Preheat oven to 350F with oven rack on lower middle position. Grease and flour a 9x4 inch loaf pan. Set aside.
In a large bowl, whisk together the sugar, egg, lemon extract, and oil. Stir in zucchini and lemon zest until combined. In a separate bowl, whisk together the flour, cinnamon, salt, baking soda, and baking powder until well combined. Using a rubber spatula, gently fold wet mixture with dry mixture just until no white flour streaks remain; do not over work the batter. Pour batter into greased/floured pan.
Bake 45 minutes or just until a toothpick inserted in center comes out almost clean, with a few tender crumbs attached (not fully clean.) Allow bread to cool in pan on a wire rack 15-30 min.
To make the icing, combine the powder sugar with lemon juice and whisk until incorporated. Add more powdered sugar if it's too runny. Add a bit more lemon juice if it's too thick. Once it's to your desired consistency, place in a Ziploc bag and cut off a tiny bit of the corner to make a drizzle-pouch. Carefully remove warm bread from pan and place on serving platter. Drizzle icing over the bread, allowing it to soak into warm bread. If desired, sprinkle top with extra lemon zest. Wait until bread is completely cool before slicing.