It's much more likely that whoever had the house before you did had cats that peed down there. Or, if it was a new house, cats or other animals may have hid out in the basement when the house was open. Either way, do you have carpet or panneling down there? If you do, the cat pee may have soaked in under it and you may have to just take it out. It's REALLY hard to get rid of that particular smell. Don't use bleach, as it will just react with the ammonia. You can try something like Nature's Miracle, which you can get at any pet supply store, but even that doesn't always work. Can you pinpoint where the smell is exactly and start from there? Like I said, if you have carpet or paneling you may have to take it out to get to the source of the smell. Good Luck!