Are free domain offers from website builders really free?

Only for the first year. Wix gives a free-domain voucher with most yearly plans, for a new Wix domain only. Squarespace includes one free year on annual plans, then renews at the standard rate.

• Wix: most yearly Premium plans include a one-year voucher, valid only for a new domain bought from Wix.
• Wix limits: redeem within 60 days of purchase. It cannot be used for renewals.
• Squarespace: annual plans include one year of free domain registration, which then renews at the standard rate.
• Squarespace limits: claim it within a year, one free domain per site.