I seem to remember check in being 4pm but they held our luggage and said if we had an inland phone they would notify us if our room became available sooner. If we planned to stay on the property to swim they would come find us. We were actually gone visiting elephant sanctuary all day until almost 10pm. They put our luggage in the room and held our keycards at the desk. :)
Get an answer from Siripanna Villa Resort and Spa Chiang Mai staff and past guests.