How Much Does a House Extension Cost in London?
If you’re a homeowner thinking about extending or refurbishing your property, one of the first questions that naturally comes up is one of costs. So how much does an extension actually cost?
It’s a good question, but one that doesn’t have a single, simple answer. Costs will vary depending on the type of project, the character of your home, the standard of finishes, and the quality of the builder.
If you want some baseline costs that you can use to get started budgeting and planning your project, read on below.
Outline Cost Breakdown
The three most common categories for home extensions are rear or side extension, loft conversion, and internal refurbishment. An extension requires ground works and new structure. A loft conversion requires insulation and fabric upgrades. An internal refurbishment can involve anything from upgrading finishes, to extensive remodelling.
These figures offer a helpful benchmark, but it’s important to remember that no two projects are the same. Site conditions, access, and design choices all play a big role in shaping the final cost.
| Type of Project | Typical Cost (per m²) | Description |
|---|---|---|
| Rear or Side Extension | £2,500 – £3,500 / m² | Includes new kitchen or living area extensions with full finishes. |
| Loft Conversion | £2,000 – £2,500 / m² | Suitable for adding an extra bedroom, ensuite, or home office. |
| Internal Refurbishment | £1,200 – £2,500 / m² | Covers reconfiguring existing layouts, new finishes, and minor structural changes. |
Construction costs is generally quoted ex VAT, so this will need to be added to the figures above. For normal extensions, loft conversions and refurbishments, add 20% VAT. If the work involves creating new units in an existing building, it is normally 5%. New builds, however, are currently zero-rated.
You will also need to factor in professional fees for the architect, engineers and surveyors. Depending on the complexity and the scope of their involvement, this usually range between 10-20% of the construction cost (ex VAT).
Why Costs Vary So Much
London homes are wonderfully diverse, and range from Victorian terraces and Georgian townhouses to modern flats, and each comes with its own set of opportunities and constraints.
A larger straightforward extension at the rear of a detached home will usually cost less per square metre than a smaller extension, built in a tight rear garden of a terraced house, where materials must be carried through the house.
The level of finish can also make a significant difference. High-end glazing systems, specialist finishes, and expensive materials can elevate the result, but will also add a fair amount to the cost.
Similarly, if your home is listed or in a conservation area, you may need additional permissions or structural input, which affects timelines and fees.
Sustainable upgrades, such as extra insulation, air-source heat pump, and natural materials, could also cost more than the standard – but these are upgrades we really think you should consider!
Design complexity matters too. A well considered scheme can make big impacts on the result without making too many big changes to the building. Often, structural changes can be omitted by looking at ways to work with what is already there. The key is balancing ambition with practicality. That way we can create a design that adds real value without unnecessary expense and an inflated carbon footprint.
The Value of Professional Guidance
As architects with years of experience in residential extensions and refurbishments across London, we work with homeowners from the very first sketch through to planning approval and construction.
One of our first steps is to establish a realistic budget and help you understand how your aspirations translate into cost, get a sense of what’s possible — and where to prioritise your investment.
At first, costs are based on experience from similar past projects. As the project progresses, more detail is added to the design, which translates into more detailed understanding of costs.
We recommend engaging with a builder early on. This is the best way to get cost certainty. From years of working on extension projects, we have good connections with excellent builders across London.
Talk to Us About Your Project
Every home, site, and family is different — and so are the costs involved.
If you’d like to get a better understanding of what your project might cost, we’d be happy to talk it through and share how certainty around budget can be gained at various stages of the project before all the decision are made.
Our team can review your property, discuss your goals, and provide a realistic estimate based on your specific circumstances.
There’s no obligation — just straightforward, practical advice from an experienced architectural practice that understands how to make the most of London homes.
Contact us today to arrange an initial consultation and start planning your home extension or refurbishment with confidence.
Selected projects
Flexibility in Architecture
One of the key advantages of hiring a professional to do your design is their ability to design in flexibility. This is crucial for addressing the pressing challenges of high land costs and environmental breakdown. By making sure adaptable design principles are understood and followed, we can create efficient, multifunctional spaces that has the potential to cater to a variety of needs, both present and future, while minimising resource use.
Overlapping Use
Living in London is a great example of the problem of cost. Most people would love to have more space, but the high pressure on the available properties mean that most of us have to compromise on size. On average, one square meter in London costs around £7,000. By these measures a normal corridor can cost £40,000. We therefore need to make sure that no space is wasted and where possible, we need to find ways to have overlapping use of the same space. Multifunctional space is therefore one of the key aspects to design flexibility into your home and make the most out of your space.
