When putting a permanent ramp outdoors, keep in mind a visible ramp may advertise to the bad guys that a vulnerable person lives in the house. Put the ramp in the back of the house rather than the front. Better still, build it in the garage where it cannot be seen with the garage door closed.
For a permanent ramp, codes usually require a foot of ramp for every inch of drop. By this I mean if the platform is 15 inches from the ground you will need a 15 foot ramp. This will help prevent the ramp from being too steep and putting a wheelchair occupant at risk of falling out of their chair.