Generally I include a conservatory with the regular window clean (at an extra few pounds depending upon it's size) - but this only includes glass on the sides.
If someone wants the roof cleaned (whether a regular customer or a one off) I charge far more as you are usually getting months (years?) of green algae, bird's muck etc off by scrubbing it with a brush on a pole and sluicing it off afterwards. You will invariably have to do the sides because of the muck that runs off the roof and you will end up cleaning the glass as a final thing with w/c gear.
So - if I were to charge £5 to add on a conservatory to clean it's glass I would charge about £30 to clean it's roof as described above (if it was a lean-to) and about £60 if it was an Edwardian look-a-like roof with finials etc. But size makes a difference!
Hope this helps.