A home with an irrigation system asks the treatment equipment to do two jobs that are not the same. Indoor use is steady and predictable. Sprinkler zones can open all at once and pull a large volume of water in a short time. Sizing a system well means planning for the busiest moment in the house, not the average day.
Count what can run at the same time
List the fixtures and appliances that could run together during a normal busy hour: showers, laundry, dishwasher, kitchen and bath faucets, and any irrigation zone. Include the zones that open in sequence on the watering schedule. The provider will need that list to judge whether the equipment can keep up at the pressure your home actually has.
Decide whether the outdoor line needs treatment
Many homeowners route outdoor lines around the treatment equipment, because treating water for lawns is a separate decision from treating water for the house. Ask the provider how a bypass for the outdoor line would work, whether it changes the flow reaching the equipment during watering, and whether any sediment or hardness in the lawn water will cause a problem for the sprinkler heads or the controller. Confirm the answer for your specific layout.
Test the fit against real demand
Ask the provider what flow rate the equipment is rated for, at what pressure, and what happens when a sprinkler zone opens during a shower. A good answer describes the drop in pressure or flow and says what the homeowner should expect. A vague answer is a reason to ask for the sizing calculation in writing.
Set the schedule around the equipment
If the irrigation controller can be adjusted, avoid starting zones at the same time as heavy indoor use. Ask the provider whether a simple schedule change will reduce the demand peak, and whether that change is worth making before the equipment is chosen.