Engineer's Seal/Stamp for Floor Joists

How does one go about getting an engineers seal on a floor joist? The house was built a year ago and prepped for a bonus room. Basically the room is floored and framed, and it just needs final leg of the plumbing ran for the bathroom and fixtures installed, as well as hvac and some can lights. After that, just sheetrock and paint really. I tried to do right thing and get a permit, but the city wants engineers seal on the joists. Joists that they already inspected and approved last year when the house was built...the new construction plans indicated "future bonus room." It's not like I'm building a bonus room out of the blue above my garage.

Anyways, assuming the city of good ol' Baton Rouge doesn't back down from this requirement, how do I go about getting an engineers seal and stamp on the joists? I'm contracting this project out myself.

Thanks