Preventing foxes coming in sash window

[mondaytosunday]

I’m a landlord and the flat is on the first floor in London, but there’s a flat roof adjoining the hall and bedroom windows. It’s a conservation area and the windows are original single pane wooden sash. I cannot change or replace them as they are the freeholders responsibility.
The tenant left a window open (only about 3inches) and a fox came in and did a bit of damage to some items (about £200 max). She is asking what can be done so she can still open the windows but prevent foxes gaining access. I can’t affix anything externally to the windows, but what can I do internally? I don’t think a fly screen is strong enough? Any suggestions? Aesthetics are a consideration too. Thanks.

Flat cat mesh screen? They are strong enough to resist a cat climbing on them

If it isn't needed as a fire escape you can put small blocks a short distance above the lower window in the runners to prevent them from opening more than an inch. That will stop the bottom window opening enough for a fox to jump in.

Spikes along the front edge ? You can buy them on a strip from Amazon

or could she open the top sash? That might be a bit too high for a fox,

Foxes are excellent at jumping and climbing! They could get through a top sash window.

You can get fox proof mesh for chicken coops but it is pretty ugly.

You could try a decorative metal panel for radiator cupboards that can be affixed in front of the open window (and removed when not in use, if the tenant prefers). It wouldn’t stop a determined fox but unless there is something they really want inside, like a tasty chicken, they likely to go and find something to eat that doesn’t require chewing through metal to get it!

Couldn’t the tenant just close the window when she’s not in? I feel like I’m stating the obvious so maybe I’m missing something here?

She should not have the windows open when she’s not at home as it will invalidate her insurance. And foxes can get in! If a fox can so can a burglar.

We lock ours, also in a Conservation area, but they have to be unlocked at night for escape.

open the top rather than the bottom

Surely if a fox got in so could a burglar so the easiest solution for your tenant would be for them to close the window when they are not in the flat.

Or did the fox enter while the tenant was actually in the flat? If that was the case how did they not hear it opening the window?

I think this is on the tenant to be more aware of what is going on around them.

Drawing the curtains might help.