Cliveden National Trust has become the first in London and south east to welcome more than half a million visitors annually.
Cliveden, in Taplow, is the first pay-for-entry to see 500,000 visitors through the doors.
The property joins only three others who have met the figure across the whole of the National Trust.
Last year, Cliveden came very close to meeting this milestone with 499,043 visitors - but a number of elements have now helped to tip the balance.
Not least was the 2019 programming built around the former owner of Cliveden, Lady Nancy Astor, and the 100th anniversary of her being the first female MP to take her seat in the House of Commons.
Cliveden has also attracted attention as the setting of the Profumo Affair, dramatised in the BBC series The Trial of Christine Keeler.
In a bid to keep attracting hundreds of thousands of visitors each year, there are big plans to improve the facilities at Cliveden.
Managers at the site say sustainable growth is a "priority" - and plans are underway to revamp the site, starting with the £2.2 million conservation and refurbishment of the Conservatory cafe this summer.
A spokesman said: "Every visit enables National Trust Cliveden to conserve and protect the estate and collection for future generations to enjoy.
"This year, all fundraising is focused on reviving the oriental-inspired Water Garden, with plans to dredge and repair the pond system and increase the plant stocks in the garden which will encourage biodiversity."
The announcement of the major plans comes as Cliveden celebrates its 125th birthday this year.
To celebrate, Cliveden has launched a list of ‘50 Things to do at Cliveden before you’re 125¾'.
The list was compiled from suggestions from estate staff and volunteers, as well as the general public, who were asked on social media to submit their favourite things to do on the estate.
The list can be enjoyed by all ages and comprises suggestions for things to do across the whole estate, such as ‘Choose your favourite scent in the Water Garden’, and ‘Strike a pose and mimic one of the sculptures you find in the gardens’.
Alicia Peacock, senior visitor experience officer at Cliveden, who led the project said: “Our aim with this project is to help our visitors get the most out of their visit to the estate the whole year round.
"We hope that ticking through the list, created by those who know and love Cliveden best, will help people of all ages to reconnect with nature, rediscover their inner child and fall in love with all Cliveden has to offer, come rain or shine.”
The list of 50 Things to do at Cliveden before you’re 1253/4 is available for visitors to pick up, free of charge, from the Info Centre in the Walled Garden car park. It is also available to download on the Cliveden National Trust website. Visitors can share their experiences of 50 Things on social media by using the hashtag #50ThingsatCliveden.