This sauna is exceedingly easy to find. Despite this, my bf and I wandered around for over an hour trying to remember where it was! Now we know, and we will never stay anywhere else in Hongdae. If you leave the Hongik University subway stop (Hongdae-ip-gu, line 2) from exit 9, just walk straight along that street until you get to the large, tall Benikea Premier Marigold Hotel building (베니키아프리미어메리골드호텔) about 10 min, on your left. Alternately, exit Hapjeong station exit 3, walk the same distance, it will be on your right. The jjimjilbang is down one level. Easy.
Inside, you'll find some 3 floors of decent rooms and baths, areas to sleep, and separate levels for the sauna and jjimjil. If you're staying overnight, I'd recommend getting there earlier as it can fill up on weekend nights. I find that the yoga room is the quietest, darkest place and they often leave it last for morning cleaning.
Recently, there are a lot of negative reviews on Korean sites, mainly because it is so popular with young people for a cheap night over in Hongdae and as a result, super noisy and crowded at night. I would caution against spending the night here unless you really have no other option, or you just want to clean up and stay up all night. Also seems you may need to pay extra for mats/bedding now too :(
서울시 마포구 서교동 371-10 번지 B1층
Seoul, Mapo Gu, Seo-gyo Dong 371-10, B1
Open 24 hours a day/ 7 days a week
(note: This information has been provided by recent visitors. All web sources seem to be out of date.)
Sauna + Jjimjilbang:
12,000 won - If you arrive between 5am & 8pm (for daytime or overnight use)
15,000 won - If you arrive between 8pm & 5am (for overnight use)
***Weekend costs may be slightly higher, no information available***
This cost covers 12 hours from the time you arrive, as usual. They also offer 4 hours free parking near the building.
Wikimap of Happy Day Jjimjilbang