Sustainable home extensions in London: the complete guide
Most people who come to us have already made a decision; they want more space, they want to do it well, and they don’t want to build something they’ll regret in years to come. What they’re often less sure about is what ‘doing it well’ actually means in practice in environmental terms — beyond buying some triple glazing and hoping for the best.
This guide covers everything you need to know before starting a sustainable home extension in London. Not as a list of virtuous gestures, but as a genuine approach to building something that performs well, lasts well, and is a beautiful place to call home.
First, what do we mean by sustainable?
The word gets used loosely. We’d argue it has two distinct but related meanings in the context of a home extension.
The first is embodied carbon: the carbon released in manufacturing, transporting and assembling the materials in your building. A standard London extension — concrete slab, blockwork walls, brick facing, foam insulation — carries a significant carbon cost before a single person has moved in. That carbon is already spent. It can’t be offset by efficient heating later.
The second is operational carbon: the energy the building uses in use, day to day, year to year. Better insulation, better glazing, less heat loss. This is the side of sustainability most people think about — the running costs, the warmth in winter, the energy bills.
A genuinely sustainable extension addresses both. At New Makers Bureau, we’d argue the embodied side is the more urgent one right now, because it’s the carbon you can’t take back, and as the gird becomes decarbonised - embodied carbon becomes a bigger part of the puzzle.
Before you extend: ask whether you need to
This might sound like a strange thing for an architecture practice to say. But the most sustainable square metre is the one you don’t build.
Often the problem isn’t that the house is too small — it’s that the existing space isn’t working hard enough. A well-designed intervention in what’s already there can sometimes unlock more liveable space than a new extension would. It’s always worth asking the question.
When you do extend, designing efficiently matters. The bigger the extension, the larger the carbon footprint. Good design makes the most out of less floor area, and a smaller extension that unlocks existing dead space often performs better than a larger one that simply adds square metres. Also potnetially saving money too.
Reducing embodied carbon: the key decisions
Structure and walls
Concrete and blockwork are the standard. They’re durable and familiar to contractors, but they’re also high in embodied carbon — concrete at around 229 kg CO₂ eq/m³, brick at around 565 kg CO₂ eq/m³. At a typical London extension scale, that adds up quickly.
Timber frame is our preferred alternative. Sustainably sourced timber actually stores carbon rather than releasing it, and modern timber panel systems (including SIPs — structurally insulated panels) are fast to build, thermally excellent, and involve far fewer wet trades on site. At our Downton Avenue project in south London, the move to a timber frame with recycled wood fibre insulation saved a substantial amount of concrete and the associated carbon.
Floors
Concrete ground slabs are the default, and avoiding them entirely is often tricky. But there are ways to reduce the amount needed. A timber floor sitting above a ventilated ground cavity, for instance, can save several cubic metres of concrete on a typical extension footprint — which translates to hundreds of kilograms of CO₂.
The facade
The facade is often the single highest source of embodied carbon in an extension. The choice of facing material matters enormously, and it’s also where the building makes its character.
Reclaimed brick is one of our favourite solutions. Its carbon cost was already paid decades ago — by reusing it, we save it from landfill and give it a second life. Reclaimed brick brings a richness and depth that new bricks rarely match, and the right material from the right source can make an extension feel completely at home in a Victorian or Edwardian London terrace. It takes more time to source and can cost a little more, but it’s almost always worth it.
Downton Avenue: a completed example of reclaimed brick facade on a London extension
Improving energy performance
Energy in use accounts for roughly a third of a building’s whole-life carbon. So while it’s not the whole story, it’s still significant, especially over a 30-50 year building lifetime.
For an extension, the big wins are high-performance insulation throughout (walls, floor, roof), well-specified glazing that minimises heat loss without losing light, and airtightness — eliminating the draughts and cold bridges that undermine even the best insulated walls.
The biggest energy improvements in a home extension project are often found in the existing building, not the new part. If you’re opening up walls and floors anyway, that’s the moment to insulate under existing ground floors, improve loft insulation, and address the thermal leaks that have been there for a hundred years. That work costs relatively little at the point of a building project and saves significantly more energy over time.
The London planning picture
Planning policy across London boroughs is gradually shifting to require sustainability credentials in new buildings and extensions. Some boroughs have more demanding requirements than others, particularly around energy performance and materials in conservation areas.
In conservation areas, the choice of reclaimed or natural materials can actually help, not hinder, a planning application, because the visual quality argument is easy to make and the materials are sympathetic to the historic context. We find that well-presented sustainability arguments in planning statements are generally always welcomed and help to strengthen applications.
Planning a Sustainable Extension in London: What the Boroughs Actually Require
What does a sustainable extension actually cost?
More than a standard extension — but not as much more as people assume, and the difference narrows over time.
The premium for a sustainable extension in London varies depending on the specification, but in our experience it typically adds 5–15% to the build cost. Timber frame construction can actually offset some of that because it builds faster than blockwork, involves fewer on-site trades, and reduces the programme. Reclaimed materials cost more to source and lay, but the finished quality can justify the spend.
The more useful calculation is the whole-life cost: a building that’s cheaper to heat, requires less maintenance, and performs better over decades represents genuine long-term value. And for clients who care, as ours increasingly do, the knowledge that it was built well is itself part of what they’re paying for.
How to find and brief a sustainable architect
Look for a practice with completed sustainable extension projects, not just intentions. Ask to see real buildings, real material decisions, real carbon numbers if they have them. Anyone can put ‘sustainability’ on their website; far fewer can show you how they reduced embodied carbon on a specific project and explain why they made each decision.
Brief them early on what matters to you. A good sustainable architect will push back if they think the specification can go further, help you understand the trade-offs between budget and carbon, and bring material knowledge that makes the design better, not just greener.
How to Brief an Architect for a Sustainable Home Extension in London
