Letchworth's garden city design principles restrict most homes to having no more than 50 per cent of their front yards paved over as part of measures to retain the city's special character.
But local newspaper The Comet reports companies have been knocking on doors and telling homeowners they can do additional work without consent.
The foundation's Chris Shipman has called for locals to check with its heritage advisory team before adding or replacing hard surfacing or even putting in artificial grass.
"It’s a garden city and we need to protect its appearance. Excessive hard surfacing can also lead to problems with rainwater drainage," Shipman said.
He told the newspaper he had seen 22 cases in the past year which were carried out without approval. Some homeowners were now faced with expensive bills or even enforcement action, he added.