When you're promised a "rate lock" from the lender, it means that you are guaranteed to get a particular interest rate for a determined period while you work on the application process. This means your interest rate can't go up during the application process.
While there are several lengths of rate lock periods (from 15 to 60 days), the extended ones are generally more expensive. You can get a longer period for your lock, but in doing so, will likely have a higher rate than you would with a shorter span of time
There are more ways to get a low rate, besides choosing a shorter rate lock period. The larger down payment you can pay, the better the rate will be, because you will have more equity from the beginning. You may choose to pay points to lower your interest rate over the life of the loan, meaning you pay more up front. One strategy that is a good option for many people is to pay points to improve the rate over the life of the loan. You'll pay more initially, but you will come out ahead, especially if you keep the loan for a long time.
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