YES, you can climb Mount Kinabalu in ONE day, climb up in the morning, and climb down in the afternoon. I called Sabah Parks Ranger office just now, inquiring about the possibility of 3 US citizen summit Kinabalu in ONE day. They said it is possible, but there are few things that you have to do.
1. You have to present yourself (and your two other friends) in Kinabalu Park at least a day before your intention to climb. Look for the Manager of the Park, Mr. Haji Abdul Wahab and ask to get the permission & permit to climb in one day. If he is not around, look for the Kinabalu Park Ranger On-Call, and mention the same thing. If everything is satisfactory, You will then can check in to where you have book for your night stay.
2. For ONE day climb, you will then be allowed to climb next morning after confirmation from the Rangers. Guide, insurance and permit is compulsory, i.e. guide will be around RM100-150 for the group, insurance will be around RM10 and permit will be RM100 per person. Certificate of achievement is optional (RM10). All fees to be paid next morning before the climb.
3. You need to start climbing very early next morning, usually before 7am. Better to wake up at 6, and head to the Park office and you will be allocated the guide for you. You will start the climb earlier than other regular climber.
4. You will not allowed to continue your climb to the peak, if you are not able to reach Laban Rata before noon (12pm) or if the weather is not permitting.
Remember, you need to be really FIT, maybe as fit as a climbathon runner… or maybe at least 50%of their ability…