Building a filter from standard components is a realistic project, and it produces a result as good as a professional installation because every part is doing what it was engineered to do. What separates a good build from a bad one is not skill with a pipe cutter: it is the handful of decisions about sizing, order and serviceability made before anything is cut.
The parts list
A housing sized for the flow the house draws, preferring a large diameter body over a slim one, with a bracket rated to carry it full. A cartridge matched to the job: a depth sediment element for grit, carbon for chlorine and taste, catalytic carbon if the supply is chloraminated. Two isolation valves and, ideally, a third across them as a bypass. Unions or push fittings at each connection so the housing can come out without cutting. Pressure gauges either side if you want an honest signal of when to change the cartridge. Potable rated pipe and fittings throughout.
Deciding the position
The point of entry, after the main shut off and before the water heater, the softener and every other stage, so everything downstream sees clean water. Choose somewhere a leak does no damage and where the housing can be reached without a ladder. Then measure the clearance below the sump: you need a gap equal to a cartridge length to withdraw the element, and discovering otherwise after plumbing it in is the most common self build regret. Mark the pipe and dry fit everything before committing.
The order of work
Isolate the supply and drain the line. Mount the bracket first and hang the housing on it, so the pipework is cut to suit the housing rather than the other way round. Fit the isolation valves and the bypass. Make the connections with unions so nothing is permanent. Fit the cartridge the right way round. Then open the inlet slowly, purge air through a nearby tap, and check every joint by hand and again an hour later, since a slow weep at a fitting often shows only after pressure has settled.
Where to stop
Anything upstream of the main stop valve, and any work that local code requires be done or inspected by a licensed plumber, which varies by jurisdiction and is worth checking before starting rather than after. Backflow prevention requirements are the other place to take advice. And be honest about the difference between this and an improvised filter: layered gravel and charcoal in a bottle clarifies water without making it safe. Our whole house filter guide covers combining this stage with softening.
Questions people ask about how to build a water filter
Where should a self built filter go?
At the point of entry, after the main shut off and before the water heater, softener and any other treatment stage.
What is the most common mistake?
Too little clearance below the housing to withdraw the cartridge, and a housing sized for the space rather than for the household's peak flow.
Do I need a bypass?
It is not strictly required but it is the difference between a ten minute cartridge change and a house without water when something goes wrong.
Is a homemade gravel and charcoal filter any use?
As a demonstration, yes. It clarifies water without making it safe, and it is not comparable to a build from rated components.