The drain connection can look like the easiest part of a water treatment install. A softener, filter, or reverse osmosis unit sends wastewater out, and a hose into a nearby drain seems simple. The real decision is less about tool skill and more about the rules that govern where that water goes and how it gets there.
What the drain connection has to do
Regeneration cycles on a softener, and the reject flow from a reverse osmosis unit, send water to a drain on a schedule. The outlet needs to sit above the flood level of the drain it feeds, so backed-up water cannot push into the equipment. It also needs to stay clear of the drain's trap and vent connections. Ask your installer for the required outlet height and routing for your specific unit, and write the answer down.
Ask whether an air gap is required
Many plumbing codes require a physical air gap between the discharge line and the drain, so water cannot siphon back into the system. Some setups use a standpipe or a fitting made for the purpose. Check with your local building department or your installer to learn which method applies at your address. If a standpipe is the answer, confirm its height and whether the drain can handle the flow during regeneration.
Do it yourself only if these all check out
A homeowner connection may be reasonable when all of the following are true: the nearest drain is within reach without new drain piping, your local rules allow a homeowner to make this connection, the outlet can be set above the flood level with a clear air gap, and you can leak test the fittings afterward. If any one of these fails, the drain work moves to the hired side.
Bring in a plumber when
Call a plumber if the drain sits at floor level, the nearest drain is far from the equipment, the existing drain is cracked or slow, or the fittings would need a new hole through a wall or slab. A licensed plumber may also handle the permit paperwork in your area, which is worth asking about before any work starts.
Put the split in writing
If the installer sets the unit and you handle the drain, write that split into the quote and the install notes. Ask who is responsible if wastewater backs up or leaks into a cabinet or subfloor. A clear answer now is far easier than sorting out blame after a leak.