Quick answer: Rainwater harvesting for a self-build means deciding early which uses in your home need mains water and which can run on harvested rainwater. Planning this at design stage, before groundworks begin, gives you the best chance of a compliant, cost-effective system that reduces mains water reliance from day one of occupation.
Rainwater harvesting is one of those decisions in a self-build that rarely gets the attention it deserves at the start of a project. Most self-builders spend considerable time on heating systems, insulation, and energy performance, yet the water system, how your home collects, stores, distributes, and manages water, often gets resolved late in the design process, sometimes as an afterthought. That tends to make it more expensive and less effective than it could be.
Getting your water system right from the ground up means thinking about what your home actually needs from its water supply, which uses require drinking-quality water and which do not, and what the most sustainable and cost-effective way to meet those needs looks like over the lifetime of the property.
Why does rainwater harvesting matters more for a self-build than a standard new build?
A self-build gives you a level of control over your home’s design that a standard new build simply does not. That’s the opportunity. But it also means the decisions that get made at the design stage, including the water system, are yours to own. A developer building a standard housing scheme will specify the cheapest compliant solution and move on. A self-builder can make choices that reflect how they actually want to live in the home, what their environmental priorities are, and what will serve the property well over decades rather than just on the day of completion.
Building Regulations Part G sets a maximum daily water consumption target of 125 litres per person per day for new residential buildings in England, with an optional tighter target of 110 litres per person per day available where the local authority chooses to adopt it. For self-builders, meeting or exceeding these targets is not just a compliance exercise. It’s an opportunity to design a home that relies less on the mains supply, costs less to run, and is better placed to handle the water stress that the Government National Framework for Water Resources identifies as a growing challenge across large parts of England and Wales.
What does a self-build home actually need from its water supply?
The starting point for planning rainwater harvesting on a self-build is understanding what different uses in the home actually require. Not every use needs drinking-quality mains water, yet most homes treat the entire supply as if it does.
Drinking, cooking, and bathing require mains-quality water. There is no practical alternative for these uses in a domestic setting, and the water supply to taps, showers, and kitchen appliances should come from the mains as standard.
Toilet flushing accounts for a substantial share of daily household water use. Toilets do not require drinking-quality water, and supplying them from a non-potable source such as collected rainwater is both practical and compliant with UK water regulations, provided the system is correctly designed and installed.
Washing machines also work well on non-potable water. Rainwater is naturally soft, which can reduce the amount of detergent required and may extend the life of the machine over time.
Garden irrigation and outdoor cleaning are straightforward candidates for non-potable supply. Using collected rainwater for the garden is efficient, and it avoids the frustration of hosepipe bans, which apply to mains water use but not to rainwater you have collected yourself.
Understanding this split is the foundation of a sensible rainwater harvesting system for a self-build. A home that supplies toilets, laundry, and garden irrigation from harvested rainwater, while keeping the mains supply for drinking, cooking, and bathing, can meaningfully reduce its daily mains water consumption.
When in the self-build process should you plan your rainwater harvesting system?
The answer is earlier than most people think. The water system has physical implications for the build that are easiest to accommodate when they are planned from the start. An underground rainwater harvesting tank needs to go in while groundworks are open. The pipework that carries non-potable water to toilets and washing machines needs to be routed as the build goes up, not added retrospectively. The pump and control unit needs a home inside the property, typically in a utility room or plant room, and that space needs to be allocated in the floor plan.
None of these things are difficult to accommodate at the design stage. All of them become significantly more disruptive and costly to retrofit into a finished property. This is one of the few decisions in a self-build where acting earlier is unambiguously cheaper and simpler than waiting.
What rainwater harvesting options are available for a self-build?
Rainwater harvesting systems for self-builds fall into two broad categories, and understanding the difference matters before you specify.
Garden irrigation systems collect rainwater from the roof and store it in an underground tank, from which it is pumped to garden taps and hose connections. These are simpler in design, lower in cost, and do not connect to the internal plumbing of the dwelling. They are a good choice for self-builders who want to reduce mains water use for outdoor purposes without the complexity of a full household system.
