So, Is Building a House Extension Expensive?
If you’re considering more space at home but are unsure of the financial commitment, you’re probably asking: is building a house extension expensive? The honest answer is—it can be. But how expensive it becomes depends on a wide range of factors, from the size and complexity of the build to the materials used and whether planning permission is required.
In this blog, I’ll break down what influences extension costs, how to keep spending under control, and why many homeowners still see extensions as a worthwhile investment.
What Is the Average Cost to Build a House Extension?
The cost of building a house extension in 2025 typically falls between:
- £1,400 – £2,000 per m² for a basic finish
- £2,000 – £2,500+ per m² for a mid-to-high-spec build
So, a modest 20m² extension could cost around £28,000 at the lower end and upwards of £50,000 depending on choices. Double-storey extensions cost more but may offer better value per square metre since roofing and foundations are shared.
Factors That Influence the Cost of Building an Extension
1. Size and Complexity
Naturally, the larger and more structurally complex the extension, the higher the cost. A simple rear extension is more straightforward than a wraparound or double-storey build.
2. Location and Access
Projects in areas like Kent may have slightly higher labour and material costs than the national average. Difficult access to the site or work on sloped land can also push up groundwork costs.
3. Materials and Finishes
Bespoke glazing, natural stone, and custom kitchens all raise the budget. Meanwhile, off-the-shelf units and standard materials offer savings without compromising function.
4. Planning and Design Requirements
If your extension falls outside permitted development rights, you’ll need planning permission. This adds design, application, and potentially revision costs.
We help with this through our dedicated planning services, managing applications, drawings, and communication with local authorities.
What Else Might Add to the Cost?
Be aware of hidden or additional costs that can creep in:
- Architect fees and structural engineering reports
- Party wall agreements (common in terraced or semi-detached homes)
- Drainage upgrades or utility connections
- Upgrades to your existing heating or electrical systems
- Waste removal or landscaping post-build
- VAT on labour and materials
If you’re managing the build yourself, project management time should also be considered.
How to Keep Extension Costs Down
1. Stick to a Simple Design
A rectangular or square footprint is the most cost-efficient. Complex shapes or unusual rooflines increase both labour and materials costs.
2. Limit Structural Changes to the Existing Property
The more internal walls, plumbing, or services you move, the higher your costs. Keep your new layout as close to the original as possible unless there’s a good reason to do otherwise.
3. Get a Clear, Itemised Quote
Work with experienced builders who provide detailed quotes. This makes it easier to see where money is being spent and where savings might be possible.
4. Allow for a Contingency
Always budget 10–15% above your estimated cost to cover unexpected changes. This keeps the project moving if issues arise.
Is It Worth the Expense?
In most cases, yes. Although extensions can be costly, they’re still far cheaper than moving when you factor in stamp duty, estate agent fees, legal costs, and removals.
A well-built extension can also increase your home’s value significantly—particularly if it adds a new bedroom, bathroom, or open-plan living space. Done correctly, it’s an investment in both your property and your quality of life.
Why Use Local Builders for Your Extension?
Choosing experienced local professionals offers several advantages:
- Better understanding of local planning policies and property styles
- Faster site visits and quicker problem-solving
- Reliable access to local suppliers and subcontractors
- Trusted community reputation—check our reviews on Google My Business and Trust a Trader
We also offer tailored support beyond the build, including full aftercare to ensure your extension is finished to the highest standard.
How We Can Help
We’ve helped many homeowners across Kent design and build functional, high-quality extensions that work for both their lifestyle and budget. Our service includes:
- Transparent pricing and staged payments
- Full planning and construction support
- Flexible design input based on your goals
- Ongoing assistance after the project is complete
Contact us here to get started with a no-obligation quote and expert advice for your extension project