Soul Factory is a 24-hour practice space located in Seodaemun-gu near Hongik Univ. Station. It's not for whole bands, but for individuals looking to practice alone in fairly small rooms. They say that their rooms can accommodate vocals, guitar, bass, wind and string instruments, and more, but no drums. You can pay by the hour or enter into a monthly contract.
You can contact them at 010-8007-4121.
They're located on the 1st basement floor of Dongshin Building (동신빌딩):
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