Heat oven to 375°F; butter a 9-by-13-inch baking dish.
Toss peaches with 3/4 cup sugar, cornstarch, lemon juice, cinnamon, and 1/4 teaspoon salt in dish. Bake 12 minutes while making topping.
Whisk flour, 1/4 cup sugar, baking powder, and 1/2 teaspoon salt. Cut in cold butter until pea-size pieces remain.
Stir in buttermilk just until shaggy dough forms.
Drop eight rough dough portions over hot fruit, leaving steam gaps. Sprinkle with coarse sugar.
Bake 30 to 35 minutes until biscuits brown, their centers cook through, and filling bubbles through gaps. Rest 25 minutes.