I'd go with that.
The 'secondhand' and 'all glass' aspects don't change anything - needing Planning Permission (or not) is all about the 'box nature' of the structure you are erecting - what it is made of is irrelevant in terms of the need for PP or not.
It varies between councils. I would phone up you local council planning authority and ask their advice. Our conservatory needed planning permission, but our next door neighbour didn't just because there is a little brick arch that links their house to their garage! Planning rules are weird!!