Professional who does this kind of calc for my job - basically you use calculate the occupant load (or a program load where the egress load seems high), then go to 248 CMR: 10.00 Uniform Plumbing Code. Split that occupant load - 1/2 designated female, 1/2 designated male. 248 CMR Section 10.10(18) Table 1 give you the fixture factor based on the type of occupancy. Generally in offices it’s 1 per 20 for females and 1 per 25 for males.

Say in his building, the occupant load is 40 total - 20 female, 20 male. This would translate needing at least one fixture for each gender. Since the building likely small, all the bathrooms are permitted to be located on the first floor as long it’s only a 2 story building and a person upstairs doesn’t have to travel more than 300 ft from the most remote point on the 2nd floor to the bathroom.

248 CMR Section 10.10(18)(j)(1) requires establishments provide designated bathrooms for male and female employees. Over the past few years, the Plumbing Board has been approving variances that allow for gender neutral bathrooms - it’s not explicitly permitted in the current edition of 248 CMR, but the Board has no opposition to it, it’s just a formality. I believe there is another subsection in Section 10.10(18) that allows for a 1-to-1 substitution for required fixtures for males/females with gender-neutral single user bathrooms.

For OP’s situation, the designation of the bathrooms probably goes back to when the space was built/last updated. The designated single user bathrooms probably remains if the building owner didn’t want the headache of applying for a variance, didn’t know it was an option, or it was last updated when the climate wasn’t as friendly to gender-neutral single user bathrooms.

There should be two bathrooms on each floor at least. Then having to come downstairs isn’t right.