One approach to achieving flexibility is the incorporation of movable partitions instead of traditional walls and doors. For instance, in an apartment in Belsize Park, we designed folding partitions that allow for dynamic reconfiguration of spaces. When fully opened, the main living area nearly doubles in size, enhancing natural light and providing expansive views in both directions. But sometimes a more private or intimate organisation is more favourable. A guest might stay over and the study nook becomes a private guest bedroom. Or, a dinner party is happening and the bedroom remains shut, whilst the study becomes a place for a more intimate private conversation.
Adaptability can be incorporated into the furniture as well. A fold away bed can become part of a wardrobe, or a desk can fold out of the bookshelf and fold back in again when not in use. Or how about a built up floor hiding a kings size bed beneath a hatch?
Flexibility with overlapping use can also be achieved without moving elements. For example, a corridor can usually be used for only one thing – to walk between other rooms. But with careful consideration to the layout of the spaces in the house, a corridors can be part of adjoining spaces. Overlapping the corridor with the dining room means more potential guests at a dinner party. Or a corridor can be combined with a utility cupboard housing washing machine and tumble dryer, without the need for a separate room.
Adapt over Time
Beyond immediate functionality, we believe flexible design is critical for ensuring the longevity of buildings. Environmental sustainability requires more than meeting insulation and airtightness standards; buildings must be designed to remain useful over time. A well-designed house should be able to evolve alongside its occupants’ changing needs, and to be able to adapt to future occupants.
For example, in this home we recently designed for a single resident, we have made sure that certain critical elements allows for easy modifications in the future.
A space has been allocated for a future potential platform lift, should mobility at some point become a problem when living in this three storey house. The platform lift has been situated so that all the other spaces will still be useable.
The ground floor has been organised so that parts of it can be used as a separate flat for a live-in carer, should the need arise in the future.
Incorporating the needs of potential future occupants is also important. With a few modest interventions, the house can transform from its one-bedroom layout to a four-bedroom configuration, which would be suitable for a larger family. This, of course, makes it useful to a much greater pool of people, which should significantly increases the building’s long-term relevancy.
The building also has spacious service voids in risers and in the floor slab, to make sure that it will be easy to upgrade services as technology inevitably advances. We don’t know exactly what future buildings may need, but there is a clear trend were consistently more space needs to be allocated for ductwork, cabling etc.
Flexibility means useful
Making sure that the property remains and useable and therefore desirable, reduces the likelihood of demolition and unnecessary reconstruction. This in turn supports environmental sustainability by minimising material waste and resource consumption, which is why we consider flexibility in design so important. By focusing on adaptable layouts and multifunctional spaces, we can create buildings that make efficient use of limited land, reduce carbon footprints, and remain valuable for generations. The ability to modify spaces over time ensures that buildings continue to meet the evolving needs of their occupants. We are therefore committed to designing spaces that are also practical, sustainable, and adaptable for the future.
In addition to being useful, a building should be aesthetically pleasing. Just as we take care of the things we need, we take care of the things we find beautiful. we are therefore committed to designing spaces that are also practical, sustainable, adaptable and wonderful.
Unlock Hidden Value: Turn Your London Plot into a Profitable Development
Do you own a large garden, a disused garage, or a forgotten corner of land? In London—where every square metre counts—your property may hold far more potential than you think.
With the right architectural design and planning strategy, even a small or awkward space can be transformed into a new home, a rental property, or a valuable long-term investment.
As architects in London, we help homeowners and small developers unlock the hidden value of their land through small and medium scale residential developments. Here’s how we do it.
Unlock the Potential of Your Land
You might be surprised at what’s possible. Many backland development sites, gardens, side plots, and unused garages can be sensitively developed with the right approach. With thoughtful design and intelligent planning applications, you can unlock the value of underused land.
We are experienced in working within the urban context of London and its surrounding areas, but with backgrounds from Norway and Japan, we often see things differently and identify opportunities where others may not. And our years of teaching architectural design at universities have taught us that constraints often drive innovation.
Where others see problems that they would rather not engage with, we see opportunities for new and distinctive contemporary housing design. Whether your site is narrow, sloping, or hidden behind your home, we can find inventive solutions that unlock the property’s true value. Our approach combines creativity, technical expertise, and persistence – it’s not magic, just keeping an open mind to opportunity and a rigorous approach to testing possibilities.
Depending on your goals, we can help you:
• Create a modern garden studio, guest house, or rental unit.
• Subdivide and sell part of your land with planning permission in place.
