The short answer
A bill of quantities turns a broad construction proposal into an itemised scope. It helps both client and builder understand what has been allowed for before work begins.
A useful BOQ should identify quantities, units, rates and material assumptions. It becomes a reference point for payment stages and for assessing requested changes.
A BOQ is not a substitute for site investigation or professional advice. Ground conditions, authority requirements and client variations can still affect the final investment.
Planning note
Project conditions vary. Confirm cost, structural and approval decisions with appropriately qualified professionals before construction.