cold air return vent height. is there code?
First - It is not just a COLD air return, it is a return for air and circulation.
The height is not a code item since it is dictated by the use.
For a basement, it usually is near the floor, since it will evacuate the cool air on the floor. This makes the floor warmer in the winter and reduces the load on the AC by circulating and making temperature more even in the summer.
If you are air conditioning, in some cases, a high return on upper floors is good because it sucks out the warm air and allows the cool air to fill the room. A low return on an upper floor will suck out the cool air, leaving the warm air to statify above depending on the location of the supply.
Returns and supplies should be coordinated and located properly for the functions/climate. If you have muntiple zones the locations could be different.