After receiving the one-and-a-half mat Fushin'an from Sotan, it seems that Koshin thought of rebuilding it. According to Kawakami Fuhaku(1719-1807), a tea devotee of the middle Edo period, Koshin asked Sotan's advice about the surplus space outside the tea room and changed it to the present three-and-a-half mat room. This explains how he came to change the structure. He may also have been thinking of changing to Shoan's fukasanjo-daime (three mats in a line extending inwards from the entrance plus a daime three-quarter size mat).

However, this time he changed to a hirasanjo-daime (three mats in a line looking sideways from the entrance). Probably because he wanted to keep the feeling of closeness between the host's mat and the guest's mat which he had experienced in the one-and-a-half mat room, he inserted two mats between the one guest's mat and the host's daime mat in his plan of the tea room. Accordingly the kyujiguchi (assistant's entrance) was next to the toko. However, because the katte (kitchen) was in the same position as for the fukasanjo-daime tea room of Shoan's time, the sadoguchi (host's entrance) was on the furosaki (beyond the brazier) side of the host's mat. For this reason, next to the host's mat by the katte was a board (itadatami) to make a wider space. This is because Fushin'an is on the south side of the shoin (Zangetsutei), so it is built facing the south. This special sadoguchi next to the host's mat at Fushin'an is a fusuma (paper-covered door) that opens outwards. In the old days fusuma were all sliding doors, but Sotan said, 'Fusuma that open outwards are also wabi style and are interesting.' The ceiling in front of the toko is a flat one covered with bulrushes, the one above the nijiriguchi entrance is a slanting kesho yaneura, and so is the section above the host's mat. The heaviest part of the ceiling is firmly supported by a central pillar next to the ro (hearth) which is between the host and guests. There is a horizontal bamboo kabedome at the bottom of the sodekabe (extended wall) which cuts it off about half way from the ceiling to the floor, so that the utensil mat can easily be seen from the guest mat. This became the model for the Senke daime-style tea room. From the outside Fushin'an has a regular kirizuma roof shape (like a half-opened and inverted book, the length of the ridge and the eaves are equal), with extended eaves (hisashi) on the front and the side (above the host's mat). The main roof is shingled in the yamatobuki style with the boards overlapping so that every other board is on the same level. It is a very complex structure but appears light and resembles a soan (rustic hut).

Zangetsutei is behind Fushin'an and is approached from the west side. The irotsuke shoin (coloured shoin style) tea room in the Juraku residence had a two mat upper level and a four mat middle level. Shoan skillfully reproduced the special character of the room in a slightly scaled down version by leaving out the middle level.