Our fox "Fiona," had a den under the tea-house. It sits on brick piers on paving slabs so there is between nine inches and a foot of clearance underneath. Three sides have "skirts" which don't quite reach the ground. But I left the back open so there could be a clear air passage underneath.
This is where they got in.
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Once they'd left home I concreted the area between the back fence and the building covered it with sand and cement mortar and made a sturdy wooden frame with a double layer of chicken wire, so she couldn't dig under the frame and get back in again.
She did make a visit in February, maybe she was checking it out, but we've not seen her since in the garden, but occasionally trotting along the road outside with seemingly not a care in the world at around midnight.
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These are the cubs playing, if you've not seen them. I took this with my little Lumix DMC TZ60 digital camera on "zoom," through the glass of our French windows. The audio is from a stock selection available from You-Tube you can substitute for the original sound.
We've a security light on the back of the shed facing the tea-house. It never bothered them when they set it off.