London isn’t cheap. Space is tight. And moving? It’s stressful, expensive, and rarely as simple as it seems. So you start thinking - what if you could make the home you already love work harder for you?
An extension isn’t just about square metres. It’s about possibility. More light, better flow, a home that fits you, rather than the other way around.
But here’s the thing. Good design isn’t about spending more. It’s about spending better.
So before you start knocking down walls, let’s talk numbers.
We get asked this a lot. And the answer is: it depends.
But here’s a ballpark:
(Yes, basements are expensive. Digging holes in London clay is no joke.)
These are build costs only. Professional fees, VAT, and the extras? They stack up. But they also add value - not just to your home, but to how you live in it.
A bad extension? It just makes your home bigger.
A good one? It makes your home better.
Here’s what will shape the cost:
Not all extensions are created equal. Some just add space. Others change how you live.
Before you think about costs, think about purpose. What do you want your extension to do for you?
A bigger house isn’t always a better house. But a better designed house? That’s a game changer.
Let’s break it down.
For many, a house extension means one thing: a bigger kitchen. It’s where you cook, eat, talk, laugh, work, and linger long after dinner is done.
But kitchens aren’t cheap. Plumbing, electrics, appliances, bespoke joinery - every detail adds up.
And if you’re thinking island, skylights, or slimline sliding doors? Factor that in too.
You’d think adding a bathroom would be simple. It’s not.
Drainage, waterproofing, ventilation - each detail has to be done right. And if you’re extending to add an en-suite or a downstairs WC, you’ll need to think about plumbing logistics too.
_ A budget-friendly bathroom starts at £5,000.
_ Something more refined? £10,000+.
_ If you want underfloor heating, high-end finishes, or a spa-like retreat? That’s another level.
A kitchen is full of decisions. A living space? A little more flexibility.
A rear extension that opens up to the garden can transform how your home feels. A side return extension can turn wasted space into something usable.
A well-designed living space doesn’t just look good. It feels right. The kind of space where Sunday mornings stretch longer, where conversations flow, where you can breathe.
Remote work isn’t going anywhere. And if you’re spending half your life working from home, your environment matters.
A home office doesn’t have to be a desk in the corner. It can be purpose-built, soundproofed, with natural light that keeps you focused.
And if you want a space that feels separate from home life? A garden studio might be a better investment than extending.
An extension doesn’t live in isolation. It needs to connect seamlessly with the rest of the house.
The best extensions feel like they’ve always been there. They make a house work harder, not just grow bigger.
At the start of every project, we ask:
_ What do you love about your home?
_ What’s frustrating you?
_ How do you want to feel when you walk into this space?
Because an extension isn’t just an investment in bricks and mortar. It’s an investment in how you live.
Get the plan right, and everything else falls into place.
Building an extension isn’t just about what you see - it’s about what’s hidden beneath the surface. Foundations, access, structural work, permissions - these are the things that can turn a simple project into a logistical puzzle.
A small extension on a clear, accessible site? That’s straightforward.
A two-storey addition on a tight London plot with limited access and party wall constraints? That’s a different story.
The complexity of a build isn’t always obvious at first glance, but it can add thousands to the budget. Here’s what you need to know.
Not all houses are built equal. Some sit on solid ground. Others? Not so much.
_ Soil Type Matters – If your house sits on clay-heavy soil, or worse, near a floodplain, your foundations might need extra reinforcement or piles. More work, more materials, more cost.
_ Access is Everything – A detached house with a driveway? Easy. A mid-terrace with no rear access? Suddenly, every material needs to come through the house. That means protective sheeting, manual handling, and slower progress.
_ London’s History – If your home is Victorian or Georgian, the structure might not be ready for a modern extension. Old houses = more surprises. And surprises = unexpected costs.
_ Listed Buildings & Conservation Areas – If your home falls under these rules, you’ll need special permissions. Sometimes, even the materials you use are dictated by conservation policies.
→ The Fix: A good architect will spot these challenges before you start. The right team makes all the difference.
If you share a wall with your neighbour, you’ll likely need a Party Wall Agreement.
_ This means hiring a surveyor (or two).
_ Negotiations can take weeks.
_ If your neighbour objects (dissents to the Party Wall Notice), it can delay everything.
