Just to clarify things, "The Crown Estate belongs to the reigning monarch 'in right of The Crown', that is, it is owned by the monarch for the duration of their reign, by virtue of their accession to the throne. But it is not the private property of the monarch - it cannot be sold by the monarch, nor do revenues from it belong to the monarch."
The Sovereign Grant is given to the monarch from the Crown Estate and with that funding, the monarch is footing the bill for renovations and restoration of the properties the royal family uses.
One thing too is that when the Sussexes were taking Frogmore Cottage as their UK residence, it was the personalizing of the interior that they reimbursed the Sovereign Grant for. The renovations of any of the structure of the residence would have had to be done regardless if the Sussexes moved in there or Andrew or Anne or if it was to be a playhouse for Charlotte.
Its easier to see where the taxpayer monies come into play when we take a look at the renovations being done over a period of 10 years to Buckingham Palace.
"Funding for a multimillion-pound refurbishment of Buckingham Palace has been approved by MPs. They backed changes to the Sovereign Grant - the funding formula for the monarchy's official duties - by 464 votes to 56. The grant will increase by 66% to pay for the £369m refurbishment."
https://www.bbc.com/news/uk-3802551...alace is to undergo,work, to begin next April.