Bear Mason, 74, built the 4ft stone wall this summer to replace his tatty and broken-down fence which was 6ft tall.
He built the wall lower than the original fence but too high for people to sit on in case they toppled over the steep drop onto his rockery below.
But council bosses told Bear that someone had complained about the 20m long sandstone wall and that he needed to tear it down.
Bear, who lives in the village of Greenhead a few miles from Hadrian’s Wall in Northumberland, said he did not know he needed planning permission.
The retired maths teacher and A-Level examiner said: “This summer we paid £12,000 on a beautiful new wall between our garden and the road.
“It is lower than the rickety fence it replaced.
“Northumberland County Council have told us that they are now going to bulldoze it down unless we pay to have it lowered by eight inches.
“The wooden fence was 1.8m high and was falling to bits, and it was liable to collapse in high winds.
“We built the wall it to a height of 1.2m externally for health and safety reasons as internally there is a 2m drop into the garden, onto rocks
“At 1.2m walkers and drunk people cannot sit on it, and topple over but at 1m high, which is the planning stipulation, the situation is far more perilous.
“Because the wooden fence was much taller than its stone replacement, we had no idea it or the new wall would contravene their regulations.
“We are pensioners. We don’t have £12,000 to rebuild it.
“Our wall is so much nicer than what was there before and safe. We are mortified.
“Ironic too that we live in what’s called Hadrian’s Wall Country.
“We have a good idea about who complained to the council and they have some strange ideas.
“Most of the fences and walls in the village are above 6ft in height but because they have stood for more than four years they are exempt from the planning laws.
“The whole thing is barmy.”
Bear and his wife Sharon, 64, have agreed to disconnect the wall from a Grade I listed bridge and will apply for retrospective planning permission to retain their wall at its current height.
A spokesperson from Northumberland County Council said: “We were made aware that a wall had been built at this property without planning permission.
“On further inspection it was found to be attached to a listed bridge.
“For that reason it is not acceptable and we have asked that it is removed.
“We have also given the householder the option to reduce it in height to one metre and remove it from the listed bridge.”