Installing shower for elderly mother

This does not address your question (apologies) but there are also good 'step-in' or 'walk-in' tubs on the market for people with mobility issues if she prefers to keep a tub.

I don't know the law, but... I as far as I know you can just remove the tub and install a shower. I did it as well a few elderly neighbors within the building and friends in other buildings. All reno's were approved by the building, building architect, city, etc. So I'm not sure why it should be an issue. Are you changing the footprint, moving to a different location within the bathroom? Maybe that's the issue. All those that I know that have done this, including myself, kept the same location. Shower floor in place of the tub. Perhaps that is key. If the contractor told you this maybe he just wants a bigger/more costly project. Check with knowledgeable contractors. BTW - I don't believe it reduces the value. Almost all apartments that I looked at (1 beds)when buying, that did gut renos... removed the tub. I wish the apt I finally purchased had it already done -( I'm not old either). A 2 bed might be different because it could be a family/ need tub for kids. Anyway, safety first for your mom.

You can't shower or even stand up step-in/walk-in tubs. Sit only. That I think would really be a resale issue.

You can either do it without filing if the coop will let you or file for reno of the bathroom with every thing as it is but the change the tub to shower. I believe city does not know whether it is free standing tub or fixed to the wall. Or you can show existing bathroom drawing as a shower for filing purposes rather than a tub (this is what I would do). It is not by the book but if the rules are ridiculous, what is one to do.

You can get a waiver if it is not possible to make it compliant.