If you’re planning to extend upwards or build close to a boundary, expect additional permissions, agreements, and approvals.
→ The Fix: Get advice early. A well-managed process avoids legal headaches later.
Taking out a back wall to create an open-plan kitchen? You’ll need a steel beam.
Adding a second storey? Your existing walls and foundations might not be strong enough.
Basement extension? That’s an engineering challenge of its own.
The more structural work needed, the more expensive the project becomes. Labour costs skyrocket when you’re dealing with:
_ Steelwork & Reinforcement – Bigger openings need bigger beams.
_ Load-Bearing Walls – Removing one? You’ll need calculations and structural reinforcement.
_ Roof Alterations – Raising ceilings, adding dormers - more complexity, more cost.
→ The Fix: A structural engineer is your best friend. Get the numbers right before you start knocking things down.
Everyone wants more natural light. Glass walls, skylights, floor-to-ceiling sliding doors - they look stunning.
But here’s what people don’t always realise:
_ The bigger the glass, the higher the structural demands.
_ You might need reinforced lintels or steelwork to support large openings.
_ High-performance glazing costs more - but it’s essential for energy efficiency.
Cheap glazing = heat loss = higher energy bills. Don’t cut corners here.
→ The Fix: Balance aesthetics with function. The right choices will save you money in the long run.
In London, space is tight. Your extension might affect daylight, privacy, or views for your neighbours.
And if they’re not happy? They can object to planning, delay progress, or trigger additional legal requirements.
If you’re in a terrace or semi-detached home, construction work will also impact them - noise, dust, scaffolding, builders arriving at 7 AM.
→ The Fix: Keep communication open. A well-handled conversation early on can prevent objections later.
Here’s what can go wrong (and often does):
_Hidden structural problems – Old timber joists that need replacing. Cracks that weren’t visible until walls came down.
_ Asbestos discoveries – If found, it has to be safely removed before work continues.
_ Drainage surprises – Your pipes may need rerouting or upgrading.
_ Material shortages – Prices fluctuate, and delays happen.
Even with the best planning, things crop up. The key is to budget for contingencies - typically 10-15% extra.
Difficult sites and structural challenges aren’t just obstacles. They’re opportunities to rethink how your home works.
Good design doesn’t just solve problems - it anticipates them.
_ The right structural changes can open up your home in unexpected ways.
_ The right materials can future-proof your space, making it more energy-efficient.
_ The right team can save you money by avoiding mistakes before they happen.
An extension isn’t just about adding more. It’s about making what you have work better.
And that? That’s where the real value lies.
Materials make or break a project. They dictate how your extension looks, how it feels, how it performs, and - crucially - how much it costs.
A simple brick box and a carefully crafted space made from sustainable, high-performance materials might both be called "extensions," but the experience of living in them? Worlds apart.
The question isn’t just what do you want it to look like? but how do you want it to work for you?
At its core, every extension needs walls, floors, and a roof. The materials you choose for each determine the cost, durability, and sustainability of your space.
_Traditional Brick & Block – The standard in London. Cost-effective, durable, and easy to match to existing properties.
_ Timber Frame – Faster to build, more sustainable, and great for energy efficiency.
_ Cross-Laminated Timber (CLT) – A modern, low-carbon alternative with stunning exposed wood interiors, but requires specialist knowledge.
_ Steel & Glass – Ideal for large, open spaces, but comes at a premium cost and high embodied carbon.
The key is finding the right balance between aesthetics, longevity, sustainability and performance.
→ Tip: The cheapest option upfront isn’t always the best in the long run. A well-insulated, airtight structure means lower energy bills and a more comfortable home for many years to come.
A big house that leaks heat isn’t just expensive - it’s uncomfortable.
In London, an extension must meet Building Regulations for insulation and energy efficiency, but just meeting the minimum isn’t enough.
Think of insulation as an investment in comfort and lower bills.
_ Standard insulation (basic mineral wool) → Cheaper, but bulkier.
_ High-performance insulation (PIR, wood fibre, or aerogel) → More expensive, but slimmer and better-performing.
_ Triple glazing vs. double glazing → Costs more upfront, but can drastically reduce heat loss and noise.
→ Tip: If you’re going to spend extra anywhere, spend it on insulation and glazing. A warm home in winter and a cool home in summer makes all the difference.
