Extension Budget Breakdown: Where Your Money Actually Goes
You've got a quote for £80,000 for a 25m² extension. Or maybe £120,000. Perhaps £150,000. Why is there such variation, and what exactly are you paying for? More importantly, what costs will emerge that nobody mentioned in the initial quote?
Here's a detailed breakdown based on real 2026 extension costs, so you know where every pound goes and what contingency you actually need.
The Headline Figure: £2,500-3,500 per m²
In 2026, typical rear extensions cost between £2,500 and £3,500 per square metre for a mid-spec finish. That means:
- 25m² (5m x 5m) extension: £62,500 - £87,500
- 40m² (8m x 5m) extension: £100,000 - £140,000
- 60m² (two-storey): £150,000 - £210,000
But this is misleading. The real question isn't "how much per square metre?" but "what's included and what's not?"
Critical Point: Most quotes don't include everything. Assume 15-20% on top of the builder's quote for the items they've legitimately excluded plus unforeseen costs.
Detailed Cost Breakdown: 40m² Single-Storey Extension
Let's break down a real example: a 40m² rear extension with bi-fold doors, underfloor heating, and mid-range finishes. Total budget: £120,000.
| Category | Cost | % of Total |
|---|---|---|
| Professional Fees | £12,000 | 10% |
| Architect (design & drawings) | £4,500 | |
| Structural engineer | £2,500 | |
| Building control | £1,500 | |
| Party wall surveyor (if needed) | £3,000 | |
| Project management / contingency | £500 | |
| Groundworks & Foundations | £14,000 | 12% |
| Excavation & soil removal | £3,500 | |
| Foundations (concrete, reinforcement) | £6,000 | |
| Damp proof course & membranes | £1,200 | |
| Drainage & connections | £2,500 | |
| Ground floor slab & insulation | £800 | |
| Structure & Shell | £22,000 | 18% |
| Blockwork & brickwork | £8,500 | |
| Steel beams & installation | £5,500 | |
| Wall insulation & cavities | £3,000 | |
| External render/facing | £5,000 | |
| Roof | £11,500 | 10% |
| Flat roof structure & insulation | £4,500 | |
| Roof membrane (EPDM/GRP) | £3,000 | |
| Guttering & downpipes | £800 | |
| 3x roof lights (Velux-style) | £3,200 | |
| Windows & Doors | £16,000 | 13% |
| 4m bi-fold doors (aluminium) | £8,500 | |
| 2x side windows | £2,500 | |
| Internal door to existing house | £1,200 | |
| Installation & finishing | £3,800 | |
| Electrical | £6,500 | 5% |
| Consumer unit upgrade | £1,200 | |
| LED downlights (15x) | £1,800 | |
| Sockets & switches (20x) | £1,500 | |
| Underfloor heating wiring | £800 | |
| Testing & certification | £1,200 | |
| Plumbing & Heating | £9,000 | 8% |
| Water-based underfloor heating | £4,000 | |
| Manifold & controls | £1,500 | |
| Radiator relocation (existing house) | £800 | |
| Kitchen sink & waste | £1,200 | |
| Boiler servicing & adjustments | £1,500 | |
| Plastering & Finishes | £8,500 | 7% |
| Plasterboard & skim coat | £5,000 | |
| Coving/architraves | £1,200 | |
| Decoration (paint, materials) | £2,300 | |
| Flooring | £4,500 | 4% |
| Screed for underfloor heating | £2,000 | |
| Engineered wood flooring | £2,000 | |
| Floor preparation & leveling | £500 | |
| Building Services | £6,000 | 5% |
| Scaffolding (6-8 weeks) | £2,500 | |
| Skip hire (4x) | £1,200 | |
| Site protection & cleaning | £800 | |
| Temporary weather protection | £1,500 | |
| Contingency & Extras | £10,000 | 8% |
| Unforeseen issues (realistic) | £6,000 | |
| Small variations & upgrades | £4,000 | |
| TOTAL PROJECT COST | £120,000 | 100% |
What's NOT Included (But You'll Still Pay For)
This is where budgets blow up. Items "not included" in most builder quotes:
Kitchen Fitting
Builder quotes typically exclude the kitchen itself. Budget:
- IKEA kitchen (DIY install): £4,000-8,000
- Mid-range fitted: £12,000-18,000
- High-end bespoke: £25,000+
Appliances
Oven, hob, extractor, dishwasher, fridge: £2,500-6,000 depending on brands.
Furniture & Soft Furnishings
Dining table, chairs, lighting fixtures, blinds, rugs: £3,000-10,000+
Landscaping
Builders destroy your garden. Reinstating patio, lawn, fencing: £5,000-15,000
Temporary Living Costs
If kitchen is out of action for 12+ weeks:
- Microwave, kettle, camping stove setup
- Eating out more frequently: £200-400/month extra
- Potential hotel stays during noisy phases
Reality Check: Your £120,000 extension becomes £145,000+ when you include everything. Always budget 20-25% more than the builder's quote for the complete project.
