Alan Gavet and his sister are looking after the derelict property on Candie Road, Castel, on behalf of their uncle, who is in residential care.
But Mr Gavet said their plans to sell the site to pay for their uncle's care was in tatters after planners and an independent planning tribunal said the building had been derelict for too long to be classed as a house.
'We don't know what to do next,' he said.
'They say it is just domestic curtilage.'
He criticised the States for not making clear when a property stops becoming a dwelling.
