Guide
Under-sink water filters in the UK: fit, plumbing and running costs
An under-sink filter sits in the cupboard below your sink and takes its water from the cold supply. It frees the worktop and ends the refilling, but of all the options it asks the most of your kitchen: clear space, accessible pipework and, if you rent, permission. Settle those first and the choice of model gets much easier.
Three common set-ups
“Under-sink” covers several different arrangements, and what each involves changes from model to model. Use this table to know which questions to put to a seller, not as a specification.
| Set-up | How filtered water reaches you | What it typically involves | Trade-offs |
|---|---|---|---|
| Connected to your existing tap | Through the tap you already use, where the model and tap allow it. | Teeing into the cold supply under the sink and routing filtered water to a suitable tap. Whether it works depends on how your tap is built. | No extra tap on the sink, but not every tap suits it. Check the maker’s compatibility notes first. |
| Dedicated filter tap | Through a small separate tap beside your main one. | A tap hole in the sink or worktop (a spare hole may do), plus the connection below. | Easy to understand, but drilling is hard to reverse. That matters if you rent. |
| Reverse osmosis (RO) | Usually through a dedicated tap. | A membrane system. Depending on the model it may have a storage tank, need a drain connection or a power socket; some are tankless. | More to fit and more to maintain, and the water is different from other filtered water. See the reverse osmosis guide. |
Which set-up suits which kitchen
| Your kitchen | What to look at first |
|---|---|
| Spacious cupboard, modern mixer tap, you own the flat or house | Compare all three. Which are possible depends on your tap, your pipework and each model’s requirements. Check whether your tap is one the maker lists as compatible; if it isn’t, a dedicated tap is the usual route. |
| Cupboard shared with bins, a dishwasher hose and a waste pipe | Space for housings and for removing them. Look at slimmer or single-cartridge systems, and measure before you shortlist anything. |
| Rented, and the landlord hasn’t agreed to drilling | Anything needing a tap hole is out until you have agreement in writing. A countertop filter avoids the question. |
| Hard-water area and the aim is less limescale | Check the product’s claim for scale reduction, not just “filtration”. See hard water and limescale first. |
Measure the cupboard properly
Cartridge changes come round again and again for as long as you own the filter, so measure for them as well as for the installation, and take the measurements to the manual before you buy.
- Clear height from the cupboard floor to the lowest part of the sink or waste above it, in the spot you have in mind.
- Clear width and depth between the obstacles: bins, pipes, a dishwasher or washing-machine connection, a shelf.
- Room around each housing. Some housings unscrew downwards and need space beneath; others need room to slide or tilt. The manual gives the clearance for each, and it is often more than the housing itself.
- Where the cold feed and its valve are, and whether you can reach the valve without moving everything.
- Where the filtered water will exit: your existing tap, or a spare tap hole, or a new one.
If you rent
Ask your landlord or agent before you buy, and keep the reply. Cutting into pipework, adding a tap hole or fitting a waste connection are the kinds of change a tenancy may not allow, and some are hard to undo. When you ask, say what you want to fit, whether it needs a hole in the sink or worktop, who will do the work, and that you’ll restore things when you leave if that’s the agreement. If the answer is no, the countertop guide covers options that usually need no plumbing changes.
Plumbing, approvals and quotes
In England and Wales, fittings connected to the public water supply have to meet the Water Supply (Water Fittings) Regulations 1999. Scotland has the Water Byelaws 2014, and Northern Ireland the Water Supply (Water Fittings) Regulations (Northern Ireland) 2009. Whichever applies, the installation has to meet it, and the manufacturer’s instructions for your exact model say what that model needs.
You may see WRAS approval on a product. It is a voluntary way for a manufacturer to show that a fitting or material is of an appropriate quality and standard, and it comes with conditions of use that are listed in the WRAS approvals directory. It doesn’t guarantee that your whole installation complies, and it says nothing about what a filter removes. Your installer checks where and how the product will be used. The certifications guide explains the difference.
Some installations have to be notified to your water supplier before work starts; see the reverse osmosis guide for the case that matters most for filters. For anything that connects to the cold supply, use a qualified plumber or ask your supplier what your installation must meet. The Drinking Water Inspectorate’s guidance for England and Wales covers the general principles: domestic water filters and softeners.
If you get a quote, make it comparable. Ask for it in writing and check what it covers:
| Item | What to ask |
|---|---|
| Connection to the cold supply | Is the fitting and any valve included, and is the work done to the manufacturer’s instructions? |
| Tap hole | Is drilling included, and what is your sink or worktop made of? Some materials are difficult to drill. |
| Waste connection | Needed only for some models (RO in particular). Is it included, and where will the drain line run? |
| Parts and call-out | Are parts supplied by you or the plumber? Is there a minimum call-out charge? |
| Making good and testing | Will the plumber test for leaks and clear up? What happens if it drips after a week? |
| Approvals | Will the plumber confirm the fitting and installation meet the requirements for your area? |
Upkeep and costs over 1, 3 and 5 years
The purchase price is one number among several. Add the delivery, whatever installation costs you, every cartridge change, and any extras such as a tap or fittings. A system with more than one cartridge has more than one schedule, and the schedules rarely line up.
How many cartridge changes you will pay for depends on the interval. After the set fitted on day one, the count looks like this:
| Change interval | By 1 year | By 3 years | By 5 years |
|---|---|---|---|
| Every 3 months | 3 | 11 | 19 |
| Every 6 months | 1 | 5 | 9 |
| Every 9 months | 1 | 3 | 6 |
| Every 12 months | 0 | 2 | 4 |
Multiply each count by the cost of a replacement set, add the up-front costs, and you have the total. Our running-cost calculator does the sums for you, lists every change, and handles several cartridges and litre limits. For how change dates are set, read the replacement guide.
Common questions
Will an under-sink filter slow my tap?
It depends on the model and on which tap the water leaves from. Makers state a flow rate for the filtered outlet, so compare that figure rather than guessing. A dedicated filter tap usually runs at the filter’s pace, not your main tap’s.
Can I fit one myself?
The manual for your exact model says who may fit it and what the installation has to meet. Anything that connects to the cold supply carries a risk of leaks and of an installation that doesn’t meet the rules, so use a qualified plumber unless the manufacturer’s instructions and your water supplier say otherwise. Check your warranty terms for what the maker requires.
Can I take it with me when I move?
Housings and cartridges can generally be disconnected and removed. A dedicated tap leaves a hole you will need to make good or agree to leave, so think about that before you drill.
Questions to ask before you buy
- Which exact UK model and variant is this, and do the manual’s fit and supply requirements match my home?
- What are the replacement cartridge codes, what does each cost today, and when must each be changed?
- What does the maker claim, and does an independent certificate name this exact model and the standard in question? How we label evidence.
- What will installation involve, and who does it?
Fitting and cartridge details vary by model; the maker’s instructions and a qualified plumber have the final say.