adding rear door extension

If they were an architect why didn't you ask them to include the external door originally? Strictly speaking you will need to apply for a minor amendment to the approved scheme but if the works fall within the 'permitted development' criteria you could probably 'just do it', as long as there were no objections such as overlooking, closeness to boundary etc.

This sounds like this could be done under permitted development so planning approval is not needed however it would be good to get the plans amended and submitted there should be no fee. As long as there is no loss of privacy for the neighbouring property you will be fine.

Given that you have already got Planning Approval for the extension the alteration to include a door will not usually be accepted as Permitted Development. As already suggested ask your Architect to update the plans and submit them again for Minor Amendment Approval. If this would normally have met the Rules & Criteria for Permitted Development then there should be no problem (not even from the Architect for doing this if it was his fault it was missing).