Chonghajin Marine Co., which operates the ill-fated ship, added extra rooms to hold about 180 more passengers, shortly after the ship was brought to Korea from Japan in October 2012, the Incheon Regional Maritime Affairs and Port Administration said.
The capacity of the ship rose to 921 passengers after the modification, with the number raised by 56 on the third floor, 114 on the fourth floor, and 11 on the fifth floor.
The construction is reported to have added weight to the 6,825-ton ship, a change that some speculate might have caused the ship to tilt more quickly.
But the Korean Register of Shipping said the cabin expansion was within legal bounds and it seems unlikely such a change would have caused the shipwreck.
The ship passed safety checks and weight requirements when it was empty, according to the ship classification body.
The Sewol was built in 1994 and operated under Maruei Ferry in the southern seas of Kyushu, Japan until September 2012.
By Suh Ye-seul (email@example.com)