834 fifth 3c

The one at the far eastern end of the 3rd floor is an extra for servants, who otherwise would have only one bath for four bedrooms: http://www.columbia.edu/cgi-bin/dlo?obj=ldpd_YR_1350_MH_006_001&size=large

That wing on the 4th floor was extra maids' rooms for the whole building, so can't tell. Probably same story.

The one under the back stairs was added later and must be for catering staff, so they wouldn't need to use the guest lav or one of the maid's baths. For sink they have the mop sink across the hall.

BTW, the Columbia plans show the building when it was just a mid-block lot, so B there is C as built. The developer bought the south corner lot during construction, and added the A line. Layouts didn't change, though.

There seem to be three stray toilets there not in bathrooms, two of which don't have a sink. Didn't they know then that servants' dirty hands is the reason that typhoid spread?

But I would probably take this apartment anyway

I think there was a code change after the war. In almost all prewar apartments that had them, servants' bathrooms would have a tub and toilet but no sink. Each maid's room would have its own sink. I guess those toilet-only compartments at 834 would be an extension of that.

Then there're all the no-maids-room prewars with a toilet-only compartment off the kitchen.

Regarding the mid-build expansion:
The plot on the south/west corner was acquired (a mansion standing there apparently left) allowing for the increase. Candela made the adjustments as needed internally. However, the building symmetry is not aligned. The pavilions & watertower are centered with the original plot. The only shot I could see online to notice from a direct park view perspective was from Emporis. Unless in or across Central Park the set backs of these elements aren’t noticeable from the direct surrounding street perspective - ala Candela style.
In terms of the Staff rooms the standard at the time would have been communal facilities. I’m sure proper hygiene was enforced even if sinks aren’t visible in the lab only. It would seem the maximum utilization of space (again, a Candela trait) was paramount in them appearing random. However, washing of hands even in a pantry sink would have been permissible while using an occupant toilette would be unacceptable.

FYI - the Museun of the City of NY has an exhibit on Candela currently & through October I believe. Quite interesting & full of insight. Peter Pennoyer (Architect with superb credits & a scholar of Candela’s work (they never met; he’s studied his plans since the age of 8 he states) designed the exhibit. The hope of those who produced the exhibit is Candela’s impact on the redefining of the East Side will be more widely recognized. The careful planning & style of setbacks without disregard of the 360 viewpoints is quite remarkable. When his buildings went up the landscape was one lower in height as mansions were previously the structures build. His careful attention & guiding foresight in fine examples can be attributed to much of the East Side (midtown to Carnegie Hill) area. The overall City in fact has benefited from his talents in essentially a vision of a new skyline. What he’d think today of the massive Uber-heidght projects is an interesting unknown.