Which waka form is a tanka with one extra 7-on phrase at the end?

The waka form that adds one extra 7-on phrase to a tanka is Bussokusekika.

A tanka has the pattern 5-7-5-7-7. Bussokusekika extends it to 5-7-5-7-7-7 by placing another 7-on phrase at the end. The name is associated with the “Buddha-footprint poem,” particularly poems carved on a stone monument at Yakushi-ji temple in Nara.

The form is extremely rare compared with tanka and chōka. The Man’yōshū contains one recognized bussokusekika, while the anthology preserves thousands of tanka and hundreds of chōka. Its unusual final phrase makes it easy to distinguish from sedōka, whose pattern is 5-7-7-5-7-7.

The term is also a reminder that waka originally referred to several Japanese poetic forms, not only the 31-on tanka. During the Heian period, many of these older patterns fell out of use, and waka gradually became associated mainly with tanka.