Where Can You Save Money?
Here are legitimate ways to reduce costs without compromising quality:
1. Flat Roof Instead of Pitched (£3,000-5,000 saving)
Modern flat roofs are highly effective. Save on structure, tiles, and labour while getting a contemporary look.
2. Standard Windows Instead of Bi-Folds (£4,000-6,000 saving)
Large sliding doors or French doors give similar light and access at half the price. Bi-folds are beautiful but expensive.
3. Self-Supply Major Items (£2,000-4,000 saving)
Buy your own windows, doors, and roof lights. Builders mark these up 20-30%. Just coordinate delivery carefully.
4. Simpler Roof Lights (£1,500-2,000 saving)
Fixed Velux windows instead of opening or electric ones. You get the light without the premium cost.
5. Electric UFH Instead of Water-Based (£2,000-3,000 saving upfront)
But remember: higher running costs forever. Only worth it for small extensions used occasionally.
6. Reduce Excavation Depth (£1,000-2,000 saving)
Work with existing ground levels where possible. Every additional meter down costs significantly in groundworks.
What NOT to Cheap Out On
- Structural steel – never downgrade specifications
- Foundations – cutting corners here is catastrophic
- Insulation – saves money long-term on heating
- Damp proofing – fixing later costs 10x more
- Electrical work – safety isn't negotiable
Hidden Costs Nobody Warns You About
These aren't in quotes but they're real costs that hit every project:
Upgrades During Build (£3,000-8,000)
Once you see the space taking shape, you'll want nicer tiles, better lights, extra sockets. It always happens.
Existing House Repairs (£2,000-5,000)
Knocking through reveals issues:
- Asbestos removal from old Artex ceiling: £800-1,500
- Damp treatment to exposed walls: £1,000-2,000
- Rewiring old circuits that don't meet current regs: £1,500-3,000
- Structural issues discovered when opening walls: variable
Planning & Regulation Changes (£500-2,000)
Building control might require upgrades to existing house (fire doors, ventilation) to meet current standards.
Ground Conditions (£2,000-10,000)
Clay soil requiring deeper foundations, old wells or cellars discovered, underground streams, contaminated land – these are rare but project-killers.
Neighbour Issues (£1,000-5,000)
Party wall surveyors, damage claims, access disputes. Even friendly neighbours can create unexpected costs.
The 20% Rule: Whatever your builder quotes, add 20% for excluded items and unforeseen costs. If you can't afford that 20%, you can't afford the project yet.
Payment Schedule: When Money Leaves Your Account
Understanding cash flow helps with financing. Typical payment schedule for a 12-week build:
| Stage | Payment | Timing |
|---|---|---|
| Professional fees deposit | £3,000 | Before work starts |
| Builder deposit | £12,000 | Week 0 |
| Foundations complete | £25,000 | Week 2-3 |
| Structure & roof watertight | £30,000 | Week 6 |
| First fix complete | £25,000 | Week 9 |
| Practical completion | £15,000 | Week 12 |
| Kitchen, appliances, extras | £10,000 | Week 12-14 |
| TOTAL | £120,000 |
Notice the heaviest payments come in weeks 2-6. Make sure your financing is in place before you start.
How Location Affects Cost
Same extension, wildly different prices:
- London & South East: £3,000-4,000/m² (top end of market)
- Major cities (Manchester, Bristol, Edinburgh): £2,500-3,200/m²
- Suburbs & towns: £2,200-2,800/m²
- Rural areas: £2,500-3,500/m² (can be higher due to access/logistics)
London premium isn't just labour—materials cost more due to logistics, parking charges, and congestion costs.
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Two-Storey Extensions: The Economics
Adding a second floor doesn't double the cost, but it's not cheap:
- Single-storey 40m²: £120,000 (£3,000/m²)
- Two-storey 80m²: £180,000-200,000 (£2,250-2,500/m²)
Why cheaper per m²?
- Share same foundations and groundworks
- No roof on ground floor (becomes first floor)
- Scaffolding already in place
Additional costs for second storey:
- Stronger foundations and floor structure: £8,000
- Additional scaffold and access: £1,500
- Bathroom plumbing and fitting: £6,000-12,000
- Staircase (if new access needed): £3,000-5,000
If you have space to go up, the cost per square metre makes it worthwhile—you nearly double your space for 60-70% more cost.
Final Budget Checklist
Before you commit to your extension, ensure you've budgeted for:
- ✓ Builder's quote (the base number)
- ✓ Professional fees (architect, engineer, building control)
- ✓ Kitchen and appliances
- ✓ Flooring throughout
- ✓ Decoration and finishing touches
- ✓ Furniture and lighting
- ✓ Garden reinstatement
- ✓ 15-20% contingency for unforeseen issues
- ✓ Temporary living costs
If your builder says £100,000, budget £125,000 minimum for the complete, livable space.
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