A plumber is not the expensive part. The expensive part is finding out on the day that your sink has no spare hole, your cabinet has no outlet, and your stop valve is the wrong thread.
Do it yourself
$235
The system, nothing else
Plumber fitted
$485
System plus a two-hour job
Then, every year
$119
Filters and drain water
Straight answer: for most under-sink systems, no — if your cabinet is already set up for it. The install is a drain saddle, a feed connection off the cold stop valve, a faucet in the countertop and a tank that sits in the cupboard. Two hours, a drill and an adjustable wrench.
Paying a plumber roughly $150 to $350 adds 5 months to the payback against buying bottled water. That is the entire financial consequence of the decision. If plumbing makes you anxious, buy the calm — it is genuinely not much money in context.
The reason to pay someone is not difficulty. It is that a plumber has the adapters in the van when your stop valve turns out to be the wrong thread, and you would be making a third trip to the hardware store.
RO systems come with their own small faucet, and it needs somewhere to go. Many sinks have a spare hole under a blanking plug or a sprayer you never use. If they do not, you are drilling the countertop — and in granite or quartz that is a specialist job with a diamond bit, not something to attempt on a Saturday. Check for a spare hole before you order anything.
The reject water has to go somewhere, which means clamping a fitting onto the sink's drain pipe above the trap. Cheap systems ship a saddle that fits standard 1.5 inch pipe. If you have unusual waste plumbing, or a garbage disposal in the way, this is where the job stops.
A conventional RO system has a pressure tank about the size of a small propane bottle, and it lives in the cupboard alongside the filters you have to be able to reach and unscrew. Measure the clear space, not the cupboard. Tankless systems avoid this and cost more.
Tankless and booster-pump systems need mains power under the sink. A surprising number of kitchens have no outlet in that cabinet, and adding one is an electrician, not a plumber. Check before you order — this is the single most common reason a tankless system goes back.
We do not have a regional table of plumber quotes. The $150 to $350 above is the range quoted by manufacturers and the range we have paid, not a survey — so treat it as an order of magnitude, not a price. Collecting real quotes by metro is on the list, and when it exists it will replace this paragraph and be dated.
Saying so costs us nothing except the appearance of authority, and the alternative is inventing a number, which is the thing this whole site exists in opposition to.
The one nobody quotes at all
A conventional system rejects about three gallons for every gallon it makes, and your sewer bill is calculated on metered intake rather than on what actually reached the sewer. Every rejected gallon is therefore billed twice.
At two gallons a day: 2,190 gallons a year to drain
A tankless system at a 1:1 ratio cuts that by roughly two thirds. See the full running cost.
The systems these figures describe
Check for the spare faucet hole and the outlet before you order any of these.
4.5★ 9,300 reviews · 500+ bought last month
The cheapest way to get plumbed reverse osmosis, and the one that asks least of you afterwards — a filter change roughly once a year.
Against it: 50 gallons a day means a slow trickle — fine for drinking, irritating if you fill pots from it. Tank takes up most of an under-sink cupboard.
About $85/year in filters, plus roughly 2,190 gallons a year to drain at two gallons a day.
4.6★ 21,300 reviews · 3,000+ bought last month
Six stages with a remineralisation cartridge, because RO water tastes flat to a lot of people.
Against it: Six stages means six things to replace. The remineralisation stage is the one people quietly stop changing, and then they are paying for a stage that does nothing.
About $85/year in filters.
4.5★ 4,100 reviews · 3,000+ bought last month
Tankless, 600 gallons a day, and a 1:1 drain ratio instead of 3:1 — the only thing here that meaningfully cuts the waste cost.
Against it: Needs a power outlet under the sink, which a surprising number of kitchens do not have. Check before you order, not after.
About $95/year in filters, and roughly a third of the drain water of a conventional system.
$235 for the system, $0 to $350 to fit it depending on your nerve, and $119 a year afterwards — against $832 a year for the bottled water it replaces. Run it for your own household.