The Nutter-era zoning code can’t claim all the credit for Philadelphia’s long-running housing boom, but it has certainly played an outsize role over the last eight years. Thanks to the changes adopted in 2011, developers have been able to build the kind of big, roomy houses that Americans love without having to beg the zoning board for permission. The new rules paved the way for more apartments and duplexes, which offer a less expensive alternative to those single-family townhouses. And the code slashed parking requirements, making the sidewalks friendlier and safer for pedestrians.