For most homes, install after the main shutoff and pressure-regulating equipment but before the water heater and interior branches. Well systems usually go after the pressure tank, with treatment order based on water chemistry.
City-water homes
A common layout is main shutoff → optional sediment prefilter → whole-house treatment → softener if needed → water heater and fixtures. Outdoor irrigation may be intentionally bypassed to avoid filtering water that does not need treatment.
Private wells
Well treatment is typically installed after the pressure tank so the pump and tank can operate normally. Iron, sulfur, sediment, softening and UV stages should be sequenced according to the measured water chemistry.
Keep service access in mind
Leave enough vertical and side clearance to change cartridges, replace media or remove tanks. A cramped location can make every future service call harder and more expensive.
Protect against freezing and heat
Most residential filters should be installed where they are protected from freezing temperatures, direct weather exposure and excessive heat.