Three years ago, Prince William and Kate Middleton traded their primary residence, Kensington Palace 's four-story, 20-room Apartment 1A, for a cozier family home: four-bedroom Adelaide Cottage on the grounds of Windsor Castle , about an hour outside of London .

Now, RadarOnline.com can reveal, they're looking to upgrade again, to a grand Crown Estate property nearby. Article continues below advertisement

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Fort Belvedere is a sprawling Gothic Revival mansion nestled on 59 acres in Windsor Great Park.

The couple, a source said, feel they "need somewhere more substantial" to raise Prince George , 12, Princess Charlotte, 10, and Prince Louis , 7.

The impressive

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