The short answer
On the main line after the pressure tank and any iron or sediment filter, before the water heater, close to a drain and an outlet.
The usual order
- Well pump and pressure tank (on wells).
- Sediment or iron filter, if needed.
- Water softener.
- Water heater and the rest of the house.
What the spot needs
- A drain for regeneration water.
- A power outlet for the valve.
- Room for the brine tank and salt bags.
- Protection from freezing.
Taps you may leave hard
Many owners plumb outdoor hose bibs before the softener, since irrigation water doesn't need softening and it saves salt.
Products mentioned
Questions readers ask
Should the softener go before the water heater?
Yes, so the heater gets softened water.
Can a water softener go outside?
Only if it's protected from freezing and sun as its maker allows; most are installed indoors or in a garage.