The States-owned property is on the open market and has planning permission to be converted into a four-bedroom home.
Local agent Cooper Brouard and international agent Chestertons has been appointed to sell the property.
Cooper Brouard open market sales director Ross Le Marquand said it was a tough property to value.
'There's very little to compare it with,' he said.
He also took into account how much it would cost to renovate the property – around £2m. – and how much the finished building could be worth.
Planning permission has been given for four bedrooms, a wine cellar, utility room and office over the two floors.
The top floor would have a conservatory, which would house the kitchen, dining room and sitting room.