1. Observe and protect
Log the symptoms, hold off further damage if that's safe, and pull together whatever history exists.
2. Evaluate the house and site
Before naming a fix, a plumber needs to know how the fixtures are performing, what the supply and drain lines are made of, how pressure and venting are behaving, where the shutoffs are located, how hard the area is to access, and what earlier repair work already changed.
3. Define a written scope
A written scope needs to cover preparation, protection, materials, access, permits, testing, cleanup, finish restoration, exclusions, and change-order handling.
4. Compare providers
Check each provider's credentials and compare how they reason through the job and where its boundaries are -- not just the bottom-line number.
5. Inspect closeout
Verify testing, cleanup, documentation, warranty paperwork, and remaining maintenance duties before calling it closed.