Mark the roof has to be strengthened for dormers, to take the weight. If you have a non trus roof then its a traditional with probably 3x2 rafters and purlins. Purlins are prob in way of dormers so have to be removed...the roof is then weak, so has to be strengthend usually by new rafters being placed under existing.....obviously they are cut where dormers are and again more strengthening is required. Hard to explain on here.

Deffo more work than in a simple extension.