The soan tea room had clay walls, with windows and an entrance that could be opened and closed. This kind of closed structure does not seem to fit the tradition of Japanese architecture, but private houses in mountain villages often have clay walls and a closed character.

Chanoyu has placed great importance on 'light' and on letting the right amount of it into the tea room. Too much brightness was avoided and a degree of darkness was sought. Rikyu taught that this very quality of light was the life of the tea room. Rikyu racked his brains about how to make a window, its position and size. In order not to affect the amount of light let in by the window, he made the entrance to the room small, a 'crawl-through' space (nijiriguchi) just big enough for one person to crawl in or out of the room. If the wooden door that the guest had entered through was closed, the quality of light in the room would be controlled by the windows alone. Light that passes through shoji (paper-covered wooden frames) is deeply soothing to the spirit.

Tea rooms were narrow but completely covered with tatami. That style of residential architecture was ultramodern for the time. Into this room a sunken hearth (irori or ro) was built. The ro had been a tradition of everyday life since the time of pit dwellings. Soan tea rooms brought together the old and the new and created a fresh space surrounded by clay walls.