Hello XXXXXXXXXXXX,
Welcome to Just Answer.
Landscaping garden pools are not mentioned in the building codes; this leaves the decision up to your local building official. Swimming pools are defined in the building codes, and there are specific requirements for these, but even depths are not mentioned. Swimming pools basically need to be safety fenced when they are no longer portable. In my experience I have never seen a landscaped water feature that needed to be safety fenced and I would say that this is typical. Just to make sure that you do not get into trouble with the local authorities, I would recommend running this question by your local building official, since every town has their own regulations. I am pretty sure that you will not need to fence your water feature, regardless of the depth.
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Best of Luck, Brian
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Architect
Licensed Architect- 12 years, L.E.E.D. AP, Former Remodeling Contractor- 10 years