By-laws vary from place to place as far as plans and engineer certificates go. I have worked hand in hand with many architects and engineers but do not have an appointed one of either. They either dictate to me what must be done or I tell them what I’m going to do and they approve it and put it onto plan. If your deck should require either then it would be up to you to employ the relative parties.
My clients have always paid for their own materials, whether I source them or they do, as I am not making money out of the materials, my money is in my labour which is either paid at the end of small jobs or progressively on larger ones.