Discover the full extension cost breakdown 2025 — see how much each part of a single-storey build costs, from materials and labour to fees and extras.
Most homeowners only see the final number when they receive an extension quote — a single figure that can feel intimidating and confusing. But behind every total lies a detailed structure of costs: materials, labour, professional fees, and site expenses. Understanding this extension cost breakdown in 2025 isn’t just helpful — it’s essential if you want to control your budget and avoid being overcharged.
With material inflation, labour shortages, and tighter regulations continuing across the UK construction sector, 2025 has become the year when clarity matters more than ever. Whether you’re adding a kitchen extension, expanding your living area, or building a garden room, knowing how costs are distributed helps you negotiate smarter and plan with confidence.
At Builder Expert, we analyse real-world estimating data from UK projects to show exactly where your money goes — and how to reduce waste without cutting quality.
Average Extension Cost in 2025
The national average cost of a single-storey extension in 2025 ranges between £2,200 and £3,300 per m², depending on region, specification, and design complexity.
A typical 30 m² extension (e.g., open-plan kitchen/living space) could therefore cost between £66,000 and £99,000, excluding VAT and professional fees.
| Specification | Cost per m² | Estimated Total |
|---|---|---|
| Basic Finish | £2,200 / m² | £66,000 |
| Mid-Range | £2,700 / m² | £81,000 |
| High-End / London | £3,300 / m² | £99,000+ |
These figures are drawn from current UK market analysis and BCIS Construction Cost Indices, supported by data from the Office for National Statistics showing continued labour and material inflation across 2024–2025.
- Materials: ~40–50%
- Labour: ~30–40%
- Professional Fees: ~10–15%
- Site & Contingency: ~5–10%
Each plays a crucial role in the final figure — so let’s break them down.
Material Costs (40–50%)
Materials represent the largest portion of any extension. This includes foundations, brickwork, insulation, glazing, roofing, plaster, and finishes. In 2025, material prices remain volatile due to fuel and logistics costs.
Common Material Components
- Structural materials: concrete, steel, blockwork, and timber
- Envelope materials: roofing, insulation, render, cladding
- Interior finishes: plasterboard, flooring, tiles, paint
- Fixtures: doors, windows, fittings
According to BCIS, overall construction material prices rose by roughly 5–8% over 2024 due to supply disruptions and energy costs. Items like insulation, steel, and glazing saw the steepest increases.
A typical cost range:
| Component | Percentage | Approx. Cost (on £80,000 build) |
|---|---|---|
| Structure | 20% | £16,000 |
| Finishes | 15% | £12,000 |
| Fixtures | 10% | £8,000 |
| Roofing / Windows | 5% | £4,000 |
🧱 Materials – Plan Early, Buy Smart
In 2025, material costs account for nearly half of the average UK extension budget. With prices still fluctuating due to supply and energy costs, being proactive saves thousands.
Track your usage using digital estimating tools such as Builder Expert’s Construction Estimator. You’ll see in real time how each material category contributes to your extension cost breakdown 2025.
Compare suppliers — request at least three quotes and ask about trade discounts. Smaller independent builders’ merchants often beat national chains.
Standardise sizes — bespoke glazing or odd-sized doors can add 20–30% to your total. Stick to standard components wherever possible.
Bulk buy strategically — materials like blocks, plasterboard, or insulation are cheaper when ordered together, but make sure you have dry storage to avoid waste.
🧱 Materials
- Compare supplier quotes; smaller merchants may beat large chains.
- Choose durable mid-range finishes over luxury aesthetics that add little resale value.
- Order materials per stage to reduce on-site storage and damage risk.
👷 Labour
- Vet contractors thoroughly — check qualifications and insurance.
- Use clear written contracts specifying scope and payment stages.
- Schedule inspections to confirm progress before releasing next payments.
🧾 Fees & Permissions
- Consolidate design services with one professional to simplify accountability.
- Understand your local planning zone via the Planning Portal UK before submitting.
