Article 110.26 is one of the most frequently violated codes in the field, and it’s one inspectors love to nail you on because it’s objective - either you have 36 inches of clear working space or you don’t. The rule: minimum 36 inches deep in front of the panel, 30 inches wide, and 6.5 feet high. That space has to be clear. Shelves, water heaters, furnaces, that one HVAC return that the builder crammed too close - all of it counts against you.
The part that bites people is the “dedicated” equipment space above the panel (110.26(E)) - the footprint of the panel extended to the structural ceiling has to be clear of piping, duct work, and leak-prone equipment. I’ve failed inspections on new construction because the HVAC sub ran a duct right above the panel before anyone thought to check. Get in the habit of looking up when you’re planning a panel location.
In retrofit situations, if you’re replacing a panel in an existing location that doesn’t meet 110.26, you may not be required to bring it into full compliance - but it depends on the AHJ. Some require full compliance on any permit that touches the panel; others allow existing non-conforming conditions to remain. Ask before you start the job, not after.