Quick answer: The answer is almost never what the first builder tells you.
If you’ve started researching a home extension, you’ve probably seen prices ranging from £20,000 to over £200,000. That’s a huge difference, and it leaves many homeowners wondering what they should actually expect to pay.
Reddit provides a useful snapshot of real homeowner experiences, particularly when combined with professional advice, multiple quotations and a clear project brief.
In this guide, we’ll look at the typical costs of home extensions in 2026, the factors that influence pricing, the hidden expenses many people forget, and the lessons homeowners repeatedly share online.
If you’re still at the planning stage, we also recommend reading our previous guide: Everything You Need to Know Before Starting a Home Extension.
Planning a home extension?
Create a clearer brief, compare suppliers and understand your likely project costs before committing.
Average Home Extension Costs in 2026
Although every project is unique, these are realistic starting points for standard quality work.
| Extension Type | Typical Cost |
|---|---|
| Small rear extension | £35,000 to £65,000 |
| Medium rear extension | £60,000 to £100,000 |
| Large rear extension | £90,000 to £180,000 |
| Side return extension | £45,000 to £90,000 |
| Wrap around extension | £90,000 to £180,000 plus |
| Double storey extension | £90,000 to £250,000 plus |
These figures usually include the structural work, but may not include kitchens, bathrooms, landscaping, decorating or professional fees.
Why Do Quotes Vary So Much?
Many homeowners are surprised to receive quotations that differ by tens of thousands of pounds.
This usually comes down to ground conditions, foundations, steelwork, access, finish quality, electrical work, plumbing, glazing, roof design, builder overheads and location.
A builder pricing a simple rectangular extension on flat ground may legitimately quote far less than another pricing a more complex design with difficult access.
The cheapest quote is not always the best value.
What Homeowners Say They Actually Paid
Across homeowner discussions online, several common themes appear. Many people report exceeding their original budget once additional work, specification changes or unexpected site issues arise.
Kitchens, glazing, flooring, decoration and landscaping are also frequently underestimated. These costs can make a major difference to the final figure.
The consistent message is simple: plan for contingency.
Hidden Costs That Catch People Out
Many extension budgets focus only on construction. However, homeowners often face additional expenses including architect fees, structural engineers, planning applications, Building Control, Party Wall Surveyors, drainage alterations, tree surveys, utility diversions, storage, decorating, flooring, furniture, garden repairs, patio reinstatement and fencing.
These costs can easily add another 10 to 20 percent to the overall project.
Why Comparing Quotes Is Difficult
Two quotations might both say “rear extension”, but they are rarely pricing exactly the same specification.
Before comparing prices, check whether insulation, windows, roof covering, flooring, decoration, kitchen installation, waste removal, skip hire, project management and VAT are included.
Until these points are clarified, comparing prices can be misleading.
The Importance of a Detailed Brief
Professional builders can only accurately price the information they are given. The more detail you provide, the more accurate your quotations become.
A good project brief should explain your goals, approximate dimensions, style preferences, budget expectations, finish quality, desired completion date and any special requirements.
This also reduces misunderstandings later.
Do Not Forget the Interior
Many people focus entirely on building the extension, then discover they still need lighting, flooring, bespoke furniture, wall finishes, artwork, storage, curtains and landscaping.
These elements often have a huge impact on the finished result.
Ways to Reduce Costs Without Sacrificing Quality
Homeowners often save money by choosing simpler roof designs, keeping plumbing in similar locations, using standard sized glazing where possible, planning the entire project before work begins, obtaining multiple quotations, buying certain finishes separately and avoiding unnecessary specification changes during construction.
The biggest savings usually come from careful planning rather than cutting corners.
Is an Extension Worth It?
Financially, the answer depends on your location and the value added to your property.
Many homeowners choose an extension because they love their current home, neighbours, local schools or area.
Moving can involve Stamp Duty, estate agent fees, legal costs, removal costs and renovation of the new property. In many cases, extending makes more sense than moving.
Questions to Ask Every Builder
- Are you fully insured?
- Who manages the project?
- What is excluded?
- What happens if costs increase?
- What is the payment schedule?
- Can I visit previous projects?
- How are variations handled?
- Who arranges Building Control inspections?
- What is the expected programme?
The answers often tell you as much as the quotation itself.
Need help turning your idea into a clear brief?
A clearer brief makes it easier to compare builders, designers, suppliers and specialist makers.
How Commission It Can Help
One of the biggest frustrations homeowners mention is trying to compare different builders, designers and specialists who all provide quotations in different formats.
Commission It helps simplify the early planning process by allowing homeowners to describe their project in one place, develop a structured brief, explore design ideas, estimate likely costs and request quotations from suitable professionals.
Instead of repeatedly explaining your project to multiple suppliers, you can build a detailed specification once and refine it as your ideas evolve.
Whether you are planning a rear extension, bespoke kitchen, custom furniture or specialist finishes, having a clear brief makes it easier for professionals to provide accurate quotations.
Frequently Asked Questions
How much should I budget for contingency?
Most professionals recommend allowing around 10 to 15 percent for unforeseen costs, particularly on older properties.
Is planning permission always required?
Not always. Some extensions fall under Permitted Development rights, although Building Regulations approval is still usually required.
Should I get three quotes?
Yes. Three to five detailed quotations usually provide a good understanding of the market.
Can I save money by managing the project myself?
Some homeowners do, but it requires significant time, organisation and experience. Mistakes can easily outweigh any savings.
What is the biggest budgeting mistake?
Underestimating the cost of kitchens, flooring, decorating and external landscaping after the extension has been completed.
Final Thoughts
Every extension is unique, but the same lessons appear repeatedly from homeowners who have already completed one.
Plan thoroughly. Allow a contingency. Compare detailed quotations rather than headline prices. Think about the entire project, not just the walls and roof.
Most importantly, spend time developing a clear brief before requesting quotations. Better planning almost always leads to better prices, fewer surprises and a smoother build.
If you are ready to start your own project, Commission It can help you define your requirements, compare specialist suppliers and turn your ideas into a well planned project with confidence.
