Leslie Millar had been granted planning permission to build an additional dwelling and garage on his land outside Lisburn.
But following a legal challenge by David and Elaine Lamont a judge ruled that the decision must be quashed.
Judicial review proceedings centred on approval given in 2011 for development on Mr Millar’s holding close to the Ballyclough Road.
Part of his land, including a stable building in a field, is adjacent to the Lamonts’ house.
Objections to his application were based on a claim that it did not comply with a planning policy statement which deals with sustainable development in the countryside.
Lawyers for the Lamonts argued that the Department of the Environment’s Planning Service had not properly interpreted and applied the relevant provisions.
Policy CTY10 states that if the proposed dwelling will link or cluster with a group of established buildings on the farm planning permission will be granted.
Mr Justice Treacy held that the planning application should have been refused for non-compliance.
The proposed dwelling did not link or visually cluster with a group of buildings as there was only one building – a stable block – on the development site, he pointed out.
He acknowledged that case law does not require Planning Service to slavishly follow the policy designed to achieve broader social and environmental goals on development in the countryside.
But its contents and desired results cannot be ignored, the judge stressed.
He added: “It is clear that at all material times the decision makers either did not have regard for the policy or did not understand the policy.”