Lavatory Next to a Water Closet
A locality may permit a lavatory's edge 18 inches (457 mm) minimum from the centerline of a toilet when it has a clear floor space of:
• 66 inches (1676 mm) minimum from the back wall.
• 60 inches (1524 mm) minimum from the side wall.
Though that configuration may be permitted, a wider area between a fixture and toilet makes for easier maneuvering.
*Note that the picture above has the lavatory edge 42 inches (1066 mm) from the centerline of the WC.