If you’re looking for a compact reverse-osmosis system in the UK, the biggest issue usually isn’t filtration performance — it’s whether the system will actually fit in your kitchen.
I know this from experience. We use a compact Waterdrop reverse-osmosis system in our own hard-water home, and I’ve also physically tried the much larger Waterdrop X12 beneath our standard UK sink. Despite being tankless, the X12 would not fit comfortably because of the pipework from our double-bowl sink.
I’ve also used the Waterdrop K19 countertop RO system, which requires no under-sink installation at all and is worth considering if cupboard space is extremely limited.
In this guide, I combine that hands-on experience with research into the most practical compact RO systems currently available to UK households.
Not sure how hard your water is? Use our UK Water Hardness Checker to check hardness levels in your area.
What Does “Compact” Really Mean for an RO System?
A tankless RO system is not automatically compact enough for every kitchen.
I found this out when I physically tried the Waterdrop X12 beneath our own double-bowl kitchen sink. Although the X12 is tankless, its larger cabinet would not fit comfortably around our existing waste pipes and plumbing.
By contrast, the Waterdrop G5P700A we use at home has worked well as a relatively compact under-sink system, so it gives me a useful real-world benchmark when comparing dimensions.
If under-sink space is extremely limited, I have also used the Waterdrop K19 countertop RO system. It avoids the cupboard-space problem completely because it sits on the worktop and does not require permanent under-sink installation.
For this guide, I therefore consider more than the manufacturer’s stated dimensions. I also look at:
- the space needed around the unit for pipes and filter changes
- whether a separate storage tank is required
- how easily it fits around typical UK sink plumbing
- whether electrical power is required
- how much usable cupboard space remains after installation
How Much Space Does a Compact RO System Need?
The dimensions on the product page are only part of the story. You also need room for the water connections, waste pipe, electrical cable and enough clearance to access the filters.
The three systems in this guide differ considerably:
- Waterdrop G5P700A: approximately 42.4 × 14.5 × 35.3 cm
- Waterdrop G2P600: approximately 44 × 15 × 35.5 cm
- Waterdrop K19: approximately 35 × 20 × 41.5 cm, but this sits on the worktop rather than beneath the sink
For our kitchen, the narrow shape of the G5P700A works particularly well around the pipework beneath our double-bowl sink.
My experience trying the larger Waterdrop X12 also showed why you should never assume that “tankless” automatically means compact. The cabinet itself still needs to fit around your existing plumbing, so I recommend measuring the usable space beneath your sink before ordering.
Compact Reverse Osmosis systems as they can fit beside existing plumbing instead of taking over the whole cupboard.
For full-size systems with higher output, see the best reverse osmosis systems in the UK.
Top Compact Reverse Osmosis Systems in the UK
1. Best Overall Compact RO: Waterdrop G5P700A — Our Personal Choice
The Waterdrop G5P700A is the compact reverse-osmosis system we use in our own kitchen, so unlike most products in this guide, our recommendation is based on genuine long-term use rather than specifications alone.
Its narrow tankless design was particularly important for us because our double-bowl sink leaves limited usable cupboard space around the waste pipes. It fits beneath our sink while still leaving useful storage space — something we found was not possible when we later tried the much larger Waterdrop X12 in the same cupboard.
In everyday use, we have found the G5P700A convenient, quick to dispense from and straightforward to maintain. The system also includes a remineralisation stage, which is one of the reasons we chose it for our own drinking water.
For us, its biggest advantage is the balance between RO filtration, compact size and everyday practicality. If cupboard space is a major consideration, this is the system against which I compare the other under-sink options in this guide.
Best for: households wanting a genuinely compact tankless RO system with remineralisation.
Our experience: personally installed and used in our own hard-water home.
Main drawback: it costs more than some simpler compact RO systems, and as with any under-sink unit you still need to check your pipework and available clearance before ordering.
Read our detailed Waterdrop G5P700A review for our installation experience, measurements and longer-term observations.
View Waterdrop G5P700A (Compact + Alkaline)Real-World Install: Why I Chose the G5P700A
I chose the G5P700A partly because cupboard space is limited beneath our double-bowl sink. When I later physically tried the larger Waterdrop X12 in the same cupboard, the unit itself was tankless but the available pipework and clearance made it impractical to fit.
I documented the full install with photos here:
👉 My Waterdrop G5P700A UK review (with installation photos) →
From my experience:
- It fitted neatly beside existing pipework
- The alkaline stage makes a real difference to taste
- Noise is minimal
- Flow rate is excellent for its size
If you’re in a typical UK kitchen with tight space, this is the one I recommend.
2. Best Compact Alternative: Waterdrop G2P600 — Hands-On Tested
The Waterdrop G2P600 is my second choice for a compact under-sink RO system and, importantly, this is another model I have had in my own kitchen rather than assessed from specifications alone.
Waterdrop sent me the G2P600 for review. I did not permanently install it because we already use the G5P700A, but I unboxed and inspected the complete system, checked the filters and fittings, and physically tested how it fitted beneath our own sink.
At 44 × 15 × 35.5 cm, it has a narrow tankless design that makes considerably better use of cupboard space than a traditional RO system with a separate storage tank. During my fit test, I also liked the front-access filters because you do not need lots of space around the sides of the unit when cartridges need replacing.
The G2P600 provides 600 GPD flow and uses a 2:1 pure-to-drain ratio. Its setup is simpler than the G5P700A: you get a standard mechanical drinking-water faucet rather than a smart faucet with a TDS display, and the standard G2P600 does not include remineralisation.
That simplicity may actually appeal if your priority is a compact tankless RO system without paying for premium extras.
