Experts said the structure was a fire trap waiting to happen, with insufficient emergency exits, flammable finishing materials and illegally parked cars blocking access to fire trucks.
Street surveillance video footage showed orange flames and black smoke billowing from the ceiling of a ground floor parking lot underneath the stilted building, which stood on pillars.
The fire spread quickly upwards along the outside walls of the tower, which contained cheap but highly flammable finishing materials.
"There were three or four exploding sounds and I saw the fire on the ground floor quickly jump upwards along the outside walls," said one eyewitness.
It reportedly took only seven minutes for the entire building to be engulfed in smoke and flames.
Engineering Professor Chung Sang-Man at Kongju University said the tower had cladding materials made of a cement and foam sandwich, which are widely used for insulation but prone to spreading fire.
"Inflammable finishing materials have been a great source of problem in major blazes," he said, citing the Grenfell Tower inferno in June that killed 71 people, among others.
The London fire started with a faulty refrigerator on the fourth floor of the building owned by the local authority, but rapidly spread up the 24-storey tower, which had new cladding on the outside.
In the wake of the disaster, hundreds of similar buildings across Britain were subjected to safety checks, and some residents were evacuated as a precaution.
The painstaking process to identify all the remains took months and anger is still simmering among the survivors, with most awaiting permanent housing and voicing scepticism about an official inquiry.
On Friday experts said stilted buildings such as the Jecheon fitness centre are especially vulnerable to fires that begin on the ground floor as staircases act like smoke stacks, making flames spread upwards quickly.
Vehicles in the car park under the building also ignited, fuelling the blaze.
Toxic fumes and flames were sucked rapidly upstairs to reach the first floor sauna, suffocating victims who had no escape but a narrow smoke-filled exit.
"The female sauna has a large toughened glass wall and no windows," Lee Sang-Min, head of Jecheon Fire Station, told journalists.
He also complained of illegally parked cars in the roads around the site, which hampered fire trucks in their operations.