A GRAF AA rainwater harvesting system can be installed with either the Platin flat tank or the Carat tank, so the first step is choosing your system and tank. From there the method is largely the same for both. You excavate a pit, bed the tank on a compacted gravel foundation, backfill with gravel, run the inlet and overflow pipes with a slight fall, and fit the pump console inside the property. The two tanks have slightly different installation requirements, mainly in how deep you dig and how wide the gravel surround needs to be, and this guide covers both. No specialist contractors are needed beyond a groundworker and a plumber, and on a self-build the whole process fits naturally into the groundworks programme.
Understanding how to install a rainwater harvesting system is one of those topics that puts people off before they have even looked into the detail. The assumption is that it involves deep excavation, specialist contractors, and a complicated plumbing setup that requires careful coordination across the build. For the GRAF AA system, the reality is considerably more straightforward. The AA Eco-Plus and AA Silentio are designed around an underground tank that needs no concrete, a single cover visible at ground level, and a pump console that installs inside the property like any other utility appliance. The system can be specified with either the shallow-dig Platin flat tank or the larger-capacity Carat tank, and the two come with slightly different installation instructions. This guide walks through every stage for both tanks, what each involves, what to tell your groundworker, and when in the build programme to schedule the work.
When is the best time to install a rainwater harvesting system on a self-build?
The single most important factor in a successful rainwater harvesting installation is getting the timing right within the build programme. This is true of any underground system, but it matters particularly for self-builders because the window for a low-cost, low-disruption installation is relatively short.
The underground tank needs to go in while the groundworks are open. Once the build is complete, excavating for a buried tank becomes significantly more expensive and disruptive. The pipework that carries rainwater from the roof to the tank, and rainwater from the tank to toilets and the washing machine, needs to be routed while the fabric of the building is still accessible. Waiting until after first fix to think about the rainwater system means running pipes through finished walls and floors, which is avoidable.
The practical advice is to confirm your rainwater harvesting system specification before groundworks begin, including which tank you are using. The choice between the Platin and the Carat affects how deep and how wide the excavation needs to be, so it is worth settling early. If you have already started the build, the question is not whether it is too late, but whether the groundworks are still sufficiently open to allow installation without major disruption.
For a self-builder, scheduling the tank installation to coincide with the drainage or foundation groundworks is ideal. Your groundworker is already on site, the excavation equipment is already there, and the additional time required to dig and bed the tank is a fraction of what a standalone mobilisation would cost.
What does the excavation stage involve, and how does it differ between the tanks?
The excavation principle is the same whichever tank you choose. The base of the dig is levelled to a consistent depth and compacted, then a layer of compacted round-grain gravel, grain size 10/20, is laid as a foundation before the tank goes in. The marked-out area needs to allow for the tank itself plus space all round for backfill material, and at least 1,000mm distance from any solid structures such as foundations or retaining walls. What changes between the two tanks is the depth and the working space.
The Platin is a shallow-profile flat tank, which is its main practical advantage and one of the reasons the AA system is so straightforward to install for self-builders and retrofitters alike. A traditional cylindrical tank with a 3,000 litre capacity typically requires excavation to a depth of around 1.9 metres, with significant lateral clearance for safe installation. The Platin flat tank of equivalent capacity has a height of 735mm, requiring a considerably shallower excavation. That difference translates directly into reduced machine hire time, less spoil to remove from site, and faster reinstatement. For the Platin, the area is marked out to the tank dimensions plus a minimum of 100mm clearance on all sides for backfill, and the gravel foundation is approximately 100 to 150mm thick.
The Carat is a rounded tank rather than a flat one, so it needs a deeper excavation, but it offers larger storage capacities of up to 6,500 litres in a single tank. Total tank height runs from around 2,010mm for the 2,700 litre Carat up to 2,710mm for the 6,500 litre Carat, before the telescopic dome shaft. For the Carat, the trench base is sized to exceed the tank dimensions by 500mm on each side to give room to work and backfill around the curve, and the gravel foundation is approximately 150 to 200mm thick.
For both tanks, the tank and any water-holding parts of the system should sit within the frost-free zone, usually around 600 to 800mm deep, and the ground should be assessed for load-bearing capacity and groundwater before work begins. Neither tank needs a specialist installer. The installation manual provides the required dimensions, and any competent groundworker familiar with drainage or below-ground civil work will be comfortable with the process. It is worth sharing the relevant GRAF installation guide with your groundworker before they start, so the dimensions and bedding requirements are clear before the machine arrives on site.
How is the tank placed and backfilled during installation?
Once the foundation is prepared, the tank is lowered into position using suitable lifting equipment. Both tanks are designed for straightforward mechanical handling. The telescopic dome shaft is adjusted to match the finished ground level, and before backfilling begins the tank is filled one third with water to prevent deformation during the compaction process.