Windows are one of the most transformative elements of an extension. They bring in natural light, connect you to the outside, and make spaces feel bigger.
But the bigger the window, the bigger the cost.
_ A standard UPVC window? Cheap, but won’t win design awards.
_ Slimline aluminium frames? Sleek, modern, but double the price.
_ Bespoke steel-framed Crittall windows? Timeless and beautiful, but can cost £2,000 per square metre.
The same goes for doors:
_ Basic French doors → Affordable, functional.
_ Bifold doors → Create a seamless indoor-outdoor feel, but can be heavy and require more maintenance.
_ Sliding doors → Offer bigger panes of glass for uninterrupted views, but require strong supporting structure.
→ Tip: The orientation of your extension matters. A fully glazed south-facing room might overheat without shading solutions, adding extra costs for blinds or external louvres.
Flooring is often overlooked in the budgeting process, but it’s one of the biggest cost variables.
_ Concrete floors – A sleek, industrial look, but can be cold underfoot without underfloor heating.
_ Timber flooring – Warm, timeless, but varies wildly in price depending on species and finish.
_ Engineered wood – More stable than solid timber, but avoid the cheaper versions that scratch easily.
_ Tiles or polished concrete or microcement – Durable, great for kitchens and bathrooms, but installation costs can be high.
→ Tip: If you’re considering underfloor heating, certain flooring materials conduct heat better than others. Tiles and polished concrete work well, while thick carpets reduce efficiency.
How your extension looks from the outside is just as important as how it feels inside.
_ London stock brick → Classic and helps extensions blend with period homes, but reclaimed bricks are costly.
_ Timber cladding → Beautiful and sustainable, but requires maintenance.
_ Zinc or copper cladding → High-end, long-lasting, and striking, but expensive.
_ Render → Smooth and modern, but can stain over time.
Some materials age beautifully. Others? Not so much.
→ Tip: Cheap cladding looks great on day one. Five years later, you might regret it.
More and more people are thinking beyond aesthetics - they want homes that are efficient, comfortable, and intelligent.
_ Solar panels → Reduce electricity bills, but initial costs can be high.
_ Mechanical Ventilation with Heat Recovery (MVHR) → Keeps fresh air circulating while recovering heat, ideal for energy-efficient homes.
_ Smart lighting & heating → Systems like Lutron or Nest give you control and efficiency.
Sustainability isn’t just about being green - it’s about cutting costs in the long run.
→ Tip: Investing in energy-efficient systems pays back over time through lower energy bills and a more comfortable home.
An extension isn’t just an addition to your home. It’s a statement about how you want to live.
Do you want materials that last a lifetime or ones that need replacing in five years?
Do you want a home that feels good in every season, or one that’s either too hot or too cold?
Do you want a house that’s just bigger - or one that’s smarter, more beautiful, more you?
An extension isn’t just about bricks and mortar - it’s about the people who bring it to life.
You could have the best design in the world, but if the wrong team is executing it, your dream home could turn into a costly, stressful experience.
This isn’t just about hiring professionals. It’s about building a team that understands your vision, works together seamlessly, and helps you spend wisely - not just spend more.
A great team makes a project run smoother, stay on budget, and deliver a home that exceeds expectations. A bad one? That’s where delays, cost overruns, and disappointment creep in.
So, who do you really need on your side?
A good architect doesn’t just draw plans. They solve problems, unlock potential, and protect your budget.
They help you think beyond just “more space” and focus on better space - designing a home that works harder for you.
_ Translates your ideas into a smart, buildable design
_ Navigates planning permissions and regulations
_ Maximises light, flow, and functionality
_ Helps you balance cost, quality, and design
_ Provides detailed drawings that reduce builder guesswork (and unexpected costs)
_ Ensures sustainability and energy efficiency are integrated from the start
💡 Why it matters → A well-thought-out design means fewer expensive mistakes, better use of space, and a home that feels right - not just one that ticks boxes.
→ Pro tip: Your architect should be problem-solving before problems even arise. If they’re only focused on the aesthetics and not on how your home will actually function, you may be in for surprises later.
If the architect is the visionary, the structural engineer is the realist.
They make sure your home doesn’t just look good, but that it stands strong and lasts for generations.
Any time you’re removing walls, extending upwards, or using large amounts of glass, you’ll need an engineer to calculate the loads and ensure stability.
