Affixed to the facade of 58 Sheffield Terrace, in London’s Holland Park, there is a 19-and-a-half-inch wide ceramic blue plaque. It’s in place to commemorate the detective novelist Dame Agatha Christie—creator of improbable detectives like Hercule Poirot and Miss Marple—who lived in the property between 1934 and 1941.
The plaque isn’t exclusive to the west London house. Over 900 blue plaques can be found on properties across the city—with the highest density in the boroughs of Camden, Kensington and Chelsea, and Westminster. All are there to remember a notable person that once lived inside.
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At 34 Montagu Square in Marylebone, for example, there’s a plaque paying homage to John Lennon’s tenure at one of the building’s flats; and at 3 Chalcot Square in Primrose Hill, another indicates the house was once home to poet Sylvia Plath.
The official blue plaque program, exclusive to London and run by Historic England, has been operating since 1886.
But while the blue badge of honor certainly commands interest, it does not inherently affect the value of a home, experts say.
"We wouldn't necessarily value it more because of a blue plaque," said Craig Draper, associate at Knight Frank Marylebone.
That’s not to say it doesn't help a property find a buyer.
"It's more a kudos value than a financial value," said Charlie Bubear, head of Savills in Chelsea. "You can't slap on a percentage because X person lived there. It doesn't really work like that."
"We’ve found in the past when selling, a blue plaque will increase the exposure for the house," he said.
"People love to know the history of the house. Who's lived there? Why it was built?" Mr. Bubear said. "If we can get a good story, it creates more exposure, and the more exposure, the more interest and that commands the price."
"London is a city steeped in history, steeped in fascinating people, and everyone loves a story," Mr. Bubear said.
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On May 31, the most recent blue plaque was unveiled at 50 Smith Street in Chelsea. Between the years of 1946 and 1962, the brick townhouse on which it was affixed was the home of P. L. Travers, author of "Mary Poppins."
Travers lived at the house during her fraught negotiations with Walt Disney regarding the adaptation of her books into film, according to Howard Spencer, a blue plaques historian.
"She took a picture of the house, a fairly standard London house, to show Walt Disney what [the Banks family] should have lived in," Mr. Spencer said. "That connection between the person and the building, that's what makes for a good plaque," Mr. Spencer told Mansion Global.
As opposed to property owners nominating their homes for a blue plaque, most suggestions come from people recommending that a certain individual should have a plaque in the city.
To qualify, the person nominated must have died at least 20 years ago and have a surviving and recognizable property in London in which they lived for some time or for an important period of their life, Mr. Spencer said. If a well-known person lived in more than one home in the city, the most prominent or important residence will be chosen for the plaque.
A qualifying nomination then goes before a panel to either be approved or rejected.
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"We get about between 60 to 70 valid suggestions a year," Mr. Spencer said. "And we aim to put up about 12 [plaques] a year."
The ideal individual to be honored with a blue plaque is someone historically significant whose name still carries weight. They should be famed in their field whether that’s theater, film, music, science, medicine, history, exploration, engineering, archaeology, civil rights, or any other industry.
"It’s bit of a matter of fine judgment," Mr. Spencer said. "Take the example of a poet. You can get a sense if a Victorian poet is still read if their work has been recently reprinted, or there are critical texts. If there hasn't, who’s going to see and enjoy this plaque? It won’t resonate."
On the other side of the coin, a very famous person who may have only lived at a London property for a very short time would also not be befitting a plaque.
The blue discs, besides being a historical indicator, can also be a way of preserving buildings.
While the presence of plaque does not limit construction or renovations, "they act as a sort of soft power, raising awareness," Mr. Spencer said. "They make people less likely to swing a ball through the house."
But due to the subjective nature of the historic value of a blue plaque, Mr. Spencer said, those looking to add value to their home would be better off adding a "rainforest shower or a new kitchen."