Backfilling uses round-grain gravel, grain size 10/20, rather than concrete or compacted earth, for both tanks. Gravel is placed in layers of 300mm around the tank and compacted using a manual tamper. Mechanical compaction machines must not be used at any stage, as these can damage the tank walls. The width of the gravel surround is one of the few points where the two tanks differ: for the Platin the surround must be at least 100mm wide against the trench wall at all points, while for the rounded Carat it must be at least 500mm wide all the way round so the tank is evenly supported. No concrete is required around the body of either tank, which means the tank can be accessed and removed in future without breaking out concrete, and it avoids the moisture retention issues that concrete backfill can introduce around plastic tanks.
The inlet and outlet pipe connections are made during backfilling, as each connection point becomes accessible. The inlet pipe, which carries rainwater from the roof downpipes into the tank, must be laid with a continuous fall of at least 1% in the direction of flow to prevent standing water and debris accumulation. The overflow connection directs excess water away from the tank when it reaches maximum capacity, typically to a soakaway or surface water drain, and must also be laid to a fall of at least 1%. Where an overflow is connected to a public sewer, it must be protected against backflow, using a reflux seal on a rainwater sewer or a lifting station on a combined sewer.
Once backfilling is complete and the ground is reinstated, only a single access cover is visible at the surface. On pedestrian areas, a plastic cover rated for foot traffic is included as standard, and for driveways or areas subject to vehicle loading a vehicle-rated cover is available. The Carat extends this further for heavier use. For cars up to 3.5 tonnes it takes a Class B cover, with the collar of the dome shaft supported by a ring of concrete and a minimum cover of 800mm over the shoulder of the tank, and for vehicles such as trucks up to 12 tonnes a Class D cover and a load distributing frame are used.
Where does the pump console install and what does that involve?
The pump console is the same whichever tank you choose. Both the AA Eco-Plus and AA Silentio use an external suction pump, meaning the pump is located outside the underground tank rather than submerged within it. The pump console mounts inside the property, typically on a wall in a utility room, plant room, or garage, where it draws water from the underground tank via a suction pipe and distributes it under pressure to the connected points of use throughout the home.
The integrated cistern is the component that sets the AA systems apart from a conventional indirect system. Rather than mains water entering the underground tank when levels run low, it feeds into the cistern within the console. The two water supplies remain physically separate at all times, providing the Type AA air gap that achieves Category 5 backflow protection as required by the Water Supply (Water Fittings) Regulations 1999.
Rainwater pipework from the console to toilets and the washing machine must be clearly labelled throughout to comply with water regulations, using standard “not drinking water” identification at all points of use and at regular intervals along the pipe run, in line with British Standard BS EN 16941-1:2024. Your plumber will be familiar with this requirement.
The AA Silentio console includes a digital display showing the live rainwater level in the underground tank, which can be read directly from the console. No separate monitoring hardware is required.
How long does a GRAF AA rainwater harvesting installation take?
For a new build where the tank installation is scheduled to coincide with groundworks, the below-ground element, excavation, tank placement, backfilling, and pipe connections, typically takes one day for a competent groundworker with a small excavator. A Carat installation can take a little longer than a Platin because of the deeper excavation and wider gravel surround, and longer again where vehicle loading and concrete support at the dome are involved.
The pump console installation, which is carried out by a plumber rather than a groundworker, typically takes half a day to a full day depending on the complexity of the pipe runs to toilets and the washing machine.
Commissioning, setting up the control unit, testing the switchover between rainwater and mains backup, and checking all connections, takes around two to three hours and is straightforward enough for a competent installer following the GRAF commissioning guide.
For a retrofit installation, where excavation is required in an established garden, timelines are longer and depend on ground conditions, access for machinery, and the extent of any reinstatement required.
What regulations and approvals apply to a rainwater harvesting installation in the UK?
A household rainwater harvesting installation forms part of the overall Building Regulations application for a new build rather than requiring separate standalone approval. The two frameworks that apply are Building Regulations Part G, which sets the water efficiency context for new dwellings, and the Water Supply (Water Fittings) Regulations 1999, which govern backflow prevention and the separation of potable and non-potable supplies.
For retrofit installations, Building Control notification is required where the system connects to internal plumbing such as toilets and washing machines. This is a straightforward notification rather than a full application in most cases and Building Control will want to confirm that the backflow prevention arrangements meet the regulations.
Installations involving Category 5 fluid risks, which includes harvested rainwater, will typically require notification to the local water company before work begins. Most water companies have a straightforward online notification process. It is worth initiating this early in the project timeline, as some water companies request to inspect the installation before commissioning.
The Planning Portal confirms that underground rainwater harvesting tanks are generally considered permitted development for domestic properties in England, meaning no separate planning permission is required in most cases. Exceptions apply for listed buildings and properties in conservation areas. These requirements apply equally whether you choose the Platin or the Carat, since both are installed underground.
The GRAF AA installation is simpler than most people expect
The perception that rainwater harvesting installation is complicated, disruptive, or requires specialist contractors is one of the most common reasons self-builders and homeowners delay the decision until it is too late. For the GRAF AA system, whichever tank you choose, the process is well-defined, the components are straightforward, and the trades required, a groundworker and a plumber, are already on site for any self-build.
The best time to install is during the build, while groundworks are open and the programme still has flexibility. If that window has passed, a retrofit is still achievable, but the cost and disruption are higher.
Posted by Callum Vallance-Poole, on June 1, 2026.