Water Closet Location and Seat Height (2010 ADA Standards 604)
The 18 in everyone remembers is a WINDOW of 16 to 18, and the plumbing code's 15 in minimum is not inside it -- so a closet roughed at the IPC minimum is code-compliant and ADA-noncompliant by an inch, while one set at a generous-looking 20 in fails the other end. The seat is 17 to 19 in to the TOP OF THE SEAT, not the rim, so a thick seat moves a compliant bowl either way. And the 60 x 56 in clearance is CLEARANCE, not a compartment: it must be clear of everything, which is why a lavatory that fits the room can still fail by standing in it.
Formula and source
centerline 16 in minimum to 18 in maximum from the side wall or partition; seat height 17 in minimum to 19 in maximum measured to the top of the seat; clearance 60 in minimum from the side wall and 56 in minimum from the rear wall; flush controls on the open side except in an ambulatory accessible compartment.
2010 ADA Standards for Accessible Design, 604.2, 604.3.1, 604.4, and 604.6. A US federal standard in the public domain, quoted directly.
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