Cost and Quantity Surveys

Cost and quantity surveying converts design information into measurable work items and money. Quantity takeoff measures net quantities from drawings (length, area, volume, number) following a standard method of measurement (e.g., SMM7, NRM, CESMM). Costing applies unit rates to get direct cost, then adds indirects, risk, and margin. Principle: Takeoff → Abstract → Bill of Quantities (BOQ). Methods: centre-line for walls, long-wall short-wall to avoid double-count, mean girth for trenches, average end area for earthwork V = L·(A1+A2)/2. Deduct openings per rules. Apply waste/overlap: Q_ordered = Q_net·(1+w%). Cost structure: Unit Rate = (Material + Labor + Plant)/productivity. Direct Cost = Σ Q_i·UR_i. Then: Site overheads + Head office overheads + Profit + Contingency + Provisional sums. Total Estimate = Direct Cost + OH&P + Risk allowance. Types: Preliminary estimate (area method $/m², 0-10% design), elemental cost plan, detailed BOQ estimate (±5%). Accuracy improves with design. Contingency covers known unknowns (design development), typically 5-15%. Provisional sum for undefined work, prime cost sum for nominated supply. Key variables: Q = quantity, UR = unit rate, W = waste %, P = productivity (output per hour), OH = overhead %, Fee %. Limitations: Rates time-sensitive (escalation), location-sensitive, assumes normal site conditions. Mis-measure of ground, reinforcement laps, formwork reuse, and curing time commonly cause cost overruns. Engineers use BOQ for tender comparison, valuation of interim payments, variation pricing, and earned value: Cost variance = BCWP - ACWP.