Construction projects are messy since inception. Designs and expectations keep changing, even going beyond the start of construction and awarding of contracts. 

Change orders are not just common, they are the norm. Drawing details / specs are absent or about to change. But worst of all, the details are sometimes unclear and open to interpretation.

A list of risks - the unknowns in construction estimating

1. A common ambiguity in construction drawings is when the material type is specified but maybe not the exact product / spec. This can cause significant swings down the line when things are fine tuned and decided.

2. Assumptions not written down or communicated - This is the big one. If you haven’t written down the assumptions made during the takeoff, you might not even remember it 2 months later when a change order comes.

Plus, it is very important to qualify the estimate you are submitting, to make sure there is no disagreement after you win the contract.

3. No Contingency kept for unknown job information - Often thing can change at the finalization of design stage. Small differences in quality of material demanded (higher price) that doesn’t break the contract can lead to a significant change in margins with pressure to not change the bid.

4. Site conditions at the time of job execution can also make a difference.  Change in material / non material costs, some of which you can’t fix in advance is also something you need  to consider.

Don’t overdo it though

Its important to not simply add a big margin of safety to your bid to take care of these risks. You will not win too many bids if your competition is significantly undercutting you.

Instead, focus on qualifying your bid as much as possible and making note of all assumptions and possible risks with the help of your experience and intuition. So that when change orders happen or when you submit your final bid you communicate everything and protect yourself from difficulties in future.