In a non-user-specific location in "application data":
C:\ProgramData\Autodesk\Revit\Addins\
In a user-specific location in "application data":
-
C:\Users\<user>\AppData\Roaming\Autodesk\Revit\Addins\
-
%appdata%\Autodesk\Revit\Addins\
Build user-specific "application data" path in C#.
string cSharpPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), @"Autodesk\Revit\Addins, revitVersion");
Source: Autodesk Help