overwintering plants

It depends on whether it's frost-proof. Also how much light there really is, as some plants would react to semi- darkness by growing up towards the light and therefore getting leggy. Summer bulbs and dormant plants might be fine in there.

I overwinter things in the garage. It's frost-free and I line the windows with bubble-wrap, but I also make sure I ventilate the garage well on dry days.
I also think it depends what plants you're overwintering. Some might not like the cold so I only put things that can tolerate temps down to about 1 or 2 degrees in the garage. I bring anything else into the conservatory or the house.
Could you say what plants you want to overwinter, then maybe we can answer you better.

I over winter mine in my back porch it is half glass but i always put bubble wrap all the way round & like Blodyn the one's that are to cold for them to stay in there i bring indoors.