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Estimating fundamentals

Approximate quantities estimating.

Approximate quantities estimating sits between cost per square metre and full measurement. It produces a more reliable budget than a broad benchmark without the time investment of a detailed take-off.

Who this guide is for

Contractors preparing competitive tenders early in the design stage, quantity surveyors producing cost plans, and builders who need a reliable budget before full drawings are available.

How it works

Instead of measuring every item precisely, key quantities are estimated from outline drawings or specifications. Wall areas, floor areas, roof spans, and window counts are approximated. Unit rates from previous projects or published cost data are then applied.

Miniature example

For a single-storey extension measuring roughly 6 m × 4 m: approximate external wall area = 40 m², roof area = 28 m², floor area = 24 m². Apply approximate rates: walls £120/m² = £4,800, roof £95/m² = £2,660, floor £75/m² = £1,800. Total approximate estimate = £9,260 before prelims.

Common mistakes

• Applying unit rates that are too precise for the approximate quantities used
• Omitting items that are small in area but high in cost, such as specialist doors or kitchen fit-out
• Forgetting to state that the estimate is approximate and subject to refinement