A lot of landlords write off ECO4 the moment they realise their tenant doesn't claim a qualifying benefit like Universal Credit or Pension Credit. That's only half the picture. Every unitary authority in England and Wales can, in principle, run its own "flexible eligibility" route into ECO4 alongside the national benefits route — a route usually shortened to LA Flex.
Cornwall publishes a Statement of Intent for LA Flex, which means a household in a Cornwall rental can be referred onto ECO4 using the council's own criteria (typically income, health, or fuel-poverty triggers) instead of the standard benefits test. If your tenant's household income is modest but they don't tick the Universal Credit box, that's exactly the gap LA Flex exists to close.
Cornwall covers a mix of urban and rural stock, so eligibility on a given rental depends as much on the property's EPC band and heating fuel as on the Cornwall criteria — a Victorian terrace on gas and an off-gas rural cottage produce very different funding paths.