Many architects, including McDonald, have gone well beyond the LEED standards, to create so-called Net Zero buildings, which consume no fossil fuels at all. As prices for solar panels have fallen, rootop arrays have also become common; you can even see them on the city’s old rowhouses. But few Philadelphia designs have flaunted their sustainability bona fides in quite the same way that Front Flats does. And because it wears its virtue on its sleeve, the burden of justifying the unusual design is greater.