Extension cost calculator
Estimate an early extension budget range using floor area, finish level, project complexity, contingency, and VAT. This tool is designed for early extension budget guidance rather than a formal measured estimate.
Estimate an early extension budget range.
Enter the extension area, choose the finish level and project complexity, then apply contingency and VAT to build an indicative range.
Results
This tool is for early guidance only. Final extension costs vary with groundwork, structure, steelwork, drainage, specifications, access, design development, and project-specific conditions.
Useful for quick early-stage extension budgeting.
- Sense-check an extension budget before drawings and details are fully developed.
- Compare finish levels before setting a working client budget or tender target.
- Allow for complexity where structure, services, and site constraints affect the build route.
- Export a branded summary for internal planning or client discussion.
This is a budgeting guide, not a measured extension estimate.
Extension projects can move significantly with site access, foundation design, structural openings, drainage work, retained elements, and specification choice. Use the calculator as a first-pass planning tool before a live estimate is prepared.
Useful next calculators and estimating pages.
Extension Estimating Services
Move from a quick budget range into project-specific estimating support for live extension work.
View Extension Service
Kitchen Renovation Calculator
Useful if the extension budget includes kitchen works that need a room-level early budget check.
Open Kitchen CalculatorNew Build Cost Calculator
Useful if you want to compare extension budget levels against broader new-build-style costs.
Open New Build CalculatorQuick answers about the extension cost calculator.
What does the extension cost calculator estimate?
The calculator estimates an early extension budget range using floor area, finish level, complexity, contingency, and VAT.
Why is complexity important in an extension budget?
Complexity matters because access, structural openings, foundations, drainage, sequencing, and retained elements can all affect the build cost.
Can this calculator replace a formal extension quote?
No. It is intended for early-stage budgeting only. Formal pricing still depends on drawings, specifications, structure, services, and project-specific conditions.
Is this useful for domestic house extensions?
Yes. It is designed as a quick budget guide for domestic extension work before a fuller live estimate or tender review is prepared.
Need a formal extension estimate beyond a quick budget check?
Use the calculator as a starting point, then get in touch if you need project-specific take-off, estimating, or pricing support on live extension work.