Most “simple swap” surprises trace back to the purchase, not the plumber. A few checks before you buy keep installation day boring — which is what you want.

Match the fixture to what’s there

Faucets: count the holes in the sink or counter (1 vs. 3) and their spacing before falling in love with a model.
Toilets: measure the rough-in — the distance from wall to the closet bolts (12" is standard, but older homes vary).
Disposals: check horsepower against your usage and confirm the outlet under the sink is switched.
Shower valves: a trim swap is cosmetic; changing the VALVE means opening the wall — know which one you’re buying.

The hidden condition that changes the job

The fixture is often fine — it’s the fifty-year-old shutoff valve under the sink that won’t close, or the corroded flange under the toilet, that turns a swap into a repair. A pro checks these first and fixes them while everything is already apart, which is exactly when it’s cheapest.

When DIY is fine — and when it isn’t

Swapping a faucet with good shutoffs is a reasonable DIY job. Anything involving the gas line, the drain in the wall, a shower valve, or a toilet flange that doesn’t look right is where a licensed plumber earns their fee — water damage from a small mistake costs far more than the install.

Measure before you buy, and budget for the possibility that what’s behind the fixture needs attention too. That’s the whole secret.