I'm pretty sure those are 4" tiles, so the bottom of the niche is at 12". We had the clients choose the height based on what was comfortable for them. This is another one that's more recent, and set at +-15". Others opted for corner shelves instead, and some of those were set higher based on what clients wanted.

Thanks for responding! That was my guess as well. (BTW - IMHO I feel this was a really lovely detail that I think is a more elegant solution than the corner shelves!)