_ Ensures walls, beams, and foundations can handle new loads
_ Prevents future structural problems (subsidence, cracks, sagging roofs)
_ Specifies steel beams and reinforcements for open-plan designs
_ Advises on foundation solutions, especially for tricky London soil conditions
💡 Why it matters → Skimping on engineering could mean costly structural failures down the line. Get it right from the start.
The quantity surveyor (QS) isn’t just for big projects. If your extension budget is upwards of £250,000, a QS can save you thousands by keeping costs in check.
They help you understand where your money is going, so you can spend on what matters and cut back where it doesn’t.
_ Provides cost estimates early in the design phase (so you don’t get surprises later)
_ Breaks down costs for materials, labour, and contingencies
_ Identifies cost-saving alternatives without compromising quality
_ Prepares tender documents to get accurate builder quotes
_ Prevents overspending by tracking costs during construction
💡 Why it matters → Many homeowners set a budget based on guesswork. A QS helps turn that guess into a real financial plan - avoiding “hidden costs” and ensuring your money is working for you.
Your builder is on site every day. They bring the project from paper to reality.
A great builder doesn’t just follow drawings - they problem-solve, manage tradespeople, and keep the project moving efficiently.
_ They come recommended from previous projects (ideally ones similar to yours)
_ They provide a detailed, transparent quote (no vague estimates)
_ They have a clear project timeline and contingency plan
_ They communicate well and don’t cut corners
_ They work well with your architect and engineer
💡 Why it matters → The cheapest quote is rarely the best quote. A low-cost builder might be cutting corners, using lower-quality materials, or underestimating labour - leading to delays, cost overruns, and headaches down the line.
👉 Pro tip: Always check a builder’s previous work, client reviews, and contract details before hiring.
Even a simple extension has a lot of moving parts. Without good coordination, projects run over budget and take longer than expected.
That’s where a project manager (PM) comes in.
For smaller projects, your architect or builder may handle project management. But for more complex builds, having a dedicated PM can be a game-changer.
_ Manages timelines, budgets, and workflows
_ Ensures the right people are in the right place at the right time
_ Handles supplier delays and on-site troubleshooting
_ Acts as a bridge between the client, architect, and builders
💡 Why it matters → A well-managed project stays on budget, on time, and as stress-free as possible.
→ Pro tip: If you’re managing the project yourself, be prepared for a second full-time job. Even the best teams need someone to keep everything running smoothly.
A well-executed extension is never just about one person. It’s about a team that works together to make sure the design, budget, and build all align.
When things go wrong, it’s usually because one of these key players was missing - or because they weren’t working collaboratively.
→ Aligned Vision – Everyone understands the goal of the project, not just their part in it.
→ Clear Communication – Problems are solved before they become delays.
→ Respect for the Budget – No unnecessary costs, no surprise overspending.
→ A Commitment to Quality – Every decision is made with long-term value in mind.
Your extension isn’t just another project. It’s your home, your money, your time.
Who you choose to work with will determine whether it’s an exciting journey or a stressful, costly mistake.
So don’t rush it. Choose people who:
→ Get what you’re trying to achieve
→ Have the right experience
→ Care about the details
→ Are honest about costs and challenges
→ You actually enjoy working with
Because the right team doesn’t just build a house.
It’s not just bricks and mortar. The hidden costs can be just as important.
_ Planning Permissions – £206 (or £103 for a Lawful Development Certificate)
_ Party Wall Agreements – If your neighbours need to sign off on the work
_ Insurance – Standard home insurance won’t cover major works
_ VAT – 20% (unless your home has been vacant for 2+ years, then 5%)
The best home is the one that works for you.
The one where the morning light hits just right in your kitchen.
Where your kids have space to grow, where you can work without being in the middle of the living room or on the kitchen table.
Extensions aren’t just about adding square metres. They’re about crafting a home that fits the way you live and adapting to future needs.
And if done right? The value isn’t just financial - it’s how it makes you feel every day.
💡 Do you want more space, or do you want better space?
If it’s the second one, let’s talk.
We build homes that last. Homes that matter. Homes that make life better.
And that? That’s worth every penny.
If you would like to talk through your project with the team, please do get in touch at architects@risedesignstudio.co.uk or give us a call on 020 3947 5886
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