Differential Voltage Measurement
If you have a floating supply (battery) then there are no absolute or earth ground stresses of offsets or common mode voltage issues to deal with.

The usual limitation is that the signal must be inside the supply voltage rails or even inside 2V away from the rails. Some are design to work a bit below ground (PNP inputs) or above V+ rail somewhat (NPN inputs) but not likely 250V above the supply voltage to the chip. ( but possible at higher expense.)

It is possible however to design a custom solution to perform this task in a ground referenced system or use a positive supply reference with a small negative rail, or AC couple or attenuate within the supply range , then amplify the difference.  There is one chip I know with ADC with (DC isolated) differential inputs for high voltage measurements. 

Also here is a Diff Amp with +/-270V CM linear range and  +/-500V CM protected range.
www.analog.com/media/en/technical-documentation/data-sheets/AD629.pdf

This works due to 20:1 reduction of  input signals with precision matched R's.
77 dB minimum CMRR @ 500 Hz