House Extension Cost Calculator
Plan a UK extension using total new floor area across all storeys.
Market-specific data
Low, typical & high range
Assumptions shown below
Project measure
Project details
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Your planning estimate
UK estimate · 30 m²
Typical project allowance
£72,000
Low£54,000
High£90,000
This range updates from the project details you enter. It is a planning estimate, not a contractor quote.
Typical cost breakdown
- Base project allowance (30 m²)
- £72,000
Review the methodology below for included work, exclusions, assumptions, and source information.
Understand the range
Methodology, assumptions & sources
How this estimate is calculated
Total new floor area uses reviewed full-extension rates excluding VAT. A two-storey project uses the same per-m² benchmark when total floor area is entered; kitchen and bathroom are explicit add-ons.
This is a planning estimate, not a contractor quote. Actual quotes vary by location, specification, site conditions, labour, and project requirements.
Included assumptions & exclusions
- Base extension benchmark excludes VAT.
- Professional fees, planning, Building Control, party-wall, ground conditions and unusual structural work are excluded.
Data sources & review information
Source reference
Checkatrade House Extension Cost 2026
Source references provide benchmark context. HomeCostNest applies its own calculator structure, ranges, assumptions, and allowances; listing a source does not imply endorsement of HomeCostNest or verification of every calculated price.
No source URL is stored in this calculator's current data record; no link has been inferred or fabricated.
- Review date:
- September 19, 2026
- Dataset:
- 1.0
- Data record status:
- Reviewed internally
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