Levels of estimate accuracy.
Not all estimates are equal. As a project moves from concept to construction, the expected accuracy range tightens. Understanding these levels helps you choose the right method and set client expectations.
Who this guide is for
Builders pricing work, quantity surveyors preparing cost plans, and project managers who need to communicate estimate reliability to stakeholders.
Estimate accuracy classes
Order of magnitude (±30–50%) — Based on cost per square metre or similar historical data. Used at concept stage when no detailed information exists.
Budget estimate (±15–25%) — Uses approximate quantities and known rates. Suitable for feasibility and early financial planning.
Detailed estimate (±5–10%) — Itemised quantities measured from drawings with current supplier and subcontractor quotations. Used for tender pricing and cost control.
Miniature example
For a 100 m² extension: an order-of-magnitude estimate might be £2,000/m² = £200,000 (±50%). A budget estimate uses approximate quantities: £185,000 (±20%). A detailed estimate measures every element: windows, doors, roof, finishes — arriving at £178,400 (±8%).
Common mistakes
- Presenting an early-stage estimate as though it were a detailed price
- Not stating the accuracy range alongside the figure
- Skipping stages and relying on a single method throughout the project
Related guides and tools
Read about top-down vs bottom-up estimating and budget estimates. Use the Budget Estimate generator for early-stage cost planning.