When you're offered a "rate lock" from the lender, it means that you are guaranteed to get a set interest rate over a determined period while you work on the application process. This means your interest rate cannot get higher while you are working through the application process.
Rate lock periods can be various lengths of time, anywhere from fifteen to sixty days, with the longer period usually costing more. The lending institution may agree to freeze an interest rate and points for a longer period, like 60 days, but in exchange, the rate (and sometimes points) will be more than with a rate lock of fewer days.
There are more ways to get a reduced rate, in addition to choosing a shorter rate lock period. The bigger down payment you make, the smaller your rate will be, as you will have more equity from the start. You might opt to pay points to improve your rate for the life of the loan, meaning you pay more initially. One strategy that makes financial sense for many people is to pay points to improve the interest rate over the life of the loan. You'll pay more initially, but you will save money, especially if you keep the loan for a long time.
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