Preheat oven to 425 degrees F. Line a baking sheet with parchment paper.
Place the zucchini in a strainer over a bowl or the sink and sprinkle with ¼ teaspoon of the salt. Toss to combine and let stand for 10 minutes. Rinse and drain the zucchini, then squeeze any excess liquid from the zucchini.
In a large bowl, whisk together the flour, baking powder and remaining 1 teaspoon of salt. Using a pastry blender or two knives, cut in the butter until the mixture resembles coarse sand, with no pieces of butter larger than the size of a pea remaining.
Stir in the zucchini, cheeses, sun-dried tomatoes and basil. Add the milk and stir until the mixture is evenly moistened and there are no traces of dry ingredients left in the bowl.
Drop ⅓-cup portions onto the prepared baking sheet. Bake until golden brown, about 25 minutes. Serve warm or at room temperature. The scones can be stored in an airtight container at room temperature for up to 4 days; they can also be individually wrapped in plastic wrap, placed in a freezer ziploc bag and frozen for up to 2 months.