Has any of you cat lovers ever had a cat flap fitted into patio doors? I want to get a small dog flap for my Yorkie put in our patio doors but was told by my window company that it would be very expensive??

Anyone any ideas (we don't have a proper back door).

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Proud owner of Rocky the Yorkie, born July 2008, who came to live with us at 8 weeks old
and
Ellie the Yorkie, born February 2012, who came to live with us at almost 8 weeks