'She's not sure whether to have the bash at Highgrove House in Gloucestershire or in London at Kensington Palace, but either way she wants it to be private and special,' reveals our insider.
And here comes the best bit - her romantic presents to say 'I love you'.
Our source says: 'Kate could've quite easily gone out and bought something to mark Wills' landmark year but instead she's decided to give him something far more personal as it will mean so much to him.
'Firstly, she wants to make him a scrapbook of their life together.
'It will include snaps of every holiday they've been on, memorabilia of special moments in their relationship and a heartfelt birthday message.
'Not only that, she's also making him a mixtape - well, an iPod playlist - of songs that have special meaning to them.'
The couple are no strangers to romantic gestures.
In March, they were pictured looking loved-up after Wills whisked her away on a skiing trip to Switzerland to say sorry for missing Valentine's Day, as he'd been away working in the Falklands.
Aww, wedded bliss.
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