I'm in the process of bidding out a significant addition and remodel to my house, but I'm personally struggling with financial protection. I'm dealing with reputable builders but is there anything else I can do to protect myself on a sizable cash transaction?

Too many horror stories of contractors/builders walking out on jobs with unfinished or subpar quality work.

keep looking for contractors until you can find a pay upon completion guy. Progress payments for purchased items upon delivery on site.

Customized Contract (not on form provided by contractor) outlining major items: scope, payment, change orders, flow down provisions, damage clauses, indemnification, insurance, notices, termination; M&M Lien releases promptly prepared and filed (if contractor uses subs, which it sounds like he will due to size of the job); Good references (confirmed - you talked to owner and did a site visit).

establish a draw schedule where there are milestones for payments. That way you are not getting out ahead of of work that has not been done.