A site plan is a scale drawing that shows the full extent of the site for an existing and /or proposed development.
When developing an ADU you will need a site plan to be part of your plans that shows your lot and all of its existing features in its current or remotely surveyed condition PLUS your proposed new structure (ADU) you want on your lot.
Creating a site plan usually will run as little as $1,000 as it will require taking physical measurements at your property.