In a medium saucepan add the sugar, basil, 2 cups of water, and lemon zest to make a simple syrup. Over medium heat bring the mixture to a boil. Stir the mixture until the sugar is dissolved.
Once the mixture comes to a boil reduce the heat and continue to simmer for 5 minutes. Once it has simmered for 5 minutes remove the saucepan from the heat and cover, letting it steep for 15 minutes.
Strain the simple syrup to remove the lemon zest and basil. Discard the basil leaves and lemon zest. Cool completely. Once the simple syrup is cooled add the remaining water and lemon juice. Gradually add the lemon juice until you have your desired taste.
Place in the refrigerator to chill before serving. Add lemon slices and basil leaves if desired.