What code are you under? What does your code say? I do not know of a code that requires a whole house trap and I see no reason for one and alot of reasons not to have one. What is his reasoning to add one? And I would say that the addition of a whole house trap should be about the least of your questions.
I would reconsider the decision to plumb your own place. Ask some of the inspectors about home owner plumbed houses ;-)
I have gone in with a saw-z-all more times that I like to mention and cut out ALL the drain and venting in a home owner plumbed home and thrown it away. Unless you REALLY understand the code book in regards to drainage and venting, you should consider saving money somewhere else.
There really is alot to it.