You do not need to contact ACSM for an extension.
There is an automatic six month grace period, but your certification is considered INACTIVE and you will not receive any benefits of being certified during this time.
Your certification will be re-instated once the online renewal form has been submitted along with the appropriate fee and $25 late fee. Your next recertification cycle will continue on the original schedule.
If you do not recertify within the six month grace period (which ends June 30 each year), your only option to recertifiy would be to retake the examination. However, you must meet the current minimum requirements in order to take the exam.
If you do not meet the current minimum requirements, you can opt to take another ACSM certification. For example, if you let your ACSM HFS certification expire, and you do not have a Bachelor's degree in Exercise Science, Exercise Physiology or Kinesiology, you would not be able to take the ACSM HFS exam for recertification. You would meet the minimum requirements for the ACSM CPT or the ACSM GEI.