There's a 3,600-square-foot rooftop plaza downtown sprinkled with cafe-style tables that has a wonderful view of the hills between the Financial District and Golden Gate Park. But, like more than a dozen privately owned public open spaces downtown, it's luxurious, yet hardly used. A decades-old city policy requires publicly accessible open spaces in new commercial buildings. But getting to the terrace that surrounds the antique dome atop the Westfield Centre shopping mall entails entering an adjoining office lobby used by S.F. State and Microsoft, so you can access the terrace only on weekdays.