The sides of downtown Detroit buildings have become hot real estate for big retailers and brands that want the attention of the city's visitors and residents.

But nearly all of these commercial wall signs from the likes of Apple, Shinola, Meijer, Fifth Third Bank and the Detroit Pistons will have to come down by Dec. 31 as the city cracks down on building owners who have been violating what was once a seldom-enforced ordinance against such advertising.

"The issue was brought to our attention by owners of legally established signs, members of City Council and others," said city spokesman John Roach. "Since then, we've been trying to get building owners to comply."

Most building signs that are up now in downtown are illegal under Detroit city code. There is a prohibition against large advertising signs, billboards and painted wall graphics within a roughly 15-mile area from the Detroit River out to East and West Grand Boulevard in all directions.

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"If you have to ask us, we are flagrantly illegal," said Andrus McDonald, president of real estate for Detroit-based outdoor ad firm Brooklyn Outdoor, which contracts with advertising agents who pay to place building signs.

"But a lot of these building owners have owned these buildings through thick and thin," McDonald said, "and there's no reason they should not be allowed to benefit from this exponential growth in demand" for Detroit sign ads.

The end-of-year deadline to remove the signs is part of an agreement between some building owners and the city. In exchange for the building owners taking down the signs, the city agreed to stop issuing fines for having the signs up.

Losing the signs will be a revenue hit for these building owners, who, according to market insiders, can make about $4,000 to $10,000 a month by renting space on their building for a wall-sized ad. This money generally comes to building owners through advertising firms, which have contracted with agents that represent the companies or brands in the ads.

"The impact of these dollars is very important," said Sharon Madison, an architect who owns the Julian C. Madison Building, 1420 Washington, which currently displays a banner for the Pistons featuring a giant-size Andre Drummond.

Madison believes that property owners like herself should be permitted to display advertisements, and says those advertising dollars allow for building improvements and renovations. Recent downtown signs, including those for the film "Detroit" and Apple's new iPhone X, have been in good taste and aren't simply visual clutter, she said.

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"For the most part, I think the signs themselves have been well done," Madison said. "I understand the concern about not looking like Times Square, but that's not what we're seeing. And I don't see that happening in Detroit."

The current restrictions for building signs date to the late 1990s. Back then, city officials criticized the proliferation of signs and billboards showing cigarettes, alcohol, scantily clad women and the soon-to-open casinos. The city allowed existing signs to remain up.

Downtown's latest signage boom began about three years ago. Around the same time, a group of city officials were working to update the city's rules for so-called off-premise signs, which advertise something that doesn't relate to the building with the sign.

McDonald said the sign industry provided input for the group's effort to update city code. The industry thought that downtown building signs were on their way to becoming legal, he said. But, ultimately, the city did not amend its code and the restrictions stayed in place.

Nevertheless, McDonald said city officials kept a "laissez-faire" attitude about downtown signs and didn't begin ticketing building owners for having illegal signs until about a year ago. Since that ticketing began, McDonald said his firm has paid tens of thousands of dollars in city fines on behalf of building owners.

McDonald said he is puzzled by the city's stance against building signs. The advertisements have meant more than $1 million a year in extra revenue for Detroit building owners, he said.

"The industry is about putting signs where advertisers want to be and where it makes sense from an aesthetic standpoint," McDonald said. The proliferation of signs, he said, "is a direct reflection that these companies want to spend their money here."

All illegal building signs must come down by Dec. 31 under agreements that city officials made with individual building owners. In exchange, the city stopped writing violation tickets for as much as $1,500.

The Michigan Opera Theatre, 1526 Broadway St., will need to take down two building signs: one for Meijer, the other for Comcast.

"We encourage the city to recognize the significant economic benefits the signs provide to local Detroit property owners, many of whom are nonprofit institutions like Michigan Opera Theatre," Patricia Walker, the organization's chief administrative officer, said in a statement. "The revenue we gain from advertising banners is a significant addition to our operating income and supports our ability to offer world-class opera and dance in the heart of Detroit."

Dennis Archer Jr., whose marketing agency represents Seattle-based advertising firm Total Outdoor, said he hopes that City Council will, at some point, update Detroit's "archaic" sign ordinance to allow for building signs.

"Detroit is hot right now, so it is in everybody's advantage to get a new ordinance in place as soon as possible," said Archer, CEO of Ignition Media Group. "The city is probably a little bit behind in the times in being commercially competitive in the outdoor space."

A draft proposal earlier this year called for allowing some off-premise commercial signs in downtown, but only in tight "zones" around Comerica Park and the west side of Little Caesars Arena.

That idea proved unpopular with the sign industry and many building owners, who would be outside the zones allowing signs for popular brands and products such as Apple's iPhone or a Shinola watch.

"We fear that if this (proposal) passes the way it is drafted now, the only people who will benefit are the stadium owners," McDonald said.

Contact JC Reindl: 313-222-6631 or jcreindl@freepress.com. Follow him on Twitter @JCReindl.