I have contributed $15000 (the annual maximum) to my 529 plan with my child as the beneficiary this year. My mother (my child's grandparent) would like to make a lump sum contribution of $75000 to my 529 plan for my child in 2018 (she has not made any contributions in the past and has no plan to make further contributions in the future). Is this allowed?
Yes, as long as the maximum account balance, as determined by the plan's rules, are not exceeded.