That depends on how low the lowest bidder is. If an estimate for a "Heavy up" comes in around $2000.00 from 3 different contractors, give or take a couple hundred, I wouldn't blame them for going with the lowest. But when Jake the snake shows up with a bid for $999.00 then the HO is asking for trouble.
4 years ago, my brother put a 2nd floor over his 3 bedroom ranch. BTW, when he started this project, he actually called contractors and showed them what he wanted done with a freehand DRAWING! (And got BIDS with no specifications. I convinced him the 1st step is an architect and it was money well spent.)
A bedroom suite, a media room, an open loft area, and a very large (unfinished but with heat, a/c and electric) room for future conversion to 1 large or 2 smaller bedrooms. He got 3 bids - $140,000.00, $160,000.00, and $80,000.00. After the shell went up and the entire house was re-sided, the work came to a grinding halt. At that point the contractor realized everything costs 2x what he expected (And copper & lumber prices were also skyrocketing.)
Long story short, it took over a year from start to completion, and getting each step in the process done was like pulling teeth.