Yep, it's up to emissions regs and whether or not you have local testing. While my home state had these regulations, I grew up in a town small enough to not matter. No testing whatsoever. Forty miles north, same vehicle code, different story. It was important to maintain my address in smallsville. Moving to Silicon Valley was a new experience. I couldn't keep the car I'd spent two years building or they'd have taken me for a one-way trip out behind the quiche factory.
For the details, search DOT and your state. Maybe DMV. Then, if you're living on the edge of regs, be prepared for different interpretations come registration time. Also, come traffic-stop time. Guys with heavily modified cars are often the most polite and respectful people when it comes to conversing with police officers.