Birkhall is a smaller private residence on the Balmoral estate, which is 50,000 acres in total, and is where the couple spent the majority of the coronavirus lockdown together.

Source: https://www.dailyrecord.co.uk/entertainment/celebrity/king-charles-avoid-balmoral-castle-30586513

He was however a regular and popular guest of his aunt, the Queen Mother, at the Royal Lodge, her home in Windsor Great Park and at Birkhall on the Balmoral estate.

Source: https://www.dailymail.co.uk/news/article-12827913/Who-Queen-Mothers-Bowes-Lyons-family.html?ns_mchannel=rss&ns_campaign=1490&ito=1490

Perhaps this is why, since that particularly special New Year nearly 20 years ago, the King and Queen have spent almost every Hogmanay since in Scotland, at Birkhall.

Source: https://www.dailymail.co.uk/news/article-12911467/The-secrets-King-Charles-Queen-Camillas-traditional-Highland-Hogmanay-Close-friends-tartan-outfits-salmon-game-menu-ceilidh-music-plenty-drink-including-50-year-old-malt.html?ns_mchannel=rss&ns_campaign=14

The King, I am told, plans to base himself at Birkhall from early August until early October, coming and going as and when official duties dictate.

Source: https://www.dailymail.co.uk/news/article-12350123/As-King-Charles-heads-Balmoral-weekend-REBECCA-ENGLISH-reveals-bittersweet-secrets-summer-Scottish-castle-without-late-Queen.html?ns_mchannel=rss&ns_campaign=1490&ito=1490