Personally, I wouldn't use creosote at all. On the outside, you will get five-year protection from the weather if you use a solvent-based wood preservative, which dries faster and is much less messy to apply. Ronseal Shed and Fence Preserver contains fungicide and woodworm preventer as well as soaking into the wood and waterproofing it really effectively if you use three coats. It does need good drying conditions between coats, however - it dries fast in summer sunshine but takes a day or so between coats in cold or damp conditions. I've just done our summerhouse with it, and then I jet washed all the wooden garden furniture and re-coated it, so now it's looking lovely and the rain just rolls off. Inside, I would use a water-based wood preservative in a light colour. Wickes Home of Colour range is good - basically a quick-drying outdoor emulsion paint which has much less odour and is a lot less toxic than creosote - followed by painting with diatom slurry, as others have advised.
And, of course, for mite prevention, the type of covering on the roof is important - mites get between felted layers and are then impossible to eradicate. Green or black plastic corrugated sheeting such as Onduline or Wickes own brand is better, and also provides good high-level ventilation under the corrugations.
Actually, although I can recommend these products from long use in our garden, this thread makes me very glad that I chose a Green Frog plastic coop - I have enough to do, keeping up with the garden sheds and buildings, never mind the coop!