If this is helpful, in Lake County IL they use the exterior dimensions of the home to determine square footage inside the home. This can mean the actual dimensions are off a bit if you have thick walls, like thick stone walls. They don't count basements that are completely underground as square footage (for tax purposes) as "living space" but I'm not sure about English or walkout basements, I think those are calculated differently.
If you have them come out to remeasure make sure you know what you're doing so they don't remeasure and you come out owing more. Also, if you decide to let them in your house to document something to reduce taxes, like that the basement is unfinished, they are more than likely to take photos. Make sure you're ok with that first.
Check all the details of your tax bill carefully for mistakes. They had my home listed as a two floor home when it's just a ranch, a detail that made a difference in my tax bill. They had exterior photos of my home on file which was obvious that it was a ranch so they just made the correction. I also found out they used an old listing of my home that was over 15 years old as the basis to make changes to my real estate taxes and increase them. The information entered by that real estate agent way back then was incorrect. In other words, you have to be constantly vigilant of changes and errors, even if never there before, because they will most likely not be in your favor.... more