I currently store my gear in a mansard, which is above my flat in the attic floor, and that I use as workroom too (Computer, NAS, and so on). In summertimes temperature may climb above 30°C/86°F and stay there for multiple days (and nights). I am concerned that plastic and rubber may become brittle and break, as well as the sensor of the camera or other (lens-/filter-)coatings may suffer. So I am wondering if these temperatures may damage my gear and which environments are best to store photo gear in general?
If you see the Canon 600D specification sheet here it says:
Operating Environment 0 – 40 °C, 85% or less humidity
If the manufacturer guarantees it will operate within those temperatures it will be safe to store it within those temperatures, especially as I believe a lot of operating specifications given by manufacturers are pessimistic, electronics will likely work outside of those temperatures even though manufacturers won't guarantee it.
That said, I would be more worried about humidity and dust than heat personally so storing within a sealed container would be my recommendation.