How Much Will My Extension Cost?
Ask ten builders what an extension costs and the honest answer will sound frustrating: it depends. That is not because builders are being vague. It is because an extension is not a product with a fixed shelf price. It is a construction project shaped by your home, your ground conditions, your design, your specification and the way the work is managed.
Price-per-square-metre guides can be useful as a first reference point. Many UK renovation websites, builder directories and architect-led cost guides give broad bands for single-storey and two-storey extensions. They can help you understand whether your budget is broadly realistic.
But relying on a simple “£X per m²” figure too early can lead to disappointment. Two extensions with the same floor area can have very different costs.
The better approach is to use early cost guidance as a starting point, then bring a preferred builder into the design conversation before every decision is fixed.
Why price per metre became popular
Price per square metre is popular because it feels simple. If an extension is 30 m² and a guide says extensions often cost a certain amount per m², the calculation looks quick and tidy.
Homeowners like it because it gives an early answer. Designers use it for feasibility conversations. Some builders use it when a project is too undeveloped for a proper quote.
It can be useful at the very start, especially when comparing broad options such as:
A small rear kitchen extension
A wraparound extension
A two-storey side extension
A loft conversion versus a ground-floor extension
A modest build compared with a highly glazed design
The problem comes when that rough figure starts to feel like a promise.
A square metre rate is usually based on assumptions. Those assumptions may not match your house. They may include a basic specification. They may exclude VAT, professional fees, structural work, drainage changes, kitchen fittings, glazing upgrades, landscaping and decoration.
A rate may also be based on a “typical” extension, yet very few real projects are typical.
What UK cost guides can and cannot tell you
Many UK cost guides from renovation websites, cost consultants, architects and trade comparison sites make the same broad point: extension costs vary widely. They often separate by build type, quality level and region.
Common themes across these guides include:
Smaller extensions often cost more per m² than larger ones
London and the South East are usually more expensive
Kitchens and bathrooms increase costs because of plumbing, electrics and fittings
Structural steel, large openings and glazing can add a lot
VAT, consultant fees and permissions need separate allowance
A contingency is sensible because unknowns are common
That information is useful, but it is not a substitute for a project-specific budget.
For example, a 20 m² rear extension with a simple flat roof, standard doors and minimal structural work is not the same as a 20 m² kitchen extension with a large steel beam, sliding doors, rooflights, underfloor heating, bespoke kitchen units and drainage alterations.
On paper, both are 20 m². On site, they are very different jobs.
A square metre rate measures size. It does not measure complexity.
Why two extensions of the same size can cost different amounts
The biggest mistake is thinking floor area is the main driver of cost. It matters, but it is only one part of the picture.
Ground conditions can change everything
Before anything is built, the ground has to support it. Some sites are straightforward. Others need deeper foundations, extra excavation, specialist drainage work or care around trees.
Clay soil, made ground, nearby drains and older foundations can all affect the build method. If the builder opens the ground and finds something unexpected, the cost can move quickly.
This is why early site investigation and honest budgeting matter. A neat drawing cannot remove physical risk from the plot.
Structure is often more expensive than expected
Open-plan living is one of the main reasons people extend. Big kitchen-diners, wide openings and seamless links to the garden are popular for good reason.
They also need structure.
Removing load-bearing walls, inserting steel beams, supporting upper floors and forming large openings all add labour, materials and design input. The extension may not be large, but if it changes how the existing house is supported, the structural cost can be significant.
A simple box with small openings may be cheaper than a smaller extension that needs complex steelwork.
Glazing choices have a major effect
Doors, rooflights and windows can shift the budget more than many people expect.
A standard window is one thing. Large sliding doors, slim-frame glazing, roof lanterns and oversized rooflights are another. They can look brilliant, but the cost is not only the product. You also need to allow for structural support, installation, weatherproofing, lead times and finishing.
Glazing affects heat loss too, so Building Regulations can influence the specification.
