Sechach that extends past the walls of a Sukkah:

Contains two extending walls:[1] If a Sukkah contains Sechach that extends past the back wall of the Sukkah, and the two horizontal walls likewise extend past the back wall, then if that area is at least 7x 7 Tefachim, it is Kosher so long as it provides majority shade.[2]

Contains one extending wall: If there is Sechach extending past one of the walls of the Sukkah and there is one wall that extends together with the Sechach, then seemingly even if that area is 7×7 that area is not considered part of the Sukkah and it is forbidden to eat under it being it contains only one extending wall.

Contains one wall :[3] If a Sukkah contains three walls and there is Sechach extending past the open side and one of the walls of the Sukkah extends together with the Sechach, then if that area is 7×7[4] that area is considered part of the Sukkah and it is permitted to eat under it even though it contains only one extending wall.

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[1] 631/8

[2] The reason: As in such a case that area contains all the criteria of a Kosher Sukkah, as it has three walls and is 7×7, and has majority shade. Now although the middle wall was not made for this Sukkah but rather for the main Sukkah, nevertheless it is valid to serve as a third wall also for the smaller Sukkah. [ibid]

[3] 631/9

[4] However the Chazon Ish rules that the area is valid even less than 7×7. [Piskeiy Teshuvos 631/3]