Usually because they are not the same job. Allowances, exclusions, and different material grades make two bids incomparable until you normalize them. A low bid frequently has a smaller scope hiding inside it.
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A 40 percent spread between bids almost never means one contractor is 40 percent cheaper. It means the bids describe different work, and the comparison has to be normalized before the numbers mean anything. Where the difference usually hides. Allowances, which are placeholder dollar amounts for items not yet selected, such as tile, fixtures, or cabinets. A bid with a $3,000 tile allowance and one with a $9,000 allowance will differ by $6,000 for reasons that have nothing to do with the builder. Exclusions, which are the items explicitly not included, often listed in small type: permits, dumpster, temporary protection, painting, cleanup, and repair of adjacent surfaces. Material grade, since two window packages or two cabinet lines can differ by a factor of three. And scope depth, such as whether the electrical is a device swap or a circuit upgrade. How to normalize. Write your own one-page scope with the actual items you want and send the same document to everyone, so they price the same thing. Require every allowance to be listed with its dollar amount. Require exclusions listed rather than assumed. Ask each bidder to price the same specified brand and model for the big-ticket items, then let them substitute afterward if they prefer. What a genuinely low bid can mean. Lower overhead, a crew already working nearby, or a slow period, all legitimate. Or an unrealistic labor estimate that becomes change orders at week three, unlicensed subs, no insurance, or an intent to renegotiate once demolition has started and you cannot easily switch. The practical rule: when a bid is far below the others, ask that contractor what they included that the others did not, and ask the others what they included that this one did not. One of those questions usually explains it.