gn I am not sure of the percentage… Off the top of my head I think it is in the 10-20% range. I converted a half bath in one of my rentals to a full bathroom back in 2003 after I bought the home. I did not get the work permitted but I did hire a contractor and had it done professionally for what it is worth. I took pictures while it was being done as well.

Also what I have seen is that even for newer homes that have not been modified there can be discrepancies. For instance there was a home that I saw a few weeks back that had a 2 car garage but the tax roll said 3. Also the number of bedrooms and baths did not match. The issue was that this home had optional floorplans from the builder but the tax roll only used the basic model.

There are other cases that are often overlooked. Buying a home with a pool for instance. Again depending on where you live you may not be able to dig up the permit for a pool. As esmith said counties only keep this information for so long.

It is a personal choice as to how much you want to be concerned over this.