Differential voltage measurement at high voltages
Using a resistive differential divider as the input to a differential amplifier with a lower voltage rail than the Vin being measured is valid BUT be aware (and some simple calculations will show you) that the common mode voltage  does not cooperate properly with the division and you get increased error.

AFAIR error effects due to errors in the divider resistors act on the full common mode voltage whereas the dividers themselves divide only the differential signal. So any errors in the divide resistors effectively increases by the error % of the resistors.  If your common mode voltage is large compared to th edifferential signal "there will be problems".

Very approximately if you have 20:1 divide and 1% resistors you can get ~= 20% errors. Using 0.1% resistors as @The Photon suggests may seem a luxury but isn't and will probably only deliver accuracy about as good as you have said is OK if unadjusted. Note that this error may be removed by either physical calibration or software adjustment and you are then just left with the time variant effects. eg a 25 ppm/K resistor may end up looking like 500 ppm/k or perhaps 1% error at 20 K variation.

All the ab0ve is approximate as you have to see how combining worst case pars works in the circuit you use, and eg temperature effects will usually track and so partially cancel to a large extent.



If you can provide a floating high side supply you can buy isolation amps that have specs better than you need.

Here are Digikeys reasonably wide range of offerings  



You can use these elegantly clever dual linear optocouplers
 to form an isolated linear interface.
 This is Vishay IL300 but there are others.
 The circuit relies on the balance in the two receive diodes- ne being used on the send side for feedback linearisation and its (hopefully) good-twin on the receive side for output. 

In simplest form



Usually done more like this, but still simple enough