Why are my builders' quotes so different from each other?

Hand the same drawings to three builders and you will get three different documents, because a quote is a set of decisions: what to include, what to exclude, what to cover with a provisional sum - an allowance for work not yet properly priced, which is a guess wearing the costume of a number - and how much risk to price in. One builder includes the groundworks complete; another excludes rock, assumes good ground, and wins the comparison right up until the digger finds otherwise. The spread between your quotes is not noise - it is information about what each builder saw, feared or left out. The way through is to rebuild all of them onto one basis before comparing any totals, turn every exclusion and provisional sum into a written question, and treat the answers as part of the price. The lowest number is rarely the cheapest build.