When you are promised a "rate lock" from your lender, it means that you are guaranteed to get a specific interest rate for a certain number of days while you work on your application process. This protects you from getting through your entire application process and discovering at the end that your interest rate has gone up.
While there are several lengths of rate lock periods (from 15 to 60 days), the longer ones are generally more expensive. You can get a longer period for your lock, but in choosing this option, will likely have a higher interest rate than you would have with a shorter span of time
In addition to opting for the shorter rate lock period, there are several ways you can get the best rate. The bigger down payment you pay, the smaller your interest rate will be, because you will be entering the loan with more equity. You can pay points to improve your rate over the loan term, meaning you pay more initially. For a lot of people, this makes financial sense..
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