• Develop the site yourself with full professional guidance from concept to completion.
While you can sell land without planning permission, securing consent first often increases its value significantly. It also gives you control over what gets built next to your home.
What Are Some Typical Types of Development?
There are several types of residential development opportunities commonly found in urban areas. We describe some of these below, along with key challenges and opportunities to consider.
Your site may be unique and not fit neatly into any of these categories. This is only a guide, and it’s always advisable to seek professional architectural advice so your specific situation can be considered on its own merits.
Side Plot Development for End-of-Terrace
You may have a piece of land to the side of your property. It may be a corner plot as shown on the right, or it may be a gap within a row of houses.
These plots often have awkward shapes, which may be why they were left undeveloped when the original terrace was built. They therefore require a carefully considered architectural design to create comfortable and efficient internal spaces.
They are often shallow, so particular attention must be given to providing sufficient outdoor amenity for future occupants. Private external space, such as a garden or terrace, is a planning requirement.
The flank or party wall will usually need to be fully enclosed. If there are existing windows on this wall, they may need to be removed for the new development. This can affect the daylight levels in the existing property and may raise rights of light considerations.
On the positive side, these plots benefit from direct access to the road, which simplifies access for residents, new services, and the construction process.
Backland Development of Internal Plot
Sometimes a piece of land can be found at the rear of an urban block, as shown on the map on the left.
These plots are often larger, and therefore may have the potential for several buildings, or a block of flats. However, there are also some common problems.
The first consideration is access. As the plot is usually landlocked, gaining access may be difficult. Beyond access for future residents, other factors must be addressed: how the contractor will deliver materials and equipment, how the site will connect to sewers and other services, and how emergency vehicles can reach it in the event of a fire.
With so many adjoining neighbours, it’s important to carefully consider the size and volume of any proposed backland housing avoid creating an overbearing structure, blocking daylight, or overlooking adjacent properties.
These sites can also incur significant costs in party wall agreements, as separate awards might be required for most or all of the adjoining owners.
Garden Development With Separate Access
Properties with large gardens can sometimes accommodate a new garden plot development by partitioning off part of the land. The map here shows a garden plot with a separate access road running along the rear. For end-of-terrace houses with large gardens, access may be possible via a side road instead.
It’s not possible to give a definitive answer on how large a garden must be for this to work. However, consider that the footprint of a small terraced house is typically around 70 m². In addition, you’ll need space for private outdoor amenity, bin storage, and usually car or cycle parking, depending on local authority policies.
Garden Development Without Separate Access
Many larger gardens are ‘landlocked’, with access possible only from the front.
These backland garden plots require additional space to accommodate a suitable access route.
Design That Respects Your Home and the Character of the Neighbourhood
Good design should enhance and complement what already exists. A successful development takes visual cues from its context, maintaining appropriate scale and proportions while engaging in dialogue with its surroundings. We prioritise functionality, natural light and ventilation, and a strong connection between inside and out. The new structure should be contemporary yet balanced and respectful of its setting.
When we design we ensure:
• Elegant, efficient design that complements your existing property and the context.
• Planning strategies that maximise approval chances and meet local policies.
• Sustainable, low-impact materials and construction methods.
• Complete architectural design services from feasibility to final handover.
Every proposal must work for you, satisfy your neighbours, and meet the local planning authority’s requirements.
Why Now Could Be a Good Time to Act
London’s housing demand remains strong, and councils are increasingly open to well-designed small-scale housing developments that make better use of existing land. With the current government’s commitment to building 1.5 million new homes in five years, infill and backland sites, are now seen increasingly as valuable opportunities.
Even a modest garden site can offer major potential, whether you want to:
• Sell your land with planning permission for maximum value,
• Partner with a developer through a joint venture for a share of the profits, or
• Oversee your own build with expert guidance at every stage.
From Concept to Completion – We Help You Make The Right Decisions
It may begin as a way to unlock value, but it could lead to creating a new home for yourself while selling your existing one.
Every site is unique. Some have access constraints, sensitive neighbours, or complex planning policies – but with careful analysis and creative problem-solving, hidden opportunities can be uncovered.
Once we’ve assessed your site’s potential, we’ll develop a clear proposal aligned with local planning policy to give you the best chance of success. From feasibility studies and planning applications to technical design and construction management, we can guide you through the entire process.
Discover the Potential in Your Land
If you own a property with a large garden, disused outbuilding, or underused plot, you could be sitting on untapped value. Every site is different, so contact us today to arrange a free architectural consultation. We will help you find out what’s possible and maybe turn your unused space into something truly valuable.
