House Extension Project Details
📊 Extension Construction Breakdown
Planning a single-story rear kitchen extension, two-story side extension, or wraparound extension in the UK requires establishing a realistic construction budget per square metre (sq m).
According to RICS BCIS data, average UK house extension build costs range from £1,800 to £2,500 per square metre for basic to standard finishes (rising to £3,000+ per sq m in Greater London).
⚙️ UK House Extension Cost Benchmarks per Sq M for 2026/27
1. Extension Type Cost Comparison (£ per sq m)
- Single-Story Rear / Side Extension: £1,800 to £2,400 per sq m (Basic to High-End finish).
- Two-Story Extension: £1,500 to £2,000 per sq m (Economies of scale apply: foundations and roof structure costs are shared across two levels).
- Wraparound Extension (Rear + Side L-Shape): £2,200 to £2,800 per sq m (Requires significant structural steelwork to support upper floors).
- Basement Excavation Extension: £2,800 to £4,000+ per sq m (Requires extensive underpinning, tanking, and structural engineering).
2. Regional UK Location Multipliers
Labor and subcontractor rates vary significantly across the UK:
- Greater London & South East England: Add a +25% to +35% regional cost premium.
- Midlands & East Anglia: 1.0x (National Average).
- North of England, Wales & Scotland: Apply a -10% regional discount.
3. Comprehensive Project Budget Split
A realistic extension budget allocates funds across three main areas:
- Main Shell & Core Construction: 70% of total budget (foundations, brickwork, roof, doors/windows).
- Internal Fit-Out & Plumbing: 20% of total budget (plastering, kitchen/bathroom fit-out, electrics, heating).
- Professional Fees & Statutory Charges: 10% of total budget (architect fees, structural engineer, building control, party wall).
📊 Practical Extension Cost Worked Examples
Below are two worked calculation examples illustrating house extension budgets:
- Internal Floor Area: **25.00 sq m** (e.g. 5m × 5m extension)
- Extension Type: **Single-Story Rear Extension** (£2,000 / sq m)
- Location Factor: **1.0x (National Average)**
Calculation: Main construction cost = 25.0 × £2,000 = £50,000.00 (+10% contingency = £55,000 total).
- Total Floor Area: **40.00 sq m** (20 sq m ground floor + 20 sq m first floor)
- Base Rate: **£1,750 / sq m** | London Effective Rate: **£2,275 / sq m**
Calculation: Total construction cost = 40.0 × £2,275 = £91,000.00 (+10% contingency = £100,100 total).
📑 Common Pitfalls & Extension Budget Warnings
- Forgetting 20% VAT on Contractor Quotes: Builder quotes are frequently provided net of VAT. Adding 20% VAT to a £50,000 builder quote adds £10,000 to your final bill. Always confirm if quotes are inclusive of VAT.
- Underestimating Structural Steel Costs: Opening up the entire rear wall of a house for bi-fold doors requires heavy structural steel RSJ frames (goalpost frames). Steelwork and crane hire can easily add £4,000 to £8,000 to structural costs.
- Failing to Hold a 10% Cash Contingency: Unforeseen ground conditions (such as discovering unmapped sewer pipes, soft clay, or deep tree roots requiring 2.5m trench foundations) will consume your 10% contingency reserve quickly.
❓ Frequently Asked Questions (FAQ)
Under Permitted Development (PD) rights, single-story rear extensions can extend up to **6 metres** for attached/terraced houses or **8 metres** for detached houses under the Larger Home Extension Scheme (with neighbor consultation).
Foundations and roofing are the two most expensive elements of construction. A two-story extension spreads these fixed foundation and roof costs over twice the internal floor space, lowering average cost per sq m.
Architect fees for a domestic extension typically range from **7% to 12% of total construction cost** (or £2,000 to £4,500 fixed fee for planning drawings and building control plans).
A standard 25 sq m single-story extension takes **10 to 14 weeks** from ground excavation to final plastering and kitchen installation.