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Pricing & commercial

Construction estimating mistakes that destroy margin.

Most margin erosion in construction happens before the project starts — in the estimate. Common estimating errors silently reduce or eliminate profit, even on jobs that appear correctly priced.

Authored by Alexandr Vreme & Maksym Vasylkov
Published July 2026 — Next review July 2027

Mistake 1: Not including full preliminaries

Applying a flat percentage for preliminaries without itemising what the project actually needs is the most common estimating error. A typical 8-10% mark may look reasonable, but if the project requires a full-time site manager, scaffold, welfare cabin, security, and temporary services, the real cost can exceed 15%. The gap comes straight out of margin. Always compile an itemised preliminaries schedule instead of relying on a rule-of-thumb percentage.

Mistake 2: Forgetting overhead recovery in small works

On smaller projects, overheads can consume the entire margin if not explicitly recovered. Applying 20% margin on a £5,000 job sounds healthy, but if £750 of that is materials with no overhead contribution, and the job takes three days of supervision, the effective overhead recovery may be only 5%. Track overhead recovery as a separate line item rather than assuming it is covered by the percentage margin.

Mistake 3: Optimistic gang output

Estimating assumes 100% productive time, but real site output typically runs at 60-70% due to weather, material delays, access constraints, and coordination gaps. A bricklaying gang priced at 500 bricks per gang-day may actually achieve 350-400 on a typical inner-city site. The shortfall extends the programme and inflates labour costs. Apply a realistic productivity factor based on project type, not textbook rates.

Mistake 4: Missing attendance on subcontractors

Subcontractor prices rarely include the cost of providing hoist access, welfare facilities, storage, cleaning, or skip hire. These attendance items are the main contractor’s responsibility and can add 3-5% to subcontract packages. Omitting them means absorbing the cost from your own margin. Build a subcontractor attendance schedule into every estimate.

Mistake 5: Overlooking waste and delivery charges

Materials cost more than the list price. Delivery charges, handling, storage protection, and waste factors (typically 5-10% depending on the trade) are real costs that must be included. A £10,000 materials order may cost £11,500 by the time it is on-site, allocated to the workface, and the waste skip is paid for.

Miniature example

A £60,000 project priced at a 20% margin should yield £12,000 profit. Five common estimating mistakes each consume 2-4% of the project value: under-stated preliminaries (3%), missed overhead recovery (2%), optimistic gang output (4%), missing subcontractor attendance (3%), and unaccounted waste and delivery (2.5%). The actual margin becomes 5-8%. The 22% pricing (to recover full overheads) produces a £73,200 sell price, but after mistakes the real cost is £63,000, leaving only £10,200 — a loss of £10,000 in expected profit.

How to fix it

• Use an estimating checklist that covers every line of the preliminaries schedule
• Itemise overhead recovery as a separate line, not a blended percentage
• Track actual gang outputs on completed projects and update your productivity assumptions
• Build a subcontractor attendance checklist into every bid
• Apply waste and delivery factors to all material take-offs
• Conduct a post-project margin review to identify where estimates deviated from actual costs