Kitchens and bathrooms are not priced by floor area
A 10 m² utility room and a 10 m² high-end kitchen are the same size. They will not cost the same.
Kitchens, bathrooms and utility spaces bring in:
Plumbing
Waste pipes
Ventilation
Electrical circuits
Lighting design
Tiling
Cabinets
Worktops
Appliances
Sanitaryware
The build area may be small, but the fit-out can be expensive. This is one reason a price per metre can feel misleading. It spreads the cost evenly across the plan, when the spending is concentrated in specific zones.
Access and site setup matter
Some projects are easy to build. Materials can be delivered close to the work area, skips can sit nearby and trades can move around safely.
Other homes have narrow side access, shared driveways, parking restrictions or terraced layouts where everything must be carried through the house. That takes time. Time costs money.
Scaffolding, temporary protection, site welfare, skips, storage and security all form part of the cost. These are sometimes called preliminaries, and they are easy to overlook in early budgets.
Specification decides the finish
The phrase “builder’s finish” can mean different things to different people. One quote may include plastered walls and basic electrics. Another may include decoration, flooring, fittings and a finished patio threshold.
The specification needs to be written down clearly. Without it, you may compare quotes that are not actually comparable.
What should be included in a realistic extension budget
A proper extension budget should look beyond the shell. The structure is only part of the spend.
Cost area | What it may include | Why it matters |
Design and surveys | Architect, measured survey, structural engineer | Needed before accurate pricing |
Permissions | Planning, lawful development certificate, Building Regulations | Depends on the design and location |
Groundworks | Excavation, foundations, drainage | Often uncertain until investigated |
Structure | Walls, roof, steels, insulation | Core construction cost |
External envelope | Windows, doors, rooflights, render, brickwork | Drives both cost and performance |
Services | Electrics, heating, plumbing, ventilation | Must suit the new layout |
Interior finish | Plastering, flooring, decorating, joinery | Often excluded from rough rates |
Kitchen or bathroom fitting | Cabinets, appliances, tiling, sanitaryware | Can vary from modest to premium |
External works | Patios, steps, drainage channels, making good | Commonly forgotten |
Contingency | A sensible allowance for unknowns | Reduces panic when issues appear |
This table is not a quote, but it shows why a single metre rate can hide too much.
A good builder will want to know what is included, what is excluded and what still needs choosing. If the answer is unclear, the price will either include risk or leave gaps that appear later.
Why early builder involvement can save money
Many homeowners follow a linear process. They appoint a designer, develop drawings, apply for permissions, complete technical details, then ask builders to price the work.
That can work, but it has a weakness. By the time a builder sees the design, many decisions may already be locked in.
A better route is to open a conversation with a preferred builder during the design stage. This does not mean skipping proper design. It means using practical build knowledge before the project becomes harder to change.
A builder can help test decisions such as:
Whether a large opening is worth the structural cost
Whether a rooflight layout is sensible
Whether drainage routes look difficult
Whether access will affect labour time
Whether materials have long lead times
Whether a detail is expensive to build for little visual gain
Whether the budget matches the desired finish
This early input can protect the design. It can also stop the project drifting beyond the budget before anyone has priced it properly.
How to talk to a builder before the design is finished
You do not need a full tender pack to have a useful early conversation. You do need enough information for the builder to understand the direction of the project.
Bring together:
Existing floor plans if you have them
Sketches or concept drawings
Photos of the house and access
A rough idea of the finish you want
Any known constraints, such as drains, trees or shared walls
A realistic budget range
Your ideal timescale
Be open about budget. Some people hold back because they worry the builder will simply spend it all. A good builder needs the budget to advise properly. If the design is heading towards a higher-cost route, you want to know early.
The best question is not “What is your price per metre?”
Ask better questions:
“What parts of this design are likely to drive cost?”
“Where might we simplify without spoiling the result?”
