Home & Construction⏱ 6 min read
How to Estimate the Cost of a House Extension
Build costs vary enormously by region, specification, and size. Here's how to use cost-per-m2 benchmarks, adjust for your specific project, and understand the factors that blow budgets.
Most extension cost estimates wildly underestimate the final total because they focus only on build cost and ignore professional fees, contingency, fitting out, and planning. Here's a realistic framework.
Build Cost Per m2 Benchmarks (2025, UK)
Specification LevelCost per m2 (GIA)Includes
Basic (budget finishes)£1,800-£2,200Structure, basic fit-out, standard kitchen/bath
Standard£2,200-£2,800Mid-range finishes, good insulation
Premium£2,800-£3,800High-spec finishes, underfloor heating, bifold doors
Luxury£3,800+Top-end everything, bespoke joinery
London and South East costs run 20-40% above these figures. Scotland and North East typically 10-20% below.
Full Cost Calculation: 25m2 Rear Extension
Build cost (standard spec, 25m2 at £2,500/m2):
= 25 x £2,500 = £62,500
Add: Professional fees (10-15% of build cost)
Architect: £4,000-£6,000
Structural engineer: £1,500-£2,500
= £5,500-£8,500
Add: Planning and building regs
Planning application (if required): £258
Building regs application: £800-£1,500
Add: VAT on labour (5% on new build, 20% on refurb work)
For new extension structure: 0% if new dwelling or 20% for most work
Practical: budget 20% on all labour for safety
Add: Fitting out (not always in build cost)
Kitchen or utility room: £3,000-£15,000+
Flooring: £1,500-£4,000
Decoration: £1,500-£3,000
Add: Contingency (10-15% of everything above)
Total realistic range: £80,000-£110,000 for a quality 25m2 extension
What Affects Cost Most
- Groundworks: Poor ground conditions, old drainage, sloping sites — this is where costs escalate most unpredictably. Always get a ground investigation for larger projects.
- Glazing: Bifolding or sliding doors, skylights, and structural glass add £3,000-£15,000 vs standard windows
- Structural steel: Open-plan extensions often require RSJ beams — budget £2,000-£5,000 for steel and installation
- Heating extension: Extending central heating to a new room costs £500-£2,000 depending on complexity
Does an Extension Add Value?
Approximate added value by extension type:
Kitchen/dining extension: 5-15% of property value
Additional bedroom: 10-20% (especially if adds en-suite)
Loft conversion: 10-20%
Garage conversion: 5-10%
Example: £400,000 property, kitchen extension costing £90,000
Value added at 12%: £400,000 x 0.12 = £48,000
Net position: -£42,000 (costs more than value added)
At 15% uplift: £60,000 value added → -£30,000 net
Extensions are usually lifestyle decisions that partially
recoup their cost — rarely pure investment plays.
Exception: adding a bedroom to a 2-bed pushing it to 3-bed
in a sought-after area often recovers more than cost.