When you are promised a "rate lock" from your lender, it means that you are guaranteed to keep a certain interest rate for a certain number of days while you work on the application process. This ensures that your interest rate cannot grow while you are going through the application process.
Rate lock periods can vary in length, between fifteen to sixty days, with the longer ones typically costing more. You can get a longer period for your lock, but in choosing this option, will most likely have a higher interest rate than you would have with a shorter span of time
In addition to going with a shorter lock period, there are other ways you may be able to attain the lowest rate. The more the down payment, the smaller your interest rate will be, because you will be starting with more equity. You could opt to pay points to lower your interest rate over the life of the loan, meaning you pay more up front. For many people, this makes financial sense..
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