Architect here. What you said is absolutely untrue. Some counties may not require a permit for residential, but typically non-residential structures require a permit. Your county's permitting or records department has a copy of all permitted plans including structural, mechanical, electrical, and whatever else is required to get the permit. Now, how well they kept the records is another story. I've seen the historical stuff stored on microfiche or microfilm and you need to use a special machine to see it, however I believe some counties are making efforts to digitize their records. Also the accuracy of where things are is a hit or miss due to the nature of construction.