How Much Does a House Extension Cost? Why There’s No One-Size-Fits-All Answer 🏡
It’s one of the most common questions we get asked:
“How much does an extension cost?”
And whilst we completely understand why people ask — the honest answer is:
It depends entirely on the project.
We know that’s not the exciting answer people want to hear, but giving a fixed figure without understanding the specifics of your property can be hugely misleading.
Every project is different, and there are multiple factors that can significantly impact overall cost.
📐 Size Is Just One Part of the Equation
Naturally, larger extensions tend to cost more — but square footage alone doesn’t tell the full story.
A straightforward rear extension will look very different in cost compared to:
A double-storey extension
A project involving major structural alterations
A build with premium finishes
Properties with difficult access
Two extensions of the same size can vary significantly depending on complexity.
🏗️ Structural Complexity
This is often where costs can increase.
Examples include:
Steel beams
Removing structural walls
Complex roof designs
Large glazing systems
Bespoke structural elements
The more structural work involved, the more planning and labour is required.
🚧 Groundworks & Hidden Site Challenges
Groundworks can vary massively depending on the property.
Things like:
Existing foundations
Drainage routes
Ground conditions
Utility infrastructure
We’ve worked on projects involving approvals and coordination with organisations such as:
Severn Trent
Cadent
National Grid
Local authorities
These requirements can add both cost and time depending on the project.
🎨 Specification & Finishes
This is one of the biggest variables.
There’s a huge difference between:
Standard windows vs bespoke glazing
Basic kitchen finishes vs high-end bespoke kitchens
Standard flooring vs premium materials
Your chosen finish level will heavily influence final costs.
📋 Planning, Building Control & Professional Fees
Many homeowners forget to factor in:
Architectural drawings
Structural engineer calculations
Building control
Planning-related costs (where applicable)
Party wall considerations
These are important parts of the wider project budget.
So... What’s the Rough Cost?
Whilst every project is different, as a very broad guide:
Smaller extensions often start from £40,000+
Mid-size projects often fall between £60,000–£120,000+
Larger, high-spec or more complex projects can exceed this significantly
These figures are broad and should never be treated as fixed project costs.
Why We Carry Out Surveys
The only way to provide accurate guidance is by understanding your property properly.
This is why we offer on-site surveys with Lewis, our Managing Director.
During the survey we can assess:
The property itself
Access limitations
Structural considerations
Your goals for the space
Any potential challenges early on
This allows us to provide far more accurate guidance based on your actual project — not guesswork.
🔨 Every Project Is Different
Extensions are significant investments and no two are ever the same.
The right planning, design, and construction team can make the difference between a smooth project and a stressful one.
Thinking About Extending Your Home?
If you’re considering an extension, renovation or structural alteration and want realistic advice based on your specific property, get in touch to arrange a survey.
📱 Follow our work: @loreillycarpentry
📧 Book a survey directly via our website
— L.O’Reilly Carpentry & Joinery