Rainwater harvesting is growing in popularity among UK homeowners and self-builders as a way to live more sustainably and save on water bills. By capturing rainwater, you can reduce demand on mains water and help mitigate issues like droughts and storm runoff flooding. One key decision when planning a rainwater harvesting system is whether to use an aboveground tank or an underground tank. Both options can store rainwater for uses such as garden irrigation, toilet flushing, or even laundry, but they come with different advantages and considerations.
Aboveground Rainwater Tanks
Aboveground rainwater tanks are installed on the surface (in a garden or alongside a house) and are often the simplest form of rainwater harvesting. Many homes already use small above-ground barrels or water butts connected to gutter downpipes to collect rain for watering plants. However, above-ground tanks can also be much larger, up to tens of thousands of litres, to serve bigger gardens, farms, or even some household uses. Here are the key pros and cons of above-ground systems:
Advantages of Aboveground Rainwater Tanks
Lower Cost & Easy Installation
Aboveground tanks are generally cheaper to buy and install than underground ones. No digging or heavy machinery is required, if you have a firm level base, you can often set up an above-ground tank as a DIY project with basic tools. This makes them accessible for homeowners on a budget.
Simple Maintenance and Access
Because the tank is visible and not buried, it’s easy to inspect and clean. Any leaks or cracks can be spotted quickly and repaired without excavating. Routine maintenance (like cleaning out debris or checking filters) is straightforward when everything is above ground.
Flexible and Portable
Aboveground units can be relocated or expanded with relative ease. If you move home, you could even empty and take your tank with you, something not feasible with an underground tank. You can also link multiple above-ground tanks together to increase capacity if needed.
Great for Gardens (Hosepipe Ban-Friendly)
An aboveground tank is ideal for garden irrigation. During UK hosepipe bans or dry spells, you can still water your plants using stored rainwater and stay within the rules. Many above-ground designs allow gravity feeding with a tap or hose, so you might not need a powered pump for watering the garden.
Variety of Sizes and Styles
There’s a wide range of above-ground tank designs, from slimline water butts that fit small patios to large vertical tanks for farms.
Drawbacks of Aboveground Tanks
Exposure to Weather & Temperature Fluctuations
Because they sit outdoors, above-ground tanks are exposed to the elements year-round. In winter, water inside can freeze and expand, risking cracks in the tank if it’s not insulated. In summer, direct sun can heat the water. Without insulation or regular turnover, warm stagnant water can encourage algae or bacteria (such as Legionella) growth. It’s recommended to either insulate above-ground tanks or drain them during extreme cold, and to keep them shaded if possible.
Higher Maintenance Needs
Weather exposure means these tanks may need more frequent maintenance. UV light can degrade some tank materials over time, and debris or algae may accumulate faster. Regular checks and cleaning are needed to keep the water quality good. The good news is maintenance is easier to perform (as noted), but it does require diligence.
Aesthetic and Space Impact
A large plastic tank isn’t the prettiest garden feature. Even when manufactured in neutral green or black to blend in, above-ground tanks can be an eyesore in a small garden. They also take up valuable outdoor space, which can be a problem on compact plots. You might end up installing screens or fencing to hide the tank, adding to the cost. Neighbours might complain about big tanks in view, especially in front gardens.
Vulnerability
Above-ground installations are more susceptible to accidental damage, impacts, or even vandalism. A surface tank could be damaged by garden tools, severe weather, or, in remote areas, tampering. Any damage could lead to leaks, so you’ll want to position the tank safely and perhaps secure it if vandalism is a concern.
Limited Water Pressure
If you plan to use the water for anything beyond filling a watering can (for example, feeding an irrigation system or supplying a toilet), you might need a pump. Above-ground tanks can provide gravity pressure for small uses, but since they’re at ground level, the water pressure may be low. Elevating the tank on a stand can improve gravity flow slightly, but pumps or booster kits are often needed for higher-pressure requirements.
Underground Rainwater Tanks
Underground rainwater tanks (also called below-ground or in-ground tanks) are installed beneath the surface, typically buried in the garden or under a driveway. These systems collect rainwater and store it out of sight below ground. The water is usually pumped up when needed for use. Underground tanks are common in more advanced rainwater harvesting setups, for example, where the rainwater is plumbed into the house to flush toilets, supply washing machines, or feed garden taps. They range in size, often from around 1,000 litres up to 10,000+ litres for domestic systems. Here are the main pros and cons:
Advantages of Underground Rainwater Tanks
Space-Saving & Hidden Installation
Placing the tank underground frees up your above-ground space. This is ideal for small gardens or urban plots where a bulky tank would be in the way. You can build a patio, lawn, or even a driveway over a buried tank (many underground tanks are structurally designed to support vehicle or pedestrian weight). Plus, the entire system is out of sight, no ugly plastic visible, which keeps your landscaping looking neat.
Stable Water Temperature & Quality
Underground storage provides natural insulation. The soil keeps the water cool and at a relatively constant temperature year-round. This means no worrying about freezing in winter or overheating in summer. The cool, dark environment also discourages algae and harmful bacteria growth, helping maintain better water quality over time. Many below-ground systems still include filters and calmed inlets to further protect water quality.
