Combine sugar, yeast and 75ml warm water. Let stand until creamy, about 10 minutes.
In a large bowl, combine the yeast mixture with flour and salt; stir well to combine. Stir in additional water, 1 tablespoon at a time, until all of the flour is absorbed. When the dough has pulled together, turn it out onto a lightly floured surface and knead until springy, about 4 to 6 minutes.
Lightly oil a large bowl, place the dough in the bowl and turn to coat with oil. Cover with a damp cloth and let rise in a warm place until doubled in volume, about 30 minutes.
Preheat oven to 240 C / Gas 9.
Deflate the dough and turn it out onto a lightly floured surface; knead briefly. Pat or roll the dough into a sheet and place on a lightly greased baking tray. Brush the dough with remaining oil and use your fingertips to make dimples across the surface. Sprinkle with rosemary leaves and extra salt to taste.
Bake focaccia in preheated oven for 10 to 20 minutes, depending on desired crispness. If you like it moist and fluffy, then you'll have to wait just about 10 minutes. If you like it crunchier and darker in the outside, you may have to wait 20 minutes.