I converted a tub to a handicap accessible walk-in shower for my mother, who lives with me.

Yes, this could be claimed as a Medical Expense which is entered under 

Deductions & Credits

Medical

Medical Expenses

You are limited to only claiming the difference in the cost versus the increase in value. In other words, if the shower cost 5,000 but increased the value of your home by 2,000, you could only claim 3,000 as a medical expense. 

Medical expenses only count if you Itemize Deductions rather than using the Standard Deduction. 

Additionally, medical expenses need to reach a certain threshold before they are added. 

Enter the cost (less increase in value) in the program and TurboTax will let you know if it is worth claiming as an Itemized Deduction. 

More on Itemized Deductions 

If she is your dependent the expense is a medical expense deduction if you itemize. You can only deduct a total amount of medical expenses greater than 7.5 percent of your AGI.