trades involved.

Occasionally, you’ll have to move out temporarily if significant structural work cuts essential services or tears down major load-bearing walls. Even if you remain, dust, noise, and some limited room use may be part of daily life.

Include extra margin in your schedule. Delays can arise from poor weather, inspector requests for minor modifications, or late deliveries. Talking through a realistic plan with your contractor helps reduce stress, and finishing sooner than expected is always a bonus. Likewise, holding a budget contingency is recommended for hidden structural problems or design shifts discovered mid-project.

Bromley roads, especially near popular commuting routes, can hamper skip drop-offs or large deliveries, so midday typically works best. If you opt for special materials or decide to revise finishes halfway, your completion date might be extended further.

House Extensions Step By Step

Putting up an extension is significant, but with methodical planning, skilled trades, and a balanced financial plan, there’s little reason you can’t succeed. Whether you’re planning more kitchen-dining space, a loft room, or extra bedrooms upstairs, it helps to grasp the various stages and common hurdles.

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. Any unresolved defects should be handled before final payment. Keep all records (compliance certificates, warranties) secure, in case you plan to refinance or sell.

After you pass the final checks, you’ll have a bigger, better-suited home. Whether you added a light-filled rear living zone or extended upward with an extra floor, your property typically gains both comfort and prospective market value. Bromley families in particular benefit from expansions that let them remain in a borough known for green spaces, good schools, and commuter-friendly stations. Conducting a snagging survey ensures paint flaws or minor cracks can be fixed before you settle the last invoice. Retaining building control approvals and warranties proves your extension followed proper rules. With all that done, you can enjoy your newly enlarged Bromley property for many years.