What are Sanborn maps?
Sanborn maps are detailed fire insurance maps of US cities and towns, made from the late 1800s through much of the 1900s. They show each building's outline, construction and use, down to gasoline tanks.
In more detail
The Sanborn Map Company mapped thousands of towns for fire insurers. Maps were updated as towns grew, so a set for one place can show decades of change.
For environmental work they are a key historical source: they mark filling stations, garages, dry cleaners, plants and tanks long before any database existed. The Library of Congress has a large digitized collection.
Coverage is best in towns and city centers. Rural land and newer suburbs often have none.
What it means when you buy
If your property is in an older town, a Sanborn map can show a past use nobody remembers. Phase I consultants check them as a standard source.
How the pre-screen handles it
Where Sanborn maps cover the property, the pre-screen reads them alongside historical aerials and old topographic maps to build the past-use timeline.
Pre-screen a property before you spend on a Phase I
A desk review of one address: past uses from historical photos and maps, federal and state environmental records, and flood risk, ready usually within the hour. $100 for one report, or $100/month for 3.