LAWS OF SUKKAS

 It depends on the size of the beam—some say it's 9¼", while others believe it's 12" 

 If the gap between the skylight and the wall is less than 9¼" or 12" (as discussed regarding the beam), it is considered part of the sukkah, and you may eat and sleep beneath it.

If the distance is up to 5 feet, the wall is still considered valid; however, you may not eat or sleep under that section of the Sukkah. 

 If your space doesn’t have three walls, a partition can be added to create the third wall.