Selected projects
Sustainability in Building Design and Construction
The construction industry is a significant contributor to energy consumption and carbon emissions in the UK. However, there is substantial potential for improvement by adopting sustainable building practices. By making thoughtful choices in design, materials, and construction methods, we can significantly reduce the environmental impact of new buildings while ensuring they remain functional and durable for generations to come. In this article we will highlight how we make sure that sustainability is a key factor when we design.
One of the most effective ways to build sustainably is by taking a fabric-first approach. This means prioritising the external envelope of the building to ensure it retains heat as efficiently as possible. In the UK’s climate, keeping heat within the building is essential to reducing energy consumption and lowering heating costs. Two key factors that influence this are the U-value, which measures how much heat is lost through walls, roofs, and floors, and the airtightness of the building, which determines how much warm air escapes through gaps and junctions in the structure. The lower the U-value and the better the airtightness, the more energy-efficient the building will be.
Another important consideration in sustainable design is the orientation of the building and the placement of windows in relation to the sun’s path. The amount of heat gained from the sun can be considerable, and this can play a crucial role in reducing heating costs during the colder months. However, it also presents a challenge in summer, when excessive heat gain can lead to overheating. By carefully designing shading elements, such as overhangs, external shutters, or planting deciduous trees, it is possible to take advantage of the sun’s warmth in winter while providing protection from its intensity in summer. Since the sun is higher in the sky during the warmer months, well-designed shading can be seasonal, blocking excess heat in summer while still allowing low winter sun to enter the building.
Beyond passive design strategies, operational energy consumption can be further reduced through the installation of energy-generating systems and efficient heating solutions. Solar panels, for example, allow buildings to generate their own electricity, reducing reliance on fossil fuels. Heating systems such as heat pumps offer a highly efficient way to provide warmth while consuming significantly less energy than traditional boilers. Additionally, mechanical ventilation with heat recovery (MVHR) systems can further minimise energy loss by ensuring that warm air is not wasted, while also maintaining good indoor air quality.
Sustainability in construction does not only concern operational energy use but also considers the embodied carbon of a building. Embodied carbon refers to the emissions associated with the production, transportation, and construction of materials used in the building. Certain materials, such as concrete and steel, have a much higher carbon footprint than others. Where possible, these should be avoided or minimised in favour of more sustainable alternatives. Timber, clay, and natural stone, for instance, have significantly lower embodied carbon and can provide durable, high-quality construction solutions. Reclaimed and recycled materials also offer an excellent way to reduce the environmental impact of a building project, as they require fewer resources to process and transport compared to newly manufactured materials.
While many sustainability principles can be measured and calculated, some aspects of environmentally responsible building design are less easy to quantify but are equally, if not more, important. One of the most fundamental principles is avoiding demolition where possible. The greenest building is the one that already exists, and by repurposing and adapting existing structures, we can avoid the high carbon costs associated with constructing entirely new buildings. This idea underpins our redevelopment cooperative, Greenest House, which is dedicated to preserving and revitalising existing structures instead of tearing them down unnecessarily.
Ensuring that buildings are designed for longevity, is another crucial aspect of sustainable design. If a building is only in use for few decades before being demolished, it means that a new structure will need to be built in its place, which requires expending further resources. While we cannot predict the exact needs of future generations, we can design buildings with flexibility in mind, making it easier for them to be adapted and modified as requirements change. This means considering future alterations when planning layouts and construction details, ensuring that walls, partitions, and services can be reconfigured without significant structural changes or material waste.
The materials used in construction also play a key role in a building’s longevity. High-quality materials that age well will reduce the need for frequent maintenance and replacement, further decreasing environmental impact. In addition to being durable, a building should also be beautiful. If people feel a positive emotional connection to their home or workplace, they are more likely to care for it, preserving it for future generations rather than allowing it to fall into disrepair or be replaced prematurely.
When all these principles come together, a building can have a long lifespan, ensuring that resources are not wasted and that environmental impact is kept to a minimum. Even when demolition eventually becomes necessary, sustainable design can still play a role. By ensuring that buildings are constructed in a way that allows materials to be easily dismantled and reused, we can further reduce waste and conserve valuable resources, closing the loop in the lifecycle of the built environment.
Sustainability in construction is not just about reducing energy use or cutting carbon emissions—it is about creating buildings that are well-designed, long-lasting, and adaptable to change. By integrating these principles into every stage of the design, we ensure that we deliver a high-quality, comfortable, and enduring architecture for people to live and work in, but crucially does not have a negative impact on our shared environment.
