A plat is a map of the property (commercial or residential) that tells you more than just where the property boundary is. Plats get recorded with the county and show important civil characteristics of a property like, easements, boundaries, assessors parcel number, lot number, etc. You can consider a plat as a "birth certificate" for the lot. Key, most residential properties do not have individual plats but instead they are apart of a plat for the entire subdivision. For instance, you may notice that you live in a subdivision that set as Phase 1 or Unit 1. Subdivision names like, Encino Heights Unit 1 or Whispering Rocks Unit 5, typically have associated plats for that unit development. The propety boundary survey that you get when you buy your house is just that a "boundary survey".
Plats are more important for commercial properties. Each commercial property typically has an individual plat. It is very common for someone to buy a commercial property and get a boundary survey that shows the limits of the purchase but later find out that the commercial property belows to a large plat that is made up of a few commercial lots. In this case, the new owner will need to amend the plat or get a new plat all together for their new individual property that they just bought. The differnece is thousands of dollars. Platting a commercial property can cost as low as $4000 up to $40,000 or more, depending on the size of the lot.
The key here is to understand if the property that you are buying or selling is platted as an individual plat or apart of a larger plat. Another key point, permits for construction are typically not granted for commercial properties that do not have individual plats for the property. A feasibility study before you purchase a property will help you understand the legal situation of the property.
