When you are promised a "rate lock" from the lender, it means that you are guaranteed to keep a particular interest rate for a determined period for your application process. This protects you from working through your whole application process and finding out at the end that the interest rate has gotten higher.
While there can be a choice of rate lock periods (from 15 to 60 days), the extended spans are typically more expensive. You can get a longer period for your lock, but in doing so, will probably have a higher interest rate than you would have with a shorter rate lock span of time
There are other ways to get a reduced rate, besides choosing a shorter rate lock period. A bigger down payment will result in a better interest rate, because you're starting out with a good deal of equity. You can pay points to lower your interest rate for the loan term, meaning you pay more up front. One strategy that is a good option for some is to pay points to bring the rate down over the term of the loan. You'll pay more initially, but you will come out ahead, especially if you don't refinance early.
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