This year's interest has been piqued by scientists gathering DNA from the loch, and Scotland's first minister saying she believes there is a monster.
SNH said it would "dust off" the plan if Nessie was discovered.
The code of practice was drawn up to offer protection to new species found in the loch, including a monster.
It stipulates that a DNA sample should be taken from any new creature, and then it should be released back into the loch.
The story of the monster can be traced back 1,500 years when Irish missionary St Columba is said to have encountered a beast in the River Ness in 565AD.
Later, in the 1930s, The Inverness Courier reported the first modern sighting of Nessie.
The creature's appearance has been described as resembling a plesiosaur, a creature that died out with the dinosaurs.
Nick Halfhide, of SNH, an organisation that promotes Scottish wildlife and natural habitats, said the 17-year-old code of practice remained relevant today.
