Add warm water, sugar, and active dry yeast to a bowl. Let it sit for about 8 to 10 minutes until it foams.
In a stand mixer, add all-purpose flour and salt. Mix well to incorporate.
Insert the dough hook attachment, start the mixer at low speed and slowly pour the yeast and water mixture.
Once the mixture starts combining into a dough, slowly pour olive oil and let the mixer run for a minute or two.
Stop the mixer, scrape down the sides and start it up again at low to medium speed until everything is well combined.
Increase the speed of the mixer to medium and let it run for 8 to 10 minutes until the dough no longer sticks to the sides of the bowl.
Stop the mixer and transfer the dough to a glass bowl.
Drizzle olive oil on top to grease the dough.
Cover the bowl with a kitchen towel or cheesecloth and place it in a dark, dry, warm place for an hour or until it rises and doubles in size.
Once the dough has doubled in size, punch it and fold it over a few times.
Place the dough on a greased rectangular baking tray. Use your hands to stretch and flatten the dough evenly into a shape you want. Let it sit for another 15 minutes.
Stretch the dough again and flatten it with your fingers. Let it sit for another 45 minutes to let it rise again.
Once the dough doubles in height, use your fingers to make indents in the dough.
Drizzle more olive oil and brush it evenly across the surface. Sprinkle sea salt and chopped fresh rosemary on top.
Place the baking tray in a preheated oven at 350 degrees Fahrenheit for about 40 minutes until its internal temperature is 190 degrees and the top turns golden brown.
Once done, remove from the oven, drizzle and brush olive oil on the focaccina again and let it cool on a cooling rack.
Cut into small squares, serve, and enjoy!