It is thought the artwork has doubled the price of the £300,000 property in Cheltenham, Gloucs, rented by Karen Smith, 48, who had never even heard of the artist.
Although Karen won't profit from the artwork as she doesn't own the building, she still appreciates the mural depicting three 1950s style secret agents using tools to listen in on a real phone box.
She told The Daily Star: "It's pretty good. It livens the street up a bit."
Experts believe it is the first time the Bristolian artist has struck in the UK in almost two years.
Karen was suspicious after hearing voices in the early hours and seeing men loading items into a van: "I thought it might be something to do with the police. When I came out I saw people staring and asked what they were looking at. I think it's pretty amazing."
Although Bansky hasn't yet confirmed his handiwork, graffiti experts say it has all the hallmarks of an original.
They believe the image is a nod to the Government spy centre GCHQ located three miles away from the property.
Bansky's famous Girl With Red Balloon was recently chiselled off a wall in Shoreditch, east London and sold for a whopping £500,000 at auction.
By Sophie Ghatreh
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