A hanok is a traditional Korean house: a timber frame, a curved tile or thatch roof, paper on the doors and windows, and a floor warmed from underneath. That floor is the ondol. In the old houses a fire under one end sent heat through flues beneath the stone floor; today most use hot-water pipes, but the idea is the same.
What you sleep on
You sleep on a yo, a padded mattress unrolled on the warm floor, with a thick quilt on top. It is firmer than a bed and, in winter, warmer than anything else you will sleep in. Most people sleep well the first night and very well the second.
If your knees or back are unsure, say so. Many hanok guest houses now have a room or two with raised beds, and we ask before we book.
What to ask for
Ask about the bathroom. Old houses were not built with them, and some still share a block across the courtyard. Several of the places we use have added private bathrooms inside the old frame. If you want one, it narrows the list, so tell us early.
Paper doors are not soundproof. Bring earplugs, take your shoes off at the step, and expect to hear the courtyard wake up before you do.