Mr Mitchell had gone to collect some farm machinery from land near Sandmoor Golf Club when he spotted 'piles' of stones and timbers that his instinct told him were unusually old.
A farmer casually told him they had once been a Jacobean manor house which had stood proudly in the ancient village to the north of Leeds before it was unsympathetically bulldozed to make way for developments to the golf course in 1969.
Amazed, Mr Mitchell immediately rescued the 17th-century building materials and took them to his farm in Gildersome, where he and his wife Louise lovingly cleaned them with the intention of reconstructing the house.
Yet three years later, the project had proved too ambitious and they decided to sell their transport business and move to Ireland, where they have lived on the coast of County Clare ever since with the pallets full of unique Yorkshire sandstone stored in their garden, having shipped them over in eight lorryloads.
The Mitchells have now admitted that time is against them, and have put the entire house - albeit in '3D jigsaw puzzle' format - up for sale.
Although there has been interest from the US and Ireland, Mr Mitchell's 'dream' would be for Alwoodley Hall to be rebuilt in north Leeds using the expertise of local stonemasons.
"When I first saw Alwoodley Hall, I could tell it wasn't just any pile of stone - I could see the gothic windows and the oak timbers, the Yorkshire sandstone. It was clearly no ordinary house. The farmer showed me some old photos of it, and told me it had just been bulldozed and left there. It was just in the way at the time."
He contacted local historian Steven Burt, who has written a book about Alwoodley and who confirmed that the walls were all built to bespoke specifications for the house and gave him information about its history and fate.
"We spent months cleaning the lime mortar off the stone and tar off the granite cobbles, but once we took it over to Ireland, the local planners weren't keen on the idea of rebuilding it. The stonework is not 'native' to Ireland, and it's a very big project."
Though iconic structures such as the original London Bridge have been rebuilt in the US, the trans-Atlantic transportation fees are high and Mr Mitchell thinks it would only cost an English buyer 'a few thousand' in shipping on top of the property's £185,000 asking price.
"If it was returned to Yorkshire and rebuilt somewhere like Alwoodley or Shadwell that would be epic. The right skills are available locally - Yorkshire has one of the highest concentrations of stonemasons. There are no original plans or drawings from 400 years ago, but there might not even have been a plan when it was built anyway."
A surveyor has previously worked out a layout and an architect could incorporate modern features into the interiors, which would not be the case with a building of similar age still standing and likely listed.
"There are no roof tiles or floors, but the stone ridges are there and there are 50 timbers that could become features. We had some of them carbon dated and they could have been there since 1066 (when a Saxon house stood on the site where the later hall was built before the materials were recycled).
"It's timber from 1,000 years ago - all pre-1400. That's incomprehensible. There are also cobbled stones from the basement, enough granite cobbles for a road several miles long.
"We've kept the stone outside and the timbers indoors. We didn't know where to start with the valuations and a surveyor said it could have even been into the millions.
"It's just got to come back to Yorkshire."
The history of Alwoodley Hall
The four-storey 'stone-cut' mansion was built in 1640 by Sir Gervase Clifton, an associate of King Charles I. The site had been occupied by a Saxon manor recorded in the Domesday Book, and it is thought he re-used some of the original materials.
The estate eventually passed to Henry Barran, who laid out the golf club in the grounds in 1926. He initially used the hall as staff accommodation, but as time went on it began to impede the course's expansion.
Leeds City Council gave permission for its demolition in 1969 on the grounds that the building was in a dangerous condition - though this has since been disputed. A local farmer acquired the stones, beams, lintels, cobbles, windows and doors, but then abandoned the lot on his land for the next 30 years until Steve Mitchell's chance discovery in 1999.
The house originally had three bays, three storeys and a basement constructed from Yorkshire sandstone and was rectangular in shape. An architect who examined 1960s photos estimated it had six or seven bedrooms and 5,000 sq.ft of living space.
Much of the detail around the windows and doors was hand-carved and could have taken years to complete.
Experts have noted that Alwoodley is a unique opportunity to build a 'modern mansion' as features such as insulation would be allowed, unlike in most listed buildings.
Although there is no catalogued record of what goes where, the outer body is relatively simple in shape, meaning it could be easily reassembled.
Included in the sale are 70 pallets of cut stone, 21 pallets of granite cobbles, approximately 400 handmade bricks, arched windows, window lintels, and much more stone such as large front doorstep, round wall cappings, and approx 180ft of rectangular cut wall cappings.
Anyone interested in purchasing Alwoodley Hall should contact Steve Mitchell on 07982445534.