Best for: smaller UK kitchens where cupboard space matters and you want straightforward tankless RO filtration.
Our experience: personally unboxed, inspected and fit-tested beneath our own sink, but not permanently plumbed in.
Main drawback: no smart TDS faucet or built-in remineralisation on the standard model.
Read my full Waterdrop G2P600 review for my own photos, under-sink fit test, filter inspection and comparison with our G5P700A.
Check Waterdrop G2P600 Price Today3. Best Countertop Alternative: Waterdrop K19 — Personally Tested
If there simply isn’t enough usable space beneath your sink, the Waterdrop K19 takes a completely different approach. It is a countertop reverse-osmosis system, so there is no permanent plumbing, no drilling a hole for a separate tap and no need to work around the pipework beneath the sink.
I have personally used and tested the K19, so this recommendation is based on hands-on experience rather than specifications alone.
For a small kitchen, rented property or anywhere that makes an under-sink installation difficult, that simplicity is its biggest advantage. You fill the removable water tank, select the water you want and the unit filters and dispenses it from the worktop.
The trade-off is that the K19 takes up worktop space instead of cupboard space, and you need to refill the tank manually. For a family using large amounts of filtered water every day, I would still prefer a permanently plumbed under-sink system such as our G5P700A.
But if fitting an RO system beneath the sink is impractical, the K19 is the compact alternative I would consider first because I have actually used it and know what living with it is like.
Best for: homes with very limited under-sink space, renters, or anyone wanting RO filtration without permanent plumbing.
Our experience: personally tested and used at home.
Main drawback: it occupies worktop space and requires the water tank to be filled manually.
Read my full Waterdrop K19 review for my own photos, water-quality measurements and experience using it.
If you literally have no under-sink space, countertop RO is your fallback.
Check K19 Price TodayCompact RO Comparison Table
| System | Our experience | Dimensions | Best for | Main drawback |
|---|---|---|---|---|
| G5P700A | Long-term use at home | 42.4 × 14.5 × 35.3 cm | Best overall compact under-sink RO | Higher cost |
| G2P600 | Hands-on inspected & fit-tested | 44 × 15 × 35.5 cm | Compact tankless alternative | No built-in remineralisation |
| K19 | Personally used/tested | 35 × 20 × 41.5 cm | No usable under-sink space | Uses worktop space; manual refilling |
Alkaline Remineralisation – Does It Matter?
Reverse osmosis removes a high proportion of dissolved minerals along with unwanted contaminants. Some people find the resulting water tastes flatter than ordinary tap water.
Our G5P700A includes a built-in remineralisation stage, and we prefer the taste of the remineralised water in everyday use. This was one of the reasons we chose that model for our own home.
Remineralisation is not essential for RO filtration, however. If you prefer a simpler system such as the G2P600, an optional remineralisation stage can be added separately if required.
Installation: What to Check Before You Buy
Before ordering a compact RO system, check more than the cabinet dimensions.
For an under-sink system such as the G5P700A or G2P600, you typically need:
- enough usable cupboard space around existing pipework
- a cold-water feed connection
- a connection to the waste pipe
- space for the drinking-water faucet
- access to a nearby electrical socket if the system requires power
- enough clearance to remove and replace filters
The biggest issue in our own kitchen was not simply the size of the RO unit. Our double-bowl sink creates a lot of pipework beneath the cupboard, which is why the larger X12 was impractical even though it is tankless.
The K19 avoids these installation issues because it sits on the worktop and does not require permanent under-sink plumbing.
If you are comfortable with basic plumbing, some systems may be suitable for DIY installation. If drilling the sink, modifying pipework or connecting the waste line is outside your experience, using a qualified installer is the safer option.
Frequently Asked Questions
Is a tankless RO system always better for a small kitchen?
Not always. Removing the storage tank can save space, but the RO unit itself can still be large. I found this when I physically tried the Waterdrop X12 beneath our double-bowl sink and the available pipework and clearance made it impractical to fit.
What is the smallest compact RO system in this guide?
Among the under-sink systems we compare, the Waterdrop G5P700A has the smallest overall cabinet dimensions. If you have very little usable under-sink space, the Waterdrop K19 avoids the cupboard-space problem entirely because it sits on the worktop.
Do compact RO systems still need electricity?
Many modern tankless RO systems use an electric pump, so they normally need access to a nearby socket. Check the installation requirements for the specific model before buying.
Is a compact RO system suitable for hard-water areas?
It can be. Reverse osmosis can reduce many dissolved substances in the drinking water it treats, but an under-sink RO system does not soften the water throughout the rest of the home. If your main problem is limescale in showers, appliances and pipework, a whole-house water softener is a different solution.
Final Verdict: Which Compact RO System Should You Choose?
For most households looking for a genuinely compact under-sink reverse-osmosis system, my best overall choice is the Waterdrop G5P700A. It is the system we use in our own kitchen, and its narrow tankless design has proved practical around the limited space and pipework beneath our double-bowl sink.
The Waterdrop G2P600 is my preferred alternative if you want a simpler compact tankless RO system. I have personally unboxed, inspected and fit-tested it beneath our sink, although I did not permanently install it.
If you have very little usable cupboard space—or do not want permanent plumbing—the Waterdrop K19 is the option I would consider instead. I have used it personally, and its countertop design avoids the under-sink space problem altogether.
One lesson from trying the much larger Waterdrop X12 beneath our sink is that tankless does not automatically mean compact. Always check the unit dimensions against your actual pipework and allow enough clearance for installation and filter changes.
My picks
Best if under-sink space is unavailable: Waterdrop K19
Best overall compact under-sink RO: Waterdrop G5P700A
Best compact alternative: Waterdrop G2P600
