Cut birdsmouth into rafter for purlin?

[Professionally nosey]

Hi

 

We're doing a 2 storey side extension with a cross gable roof to match others in the area (side and rear). We have an existing side gable which we will match anglewise for the side and mirror for the rear gable as well. The architect has designed it with 2 purlins, and a ridge beam for the rear gable, which will tie into the purlin as per attached diagram. I understand purlins are often added once the rafters are in so no birdsmouth needed, but these will be structural and flat and function almost like an additional wall plate. My question is should the rafters for the Purlin 1 side then effectively have 2 birsdmouths, one for purlin 1 and one for the normal wall plate on the wall (like the second sketch which is in no way to scale, just very quick). Purlin 2 will have diminishing rafters so no need for 2 birdsmouths per se. 

Edited September 8 by Professionally nosey