As a key witness for the South Downs Campaign, Tregay had provided evidence during a lengthy inquiry into the area and recommended the park's boundaries.
The process involved giving evidence to include the Western Weald within the park's boundaries. It had originally been excluded from proposals despite being sited within an existing Area of Outstanding Natural Beauty.
"This is great news for the South Downs Campaign," said Tregay. "I'm delighted that LDA Design has been so closely involved with the proposed South Downs National Park over the years.
"To hear that the Western Weald has been, quite rightly, included is fantastic news and the South Downs Campaign team must be extremely happy with the result after all its hard work and lobbying has paid off."
The South Downs Campaign lobbied to secure the boundaries of the National Park to ensure it was protected from development.
In the campaign, almost 20,000 postcards were sent to the Government to encourage the inclusion of Western Weald. Supporters included Bill Bryson, David Dimbleby and Floella Benjamin.
The Open Spaces Society supported the campaign. General secretary Kate Ashbrook said: "A park in the South Downs, with a new National Park authority, will help protect this unique area from development and modern pressures."