Homes built by one of the UK's largest developers were constructed without essential barriers to slow the spread of fire.
Regulations dictate the flame-resistant material must be installed in roof spaces and wall cavities.
Housebuilder Persimmon Homes found it was missing from some properties on estates in south-west England.
It has written to more than 1,000 people to say their homes need to be checked.
One resident in Truro, Cornwall, said his house "is potentially a massive fire risk".
The homeowner, who did not want to be named, said recent inspections of his five-year-old house revealed "a vast amount" of fire were barriers missing.
Some of the homes affected are on a Persimmon-built estate in Exeter where a fire last year "rapidly escalated" as it spread between properties.
Speaking about the blaze, Cornwall councillor Dulcie Tudor said it had spread "through to the roofs of the adjoining houses".
Fire safety consultant Alan Cox said blazes "could easily travel from one compartment or property to another" if there were missing barriers "at roof level".
https://www.bbc.co.uk/news/uk-england-cornwall-47613496
