That was the experience of one family on the outskirts of Glasgow - who will have to remain nameless if they want to avoid becoming a stop on one of the city's many Mackintosh bus tours.
Mr and Mrs X weren't fans themselves, but thought there was something familiar about those stained glass windows, those large stone fireplaces and the way even the gutters looped around the drainpipes in a smooth line.
They called in an expert, David Stark, who's director of Keppie Designs, itself a direct descendant of Mackintosh's company Honeyman Keppie.
During research for his history of the company, he'd visited various buildings which showed signs of Mackintosh design, but had never seen this one.
"We knew a house existed in this area but until I came here, we weren't sure this was it," he says.
Now, having had a good look round, he's quite convinced that Mackintosh had a hand in its design, pointing to the glasswork in particular as evidence.
Professor Pamela Robertson, senior curator at Glasgow University's Hunterian Art Gallery requires a bit more convincing.
She's leading the first major investigation of Mackintosh's work as an architect in over 60 years.
At the heart of it all is the Honeyman Keppie records, which meticulously detail all the buildings they ever worked on.
This house is listed - along with all the contractors who bid for work.
It's a reminder of the fact that this is a team effort - hundreds of contractors involved, from carpenters and plumbers to tilemakers and glassblowers.
The details are recorded in Mr Keppie's handwriting - as a senior partner in the firm, it was important he kept a close eye on the finances but later documents - the Hill House bill, for example, is in Mackintosh's handwriting, evidence of his promotion in the firm.
"I'm sure he had a hand in this one," says Professor Robertson," but I don't think he was the lead architect.
"He has clearly had some influence though and we have another 191 sites like this one, we'd like to look at more closely so there could be other surprises along the way."
That's a view shared by Historic Scotland - who recently listed the house.
Unable to conclusively prove it's Mackintosh, they've given it a category C listing.
For Mr and Mrs X, that's the perfect conclusion.
They can continue redecorating, knowing they have a little piece of history and perhaps a little bit of mystery on their hands. But no need to accommodate bus loads of tourists.