What is mortgage rate lock-in and why does it matter?

A rate lock freezes your interest rate for a set window while your loan moves through underwriting, protecting you from a rate increase before closing. If rates drop after you lock, some lenders offer a one-time float-down option, though not all do. Locking too early can mean paying extension fees if closing slips past the lock period, so timing it against your expected closing date matters. Ask your lender exactly when their lock window starts and what an extension costs.