“What is missing before you can price this properly?”
“What allowances would you suggest at this stage?”
“Where do projects like this usually run into surprises?”
“Would you price this as a fixed quote later, once the drawings are complete?”
Those questions lead to a proper discussion rather than a false number.
The difference between an estimate, a budget and a quote
These words often get mixed up, but they mean different things.
An estimate is an informed view based on limited information. It can help with feasibility, but it will change as the design develops.
A budget cost is a more structured allowance. It may use drawings, assumptions and known choices, but it still contains unknowns.
A quote should be based on clear drawings, specification, site knowledge and agreed inclusions. It should explain what is included, what is excluded and how changes will be handled.
Early in the design stage, expecting a fixed quote is unrealistic. Later in the process, accepting a vague estimate is risky.
The goal is to move from rough guidance to a clear cost plan as the project becomes more defined.
Why cheap metre rates can become expensive
A low headline figure is tempting. It may also be incomplete.
A cheap initial rate can leave out:
VAT
Structural steel
Drainage alterations
Kitchen fitting
Utility connections
Decoration
Flooring
Waste removal
Scaffolding
Building Control fees
Making good to the existing house
Some exclusions are reasonable, as long as they are clear. Problems start when the homeowner assumes something is included and the builder assumes it is not.
This is why comparing builders only by a square metre rate can be unfair to everyone. One builder may include a fuller scope. Another may look cheaper because key items sit outside the number.
Ask for clarity before judging value.
How design decisions affect cost without adding space
Some of the most expensive choices do not increase the floor area at all.
A few examples:
Moving a kitchen to the other side of the house
Creating a flush threshold to the garden
Choosing oversized glazing
Matching existing brickwork in detail
Adding multiple rooflights
Installing underfloor heating
Hiding steelwork within the ceiling zone
Reworking drainage runs
Upgrading insulation beyond the minimum
Adding bespoke joinery
None of these are wrong. Many may be worth it. The point is to choose knowingly.
A preferred builder can talk through these items while the design is still flexible. That gives you the chance to spend money where it improves the result, rather than where it simply solves a late problem.
A better way to plan your extension budget
Start with broad cost guides if you need a sense of scale. They are useful for early thinking. Then move quickly towards project-specific information.
A sensible process looks like this:
Define what you want the extension to achieve.
Set a budget range, including fees, VAT and contingency.
Develop concept drawings.
Speak to a preferred builder about buildability and cost drivers.
Adjust the design before it becomes too fixed.
Complete planning or lawful development steps if needed.
Prepare technical drawings and a written specification.
Ask for a detailed quote with clear inclusions and exclusions.
Keep a contingency for unknowns and client changes.
The strongest projects usually have good communication early. The designer understands the brief. The builder understands the site. The homeowner understands what drives cost.
That does not remove every surprise, but it reduces the avoidable ones.
The real answer to how much your extension will cost
Your extension cost will depend on size, yes. But it will also depend on structure, ground conditions, access, specification, services, glazing, finishes, location and timing.
A price per metre can help you start the conversation. It should not be the tool that decides the project.
If you want a reliable answer, get the right people talking while the design is still open. Share the budget. Test the expensive decisions. Ask what is missing. Turn assumptions into written choices.
The best cost advice rarely comes from a neat online average. It comes from a clear design, a realistic specification and a builder who has looked carefully at the actual work.
Ready to Extend Your Home?
We hope this has given you a clearer picture of extension costs! It's a significant investment, but one that can truly transform your living space and enhance your home life. If you're in Newton Abbot or the surrounding South Devon area and are dreaming of an extension, don't hesitate to get in touch with us.
About the Author:
Paul Moores is the Managing Director of Moores Construction, a premier residential builder based in South Devon. A time-served carpenter and joiner with over 20 years of hands-on trade and project management experience, Paul specializes in home extensions, structural alterations, full property refurbishments, and architectural collaborations.
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