As the St. Paul City Council gets ready to decide whether to allow “granny flats” citywide, the only neighborhood group to object is the realm of the carriage house.

The Summit Hill Association, which represents neighborhoods graced by some of the city’s most stately and historic homes, is urging the City Council to hold off on a Wednesday vote that would allow homeowners on lots of at least 5,000 square feet to build what are called accessory dwelling units, or ADUs.

In an Aug. 10 e-mail to the City Council, the Summit Hill Association said the city needs to adequately study how the units will affect parking, traffic, neighborhood character or property values. One longtime resident worries that decades of historic preservation will be undone.

What’s not clear is whether that opposition will make a difference.

In fact, the line behind citywide expansion has been growing for months, ever since the City Council in February asked staff members to study allowing units in additional neighborhoods as a way of adding affordable housing. Since then, the Planning Commission, a number of other district councils, the St. Paul Area Association of Realtors and the St. Paul Advisory Committee on Aging all have expressed support for allowing ADUs citywide.

Andy Rorvig, president of the Summit Hill Association, said Tuesday that the city has not taken enough time nor done enough study to determine if ADUs would be a positive for every neighborhood. While he acknowledged that there are many supporters of the idea in Summit Hill, there is very little data to show their potential benefits — or detriments.

“What we’re saying is we need some time to get everybody’s input,” he said.

Nancy O’Brien Wagner, a historian who lives with her architect husband on Linwood Avenue, said she opposes the expansion for one main reason: “For us, it’s an issue of keeping the historic nature of the neighborhood,” she said.

Homeowners in and around Summit Hill had to fight for years to establish the city’s Historic Hill District following decades of the area’s grand old homes being subdivided into inexpensive rentals, she said.

“Every neighborhood is unique,” Wagner said, pointing to the mix of century-old bungalows interspersed with apartment buildings on her block. “If the goal is mixed-use, this is an authentic mixed-use neighborhood. We already have the balance right here.”

After years of more general discussion, the City Council voted last year to allow homeowners to build accessory dwelling units, which can be stand-alone structures or apartment-type spaces above garages, in basements or in attics, but only in a zone along the Green Line on University Avenue.

While Minneapolis, Portland, Seattle, San Francisco and other cities have built scores of such units, it’s been a slog in St. Paul. Only one ADU has been built — in the Hamline Midway neighborhood.

In June, the Planning Commission recommended going citywide. And several other district councils, including Highland Park, Macalester-Groveland and Union Park, have told the City Council that they welcome the expansion.

But, while the city has for years allowed historic carriage houses in the Summit Hill and Crocus Hill neighborhoods to be rented, the idea of adding to the housing stock by finishing an attic, building atop a garage or creating a small cottage in the backyard has been slower to catch on. Accessory units proved particularly controversial in the St. Anthony Park neighborhood a few years ago, as some residents expressed fear that they could irrevocably alter the neighborhood’s character. Others worried about what will happen once the original owner decides to sell.

Rorvig said many of those same concerns exist among residents of the neighborhoods of the Summit Hill Association. Yet, despite area homeowners paying some of the highest property taxes in the city while also living in one of the most densely populated areas of St. Paul, residents didn’t find out until this summer that the ordinance might be expanded beyond those neighborhoods that want it.

“In the end, it may be a great idea,” he said. “We just want to have some more time.”