Add sugar, water, and dried lavender to a small saucepan.
Heat over low, stir until sugar dissolves. Simmer for 3-4 minutes.
Remove from heat. Let cool for 10 minutes, then strain out lavender.
Juice the grapefruit until you have 1/4 cup.
Fill a tall glass with ice cubes.
Pour in fresh grapefruit juice.
Add 2 tablespoons of the lavender syrup (or to taste).
Top with soda water and stir gently with a spoon.
Garnish with a grapefruit slice and a lavender sprig if you like.