When you're offered a "rate lock" from the lender, it means that you are guaranteed to keep a particular interest rate over a certain number of days while you work on your application process. This means your interest rate will not grow while you are going through the application process.
Rate lock periods can vary in length, anywhere from 15 to 60 days, with the longer period usually costing more. You can get a longer period for your lock, but in choosing this option, will likely have a higher rate than you would with a shorter rate lock period
There are more ways to get a better rate, besides choosing a shorter rate lock period. A bigger down payment will get you a reduced interest rate, because you will have a good deal of equity from the beginning. You might choose to pay points to reduce your interest rate over the loan term, meaning you pay more initially. One strategy that makes financial sense for some is to pay points to bring the rate down over the term of the loan. You'll pay more initially, but you'll come out ahead, especially if you keep the loan for a long time.
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