Combine sugar and 1 cup cold water in a saucepan. Bring to a simmer over medium-low heat and cook until sugar has dissolved and mixture is syrupy, stirring frequently. Remove from heat. Stir in lemon zest, lemon juice and 3/4 cup cold water, then stir in limoncello.
Pour liquid into a large, shallow pan. Place pan on a flat surface in the freezer and freeze for at least 8 hours. During the first 2 hours, remove the pan every 20 to 40 minutes and stir the contents with a fork, making sure to scrape down the sides of dish and combining the iced and still-liquid portions. When the granita has frozen completely, use a large spoon or ice cream scoop to scrape the top, collecting each serving. Place in a cocktail glass or bowl and serve.