NEW YORK ( TheStreet) -- With the Federal Reserve's decision last week to stay the course on its low-rate policy, mortgage rates have been drifting down. But they remain a good deal higher than they were last spring, and you might end up paying a good deal more than the rate you see advertised. How come? Advertised rates usually involve the best case -- the rate offered applicants with the highest credit ratings, buying homes in the best locations, and so on. Add-ons reflect higher risks associated with certain factors. To find the rate you'll end up with, you'll have to talk to the lender, and perhaps fill out an application. But here, from The Mortgage Professor website, are a few examples of how add-ons can boost the rate. Let's say you applied for a 30-year fixed-rate mortgage advertised at 4.375% for a $300,000 home with a 15% down payment. cash-out refinance, rather than a no-cash refinance, you'd also pay an additional 0.401%. For a loan on a condo rather than a single-family home, you'd pay an extra 0.1%. For an investment property rather than a primary residence, the rate would go up a full 0.502%. And opting for a 60-day lock-in period on the rate, rather than 30 days, would boost the rate by 0.033%. If you were unfortunate enough to have all these add-ons, your rate would be 5.863%, not 4.375%. rejected, try this tool on Guttentag's site.