Simon Berni, 41, a builder, was shocked to discover a secret haul of more than 30 shotguns and pistols as well as hundreds of rounds of ammunition.
He said: "It was an incredible arsenal - full, absolutely choc-a-bloc with eight shelves of fire arms and ammunition."
Police are investigating the gun stash uncovered in a semi-detached in a quiet village.
The guns were wrapped in newspaper dated March 1996, just days after the Dunblane massacre in which 16 children were shot dead. Handguns were banned in the wake of the killings.
Police believe the guns were unregistered and their owner hid them out of fear of losing them in the wake of the Dunblane shootings.
Mr Berni was renovating his uncle's house in the quiet village of Dinas Powys, near Cardiff, when he made the discovery.
He found one set hidden in the space behind a wall heater and another when he took down a plasterboard wall.
The firearms ranged from a pistol from the 1930s to a pump-action shotgun. Around 250 live bullets were also found.
He added: "I was shocked because there weren't any bricks in the wall but a lot of shotgun barrels and hand guns.
"They obviously belonged to somebody who knew what they were doing as they were all greased and beautifully wrapped up in newspaper."
Police are working on the theory that the owner of the guns died after concealing them in the house.
Following the discovery the police were called to the modern semi-detached home and carried out a search with sniffer dogs.
A South Wales Police spokeswoman said: "Police in Barry are investigating the discovery of up to 30 old firearms at a property under renovation in Dinas Powys.
"The weapons have been removed safely by trained police officers.
"It is believed that they have remained undiscovered for several years having been concealed by a previous owner of the property."
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