Almost half of the South East's historic parkland has disappeared "almost unnoticed" in the last 90 years, a rural heritage report says.
The shell of Appuldurcombe Park stands in Capability Brown grounds
The region has seen a greater loss of parkland to farming, roads, rail and housing than anywhere else in England.
However, research showed more than 50% of the South East was still "rural".
"We owe it to future generations to work with landowners to enhance rather than destroy our heritage," said SE English Heritage director Andy Brown.
The report revealed that many of the country's historic farm buildings such as barns, stables and dovecotes were under threat from disuse, dereliction and "horror" conversions.
Dr Brown said many of Kent's oast houses, now no longer used for drying hops, were the subject of "regrettable" conversions to housing in the 1980s.
"However, there is no point crying over spilt milk - in many ways it is a good thing that these buildings have been given a future by being converted."
He added that the hop pickers' cottages were as important as the oast houses.
"The little cottages where Londoners stayed when they came down and picked the harvest were all parts of the system.
"Some of those really lend character to the countryside when you come across a little group of them."
While much of the countryside lacked the legal protection offered by conservation areas in towns and cities, schemes run by English Heritage and the Department for Environment, Food and Rural Affairs could help farmers and landowners.
"The will is there. I have met many, many farmers and rural businessmen who are really keen to do the right thing by their buildings," said Dr Brown.
Interest in historic attractions was high in the South East, with 11m visits generating over £10bn in revenue per year.
English Heritage listed six parkland projects which had benefited.
Knepp Castle, near Horsham, West Sussex - Arable land returned to parkland, trees replanted where they would have been in Humphry Repton's original design and historic ha ha restored.
Godinton Park, near Ashford, Kent - Large-scale restoration of lake and return of arable fields to parkland, tree planting.
Appuldurcombe Park, Isle of Wight - Ten-year restoration of park landscaped by Capability Brown with tree planting and arable fields returned to grazing.
Hall Barn, near Beaconsfield, Buckinghamshire - Restoration of 17th and 18th century pleasure grounds where the movie Gosford Park was filmed.
Clandon Park, near Guildford, Surrey - Land converted to arable farming during World War II returned to parkland to replicate the original Capability Brown design.
Cornbury Park, near Charlbury, Oxfordshire - Grassland management, tree planting and boundary wall restoration.