⚙️ Site & Contingency
- Conduct soil surveys and drainage checks before you start.
- Keep a 10% cash buffer accessible for any surprise expenses.
For a more advanced approach, explore Builder Expert’s estimating platform — it lets you simulate materials, labour, and fees under different scenarios to forecast total project spend.
How BuilderExpert Helps You Estimate Smarter
Most homeowners still rely on spreadsheets or outdated calculators. Builder Expert simplifies this entire process.
With Builder Expert’s estimating software you can:
- Build extension cost models in minutes using real UK rates
- Adjust material specifications and instantly see new totals
- Generate printable breakdowns for clients or contractors
- Compare project scenarios and monitor profit margins
That means fewer surprises, better cost control, and a far more confident build.
🏠 Case Study: 30 m² Single-Storey Kitchen Extension in Manchester (2025)
To illustrate how the extension cost breakdown 2025 works in practice, let’s look at a real-world-style example from a typical UK home.
Project: 30 m² single-storey rear kitchen-dining extension
Location: Manchester
Spec: Mid-range finish — aluminium bi-fold doors, tiled floor, underfloor heating
Duration: 14 weeks
💷 Budget Overview
| Category | % of Total | Cost (£) | Description |
|---|---|---|---|
| Materials | 46% | £33,500 | Foundations, blockwork, insulation, windows, plaster, tiles |
| Labour | 34% | £24,800 | Builder team, electrician, plumber, plasterer |
| Professional Fees | 12% | £8,700 | Architect, structural engineer, building control |
| Site & Contingency | 8% | £5,800 | Drainage reroute, waste removal, weather delays |
| Total Project | 100% | £72,800 |
🧱 Materials
Material prices rose slightly between quotation and order, mostly on insulation and plasterboard. The homeowner locked prices early with their supplier, saving roughly £1,200 versus the later BCIS index rise. Standard window sizes instead of custom openings avoided another £1,000.
👷 Labour
Labour costs were controlled through a stage-based payment plan — foundations, walls, roof, plaster, and final fix. The homeowner hired a main contractor who managed sub-trades, preventing gaps between electricians and plasterers. That shaved nearly a week off the schedule.
🧾 Professional Fees
Using an independent architect and engineer added clarity but also cost. The design included a steel beam for an open-plan kitchen, raising engineering fees to around £1,000. However, this structural clarity avoided site rework later, saving time and preventing wasted materials.
⚙️ Site & Contingency
Unexpectedly, the old drainage line ran under the new foundations. Redirecting it cost £2,400 — comfortably within the 10% contingency buffer built into the plan. Without that buffer, the project would have stalled for two weeks waiting for additional funds.
✅ Outcome
The finished extension came in £200 under budget and added an estimated £45,000 to the property’s resale value (based on RICS regional data).
🔍 Key Takeaways from the Case Study
- Early supplier agreements protect your extension cost breakdown 2025 from inflation shocks.
- Stage-based labour payments improve accountability and reduce idle time.
- Keeping a 10–15% contingency is non-negotiable — surprises always occur.
- Integrated cost tracking through digital estimating tools ensures transparency at every stage.
Conclusion
In 2025, transparency is the difference between a controlled project and a financial headache. Understanding your extension cost breakdown — materials, labour, professional fees, and site factors — gives you a foundation to budget realistically and negotiate confidently.
Remember:
- Materials will take about half your spend.
- Labour remains variable by region.
- Professional fees protect quality and compliance.
- Always hold a contingency fund.
Use this breakdown alongside our related guides:
- 👉 Single Storey Extension Cost in 2025 – for current UK cost per m²
- 👉 Single Storey Extension Budget 2025 – for planning and cost control
- 👉 Construction Estimating Software – for instant digital estimates
And for wider estimating insights:
- Construction Estimating Solutions – Builder Expert’s main pillar
- Building Estimating Software for the UK Industry – explore how automation saves time and money
Stay informed, plan smart, and build with confidence — with Builder Expert, your estimating partner for the UK construction industry.