When you're offered a "rate lock" from your lender, it means that you are guaranteed to get a particular interest rate over a certain number of days for your application process. This ensures that your interest rate will not get higher while you are going through the application process.
While there are various 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
In addition to choosing the shorter lock period, there are other ways you can score the best rate. A bigger down payment will result in a lower interest rate, because you will be starting out with more equity. You might choose to pay points to improve your rate for the loan term, meaning you pay more up front. For a lot of people, this makes financial sense..
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