In a hidden portion of your monthly mortgage payment  (which the average person forgets about), you are paying the premiums for Private Mortgage Insurance (aka "PMI") to protect your lender against the possibility of you defaulting on your mortgage. 

While this initially allowed for a lower down payment, if your home has appreciated to where you have 20% equity based on today's market value of your home, your PMI can be stopped.   

Once a home purchaser has achieved 20% equity in their home, where is the extra risk?  The lender can never collect on the insurance policy, so why should this expense to you be continued.

Generally, lenders are under no obligation to inform you that you have the right to end this expense once you have reached the 20% equity level. 

The dollar amount of monthly premiums for PMI depends on your initial loan-to-value ratio, loan term, and the amount borrowed (it has nothing to do with your income or credit rating).  A typical premium on a thirty-year loan will range from about 1/2% to 1% of the initial amount borrowed per year.