You may have no plans to ever invite the public into a commercially-zoned facility, and in that case, making it wheelchair-accessible might make no sense.

But if you suddenly had to up and move, and sell the space, well, it's still zoned commercial, and presumably someone's running a business out of it, and they might have an employee in a wheelchair.

Commercial spaces have different requirements than residential spaces. It's not just a matter of stamping a form. It's also not surprising that it'd take some extra construction to bring a formerly residential space up to code.

AND, I get that you'd like to defer the actual cost of rezoning onto someone else, but, well, someone's got to actually pay to turn the residential space into commercial...and you're the one who wants to do it and will presumably benefit now from doing so.

$3k to make it accessible to people without two working legs seems pretty reasonable.