The three connections every tub has
A tub connects to a drain, an overflow, and a water supply reaching its faucet or valve. Swapping the tub means disconnecting all three from the old one and reconnecting them to the new one.
The drain and overflow usually share one assembly under the tub, while the supply side ends at whatever valve controls the faucet, sometimes original to the house and sometimes newer than the tub sitting on top of it.
When that crosses into permit territory
Relocating, adding to, or removing a supply or drain line pulls a permit into the job, no matter how cosmetic everything else about the swap looks.[1]
Swapping a tub for a new one in the exact same footprint, reusing the existing drain location, is the kind of like-for-like work that often skips the permit step.[1]
Changing tub style, an alcove tub to a freestanding one, for example, almost always means relocating at least one of those three connections, which pushes the job back into permit territory.[1]
What an inspector actually looks at
On a permitted swap, the drain, overflow and supply connections get checked before anything closes the wall back up, the same rough-in step a full bathroom remodel goes through.
Washington: a certified plumber, not just a registered contractor
Washington will not license a plumbing contracting business without a full-time journey-level or specialty plumber on staff holding their own certificate of competency, a credential earned entirely apart from general contractor registration.[2]
California: C-36 covers the supply and waste side
California's plumbing classification covers supplying safe water and properly disposing of fluid waste, the exact scope a tub's drain and supply lines fall under once the work goes beyond a minor fixture change.[3]
Florida: plumbing is its own contracting category
Florida law defines a plumbing contractor separately from a building or residential contractor, covering the installation, maintenance, repair or alteration of plumbing specifically.[4]
Why the trade on the card matters
That is why the pros on our city pages are grouped by trade instead of lumped into one list: a plumbing license and a bath-remodeling license are not interchangeable for this part of the job.
A bath specialist can still be exactly who you want handling the tub itself, the surround and the finish work; the plumbing connections are the piece worth confirming separately.
What to have ready when you call
Knowing your tub's current style, alcove, drop-in or freestanding, and whether you are keeping that style or changing it, tells a licensed pro in one sentence how much of this guide applies to your job.
Knowing your home's rough age helps too, since it points straight at whether EPA's renovation rule is worth asking about alongside the plumbing and licensing questions above.