A parcel of air in an isothermal layer
Isothermal, as you are likely aware means constant temperature. Your problem has the givens:


The 0 - 1 km layer has a constant temperature of 0 C
A surface parcel initially has a temperature of 0.5 C


We can likely assume the parcel is not saturated and you are aware that that a parcel with a positive relative temperature perturbation is buoyant and will rise.

To question you need to look at next is: what happens to a parcel of air as it rises?  Your 0 - 1 km layer has a constant temperature but not a constant pressure so the parcel will expand as it rises.  If the parcel expands then its temperature will decrease.  

At what rate does the temperature of an unsaturated parcel of air decrease as it rises?  This number will allow you to calculate the height that the parcels temperature decreases by 0.5 C and neutral buoyancy is attained.