Household systems go further, supplying toilets, washing machines, and garden use from the same underground tank via an automatic control unit that monitors the tank level and manages the switchover between rainwater and mains backup. These systems require connection to the internal plumbing and fall within the scope of Building Regulations and the Water Supply (Water Fittings) Regulations 1999. They are also capable of making a more significant reduction to overall mains water consumption.
Within household systems, there is a further distinction between direct systems, where mains backup water is introduced into the underground tank, and indirect systems, where mains backup feeds into a separate cistern inside the property rather than the tank. The indirect approach keeps the underground tank reserved for rainwater at all times, which means more of the tank’s capacity is consistently available for rainwater collection, particularly after dry spells.
GRAF UK’s AA Eco-Plus and AA Silentio take the indirect approach a step further. Mains water never enters the underground tank at any point. When the system needs to switch to mains backup, it draws from a small integrated cistern inside the property and feeds directly into the plumbing, leaving the underground tank fully available for rainwater storage. Both systems carry a Type AA air gap as standard, providing a widely accepted compliance route for achieving Category 5 Protection under the Water Supply (Water Fittings) Regulations 1999. The AA Silentio adds a digital display showing live tank levels, a reversible flow filter, and an automatic self-cleaning function on the filter. For self-builders who want to maximise how much rainwater their system collects and simplify the compliance process, the AA range is worth considering at the specification stage.
What tank size does a rainwater harvesting system for a self-build typically require?
Tank sizing depends on two variables: how much rainwater your roof is likely to collect, and how much your household is likely to use for non-potable purposes. A larger roof catchment area and higher household demand both point towards a larger tank, but the relationship between the two is not always intuitive.
Tank sizes for domestic self-build systems typically run from 1,500 litres at the smaller end up to 7,500 litres for a larger household system. Selecting the right size involves balancing what the roof can collect against what the household will use, avoiding the twin problems of a tank that fills and overflows before the water can be used, and a tank that empties too quickly during dry spells.
As a general principle, a roof area of around 100 square metres in a typical UK location can collect in the region of 85,000 litres of rainwater per year. That figure varies with location, roof pitch, and material, but it gives a useful starting point for understanding what a rainwater harvesting system could realistically collect.
What compliance requirements apply to rainwater harvesting on a self-build?
A household rainwater harvesting system forms part of the overall building application rather than requiring separate standalone approval. The two frameworks that apply are Building Regulations Part G, which sets the water efficiency context, and the Water Supply (Water Fittings) Regulations 1999, which govern backflow prevention and the separation of potable and non-potable supplies.
The key requirement is that harvested rainwater, which is typically treated as a Category 5 fluid risk under the regulations, must be kept physically separate from the mains drinking water supply at all times. The accepted method for achieving this is a Type AA air gap, the physical separation between the mains inlet and the non-potable supply. Systems designed around this principle tend to make the compliance and Building Control process more straightforward.
It is also worth noting that installations involving Category 5 fluid risks will typically require notification to the local water company before work begins, and non-potable pipework and outlets must be clearly labelled throughout the system.
How does GRAF UK’s range support rainwater harvesting for self-builds?
GRAF UK supplies underground rainwater harvesting systems suited to the full range of self-build projects, from compact garden irrigation setups to full household systems supplying toilets, washing machines, and outdoor use.
The Platin flat tank range offers a shallow-dig solution with tanks from 1,500 to 7,500 litres for standard domestic systems, and up to 30,000 litres for larger projects. The low installation depth makes it particularly well suited to sites where excavation depth is a constraint.
The Carat underground tank range runs from 2,700 litres up to 56,000 litres and is suited to projects requiring greater storage capacity or deeper installation.
Both ranges are available as garden irrigation systems or full household systems, and both are manufactured from 100 percent recycled plastic with a 15-year warranty as standard. The AA Eco-Plus and AA Silentio sit within the household system range and are designed around an indirect approach, keeping the underground tank reserved for rainwater at all times.
If you are planning rainwater harvesting for your self-build, the tank size calculator at grafuk.co.uk gives you a system recommendation based on your roof area and household demand in around two minutes.
Posted by Callum Vallance-Poole, on May 4, 2026.