Protection from Damage
Buried tanks are generally safe from weather damage, UV exposure, and accidents. There’s no risk of storm winds blowing the tank over, and the water is secure from vandals or animals interfering with it. The installation is permanent and very robust if done correctly.
Larger Capacities for Home Use
Because they’re hidden, underground tanks can be quite large without impacting your garden’s look. This makes them well-suited for harvesting significant amounts of rainwater to supply the home (for laundry, toilet flushing, etc.) or to provide ample irrigation water. With a sufficiently sized underground tank and a pump, you could cut mains water use by 40–50% in a home. In fact, new UK sustainable building standards increasingly encourage underground rainwater systems as part of reducing potable water demand.
Year-Round Usage & Integration
An underground system can be fully integrated into your home’s plumbing with automated control. For instance, many setups include a mains water backup, if the tank runs low, a valve switches to mains supply, ensuring toilets or appliances always have water. This integration makes underground systems a reliable part of the household infrastructure, functioning seamlessly behind the scenes. Additionally, being underground means the system is unlikely to require seasonal shutdown; you can keep harvesting through winter since the water won’t freeze.
Drawbacks of Underground Rainwater Tanks
Higher Installation Cost & Complexity
The biggest downside of underground tanks is the installation effort. You’ll need to excavate a substantial hole, which often means hiring a digger or contractor. The tank itself tends to be more expensive and built to be durable (to withstand soil pressure). There are also costs for proper base preparation, backfilling, and possibly adding a concrete surround or reinforcement depending on the soil conditions. Overall, the upfront cost of a below-ground system is significantly higher than an equivalent above-ground setup.
Professional Installation Often Required
Unlike a simple water butt, installing an underground tank is usually not a DIY job for the average homeowner. It involves technical steps like connecting inlet/outlet pipework, setting up a pump and filtration, and ensuring the tank won’t float or collapse when empty. Mistakes can be costly, so many people opt to have professionals design and install the system. This can add to the timeline and budget.
Maintenance and Access Challenges
While underground tanks need less frequent upkeep (since they aren’t exposed to debris and sunlight), when maintenance or repairs are needed it can be more complicated. Access is through a manhole cover, and working on the tank might require entering a confined space or digging if there’s an issue. Simple tasks like cleaning the tank interior or fixing a leak are harder than with an above-ground tank. You’ll want to install things like pre-filters and an accessible pump to minimise how often you need to open the tank.
Soil and Site Constraints
Your ground conditions will determine if a below-ground tank is feasible. If you have very high groundwater or a water table, a buried tank might be prone to floating or flooding. Very rocky ground can complicate digging. Also, heavy clay soil can be problematic, clay doesn’t drain well and can exert pressure; some manufacturers warn that high clay content soil may not be suitable for their underground tanks. Always check the site conditions. In some cases, an above-ground solution might be the only viable choice if the ground is unsuitable for excavation.
Less Flexibility After Installation
Once installed, an underground tank is essentially permanent. You can’t easily move it if you redesign your garden or relocate to a new house. Changing its position or upgrading capacity later would require new excavation. In contrast, above-ground systems are easier to adapt over time. This means you have to carefully plan your underground tank’s size and placement upfront to get it right the first time.
Requires Pump and Power
To use the water from an underground tank, you will almost always need a pump (electric or otherwise) to push the water up to where it’s needed. This adds a bit of ongoing energy cost and another component to maintain. (Gravity-fed options from underground tanks are rare, unless the outlet is downhill from the tank.) Above-ground tanks, by comparison, can sometimes work via gravity alone for garden watering if elevated, though pumps are still used for any fed-in systems.
Next Steps
Choosing between an above-ground and underground rainwater tank ultimately comes down to your priorities: budget, space, and how you intend to use the water. In summary, an aboveground tank is an affordable, easy starting point, great for garden irrigation and modest rainwater needs, with the trade-off of being visible and somewhat exposed. An underground tank offers a hidden, higher-capacity solution better suited for serious water reuse (like supplying toilets or multiple uses), at the cost of a more involved installation and higher upfront expense. Both can dramatically reduce your mains water usage and help make your home more sustainable.
If you’re still unsure which option is best for your project, consider reaching out for expert advice. Many UK rainwater harvesting specialists (including our team) can assess your site and water needs and recommend a suitable system. Remember that whichever route you choose, you’ll be joining a growing number of eco-conscious homeowners tapping into the free resource that falls from the sky. By harvesting rainwater, you’ll not only save money and water, you’ll also be doing your part to buffer against droughts and reduce runoff in heavy rains.
Ready to get started? A good first step is understanding how much rainwater storage your property actually needs. Our rainwater tank size calculator helps you estimate the ideal capacity based on your roof area, rainfall levels and intended water usage, so you can make an informed decision from the outset. Once you know the right size, you’ll be better placed to choose a system that captures and stores rainwater efficiently, supports your household needs and maximises long-term savings.
Posted by Callum Vallance-Poole, on November 28, 2025.
