Philadelphia is finally cracking down on Public Enemy Number One: bay windows.

City Councilman Kenyatta Johnson has introduced an ordinance that would ban these protruding windows in the neighborhoods of Point Breeze and Grays Ferry. Balconies would be verboten too. His aim, he says, is to prevent undue clashing with the traditional brownstone homes that populate the area.

"I call them pop-out windows," he told local radio station WHYY last week. "That's where we have these monstrosity developments with windows with aluminum siding that are green or orange or blue, and they don't fit on these blocks that are all red-brick rowhouses."

For some, the bay windows that have been popping up on new townhomes and condominiums throughout the city are just the most outward sign of the city's rapid gentrification.

"They are an icon of that change, and maybe for a lot of people, they are an icon of unwelcome change," Patrick Grossi of the Preservation Alliance for Greater Philadelphia tells the Philadelphia Inquirer.

Johnson's bill passed unanimously out of committee last Wednesday, and the full city council may vote on it this week. It is expected to pass easily.

The bill has nevertheless courted opposition. The city's Streets Department has said the ordinance would be essentially unenforceable, as well as a strain on limited city resources.

The Inquirer's editorial board has also opposed the measure, deriding the idea that gentrification could be stalled by selectively targeting a few of its most transparent symbols. The paper calls this the "bay windows theory"—a play on the "broken windows theory" of policing.

That editorial also notes that this kind of microregulation of buildings' outward appearance could be used to block the development of affordable housing that doesn't perfectly match a surrounding neighborhood's aesthetic.

There's a more direct way trying to preserve Philadelphia's stock of brownstone rowhomes could lead to greater gentrification and affordability problems: Historic preservation is expensive, since historic buildings require wealthier owners that have the money to prevent them from falling apart.

This is a point made by free market urbanist Alain Bertaud on a recent episode of the podcast Econtalk.

"You cannot maintain a brownstone unless you have a lot of money. And it's the same thing with any historical building," said Bertaud. "It will have to go to rich people. If not they will deteriorate very, very quickly."

Compared to other large cities, Philadelphia remains relatively affordable—in part, perhaps, because it has been more welcoming of development.

Yet the more and more minute regulations the city passes governing how home and apartments have to look, the more chances NIMBYs will have to stop needed new development it its tracks. Johnson's bill is relatively limited, but it could represent a dangerous slippery slope.