LONDON — It is not unusual for works of art to be lost and then found: It is rarer when the piece is attached to the side of a building.

But that is the case for a Banksy stencil, “Snorting Copper,” on the site of a former public toilet in the Shoreditch area of East London.

Five years ago, property developer Jonathan Ellis heard rumors that a lost work by the mysterious graffiti artist known as Banksy was hidden under a layer of paint on a brick wall in a weed-choked plot.

Two years ago, Ellis, by then the founder of Hamilton Court Developments, paid more than $4 million to buy the site on Curtain Road so he could convert it into a commercial and residential space.

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Eager to see if the rumors of a hidden Banksy were true, he wasted little time.

On the day he got the keys to the site, Ellis said, he had plywood that had been screwed to the wall removed, but “it wasn’t one of those eureka moments where we pulled the ply off and oh my God there it was.”

Instead, what he saw underneath was a section of white-painted bricks marked with numbers. “Because it had been painted over, we didn’t actually know what was going to be salvageable, or what was left,” Ellis said. “We were excited to see the wall hadn’t been removed, but at that precise moment didn’t know what we had.”

Ellis says he believes that the bricks had been numbered by someone intending to remove and reconstruct the potentially valuable stencil brick by brick. Instead, his company removed a whole section of the wall using a power tool.

“We were building there, so we had to explain to the builders — without telling them what was underneath it — that they had to protect this bit of wall,” Ellis explained. “If you looked at it, it was just a white bit of wall. You would’ve thought we had gone absolutely mental.”

After a wait of about a week, the wall was carted off to the Fine Art Restoration Co. in Carlisle, England. (The first van the company sent was too small for all the bricks.)

And it would take several more months of correspondence with the restorers to confirm Ellis’ hopes: There was a Banksy underneath.

The piece, now restored, is a little over 7 feet wide and 3 feet high, and depicts a police officer on his hands and knees preparing to snort cocaine, Ellis said.

Banksy’s authentication service, Pest Control, has not confirmed that the work is his. The local authority, Hackney Council, said its records of street art only date back seven years and so contain no mention of “Snorting Copper,” from 2005.

“We haven’t even tried to get it authenticated by Pest Control because, from what we were advised, they don’t authenticate street art,” Ellis said, “because what they don’t want us to do is get an authentication and sell it to the highest bidder.”

But Ellis said that was not his intent.

“I’ve seen a few things recently in the papers that have talked about developers, people using art to increase the value of their properties,” he said. “I don’t think it’s about that. It’s a moral obligation, isn’t it?”

He said he also doesn’t think that anyone today would choose to paint over the stencil: “If you look at Banksy’s work as an artist of our era and generation. You wouldn’t paint over a Monet, would you?”

Jo Brooks, a spokeswoman for Banksy, confirmed that there had been a piece of Banksy art in that location, and said there had been no dealings with the developer.

Ellis plans to unveil the restoration on Oct. 5 and to display it inside the new building so it can be protected but also seen by people walking by.

Hamilton Court paid about $4.3 million, for the property in 2015, according to the Land Registry. Ellis declined to comment on the sales figure, but confirmed that the artwork is insured for about $1.6 million.

He also said that he had declined offers from people wanting to buy the work for a “lot of money.”

Banksy “put it up for the right reasons and doesn’t want people profiteering from it,” Ellis said. “We’re comfortable and happy we want to put the piece back.”

Banksy, whose real identity remains unknown, has taken a dim view of efforts to profit from his public street art. When Sincura, a British concierge company, charged people to see Banksy works that had been removed from their original locations, the artist condemned the event.

“Graffiti art has a hard enough life as it is,” Banksy has said, “before you add hedge-fund managers wanting to chop it out and hang it over the fireplace. For the sake of keeping all street art where it belongs, I’d encourage people not to buy anything by anybody, unless it was created for sale in the first place.”

Communities have expressed their dismay, and buyers have become wary, but a Banksy mural nonetheless sold for more than $1 million in 2013.

Hamilton Court has, at least, stuck closer to the ethos of the artist, said Acoris Andipa, a collector of Banksy’s art. “It’s correct and quite a noble outlook on public art,” Andipa said. “I think that should be congratulated.”

“Of course, at the end of the day, there is valuable artwork and that’s not going to work against you,” he added.

Amie Tsang is a New York Times writer.