So long as they're within setbacks and building in accordance with the existing site zoning, no site planning review should be required.
They'll need a site plan drawing if you're adding to the footprint when you apply for the building permit. It will have to show the distance to the setbacks, total square footage, maybe total impervious if you're in a watershed overlay or something like that. Getting "some" drawings of the proposed work is helpful when writing the contract with the GC, if only because it helps to define what the scope of work is and what they're expected to deliver. Some GCs can do this for you, or you can do it yourself if you're so inclined. If you trust the GC and/or just like to wing it, you can just pay him by the hour and he'll build whatever.
But finding architects that do single-family residential work... especially small additions... is tough. There just isn't much money there. What usually happens is either the owner gets turned away by the idea of paying a couple thousand dollars to an architect to produce drawings for a $20k addition, or they agree to a not-to-exceed contract, and then burn up all the fee with changes to the scope.
The best way to proceed, honestly, is to ask people that live around you who they've worked with and what they've done. I don't know anybody in W-S to give you any more direction than that, though.