When you're promised a "rate lock" from the lender, it means that you are guaranteed to get a particular interest rate over a determined period for the application process. This ensures that your interest rate cannot grow during the application process.
While there may be a choice of rate lock periods (from 15 to 60 days), the extended spans are usually 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
In addition to choosing the shorter lock period, there are other ways you may be able to score the lowest rate. A bigger down payment will give you a better interest rate, because you will have a good deal of equity at the start. You could choose to pay points to reduce your interest rate for 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 bring the rate down over the term of the loan. You'll pay more initially, but you'll save money in the end.
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