Most British gardens have a water butt by the shed. An underground rainwater harvesting tank is a bigger step: you dig, you add a pump, and the water goes to the WCs, the washing machine and the garden. People usually think about it during a project, when they're building or extending, or when the water meter makes them look at what goes down the drain.
Either way, the questions are the same: what size, where, and how to connect it to the gutters and the house. Here are the sums to redo for your own house, then how to draw it on the plan of your buried services.
How much water your roof can collect
The sum fits on one line:
projected roof area (m²) × annual rainfall (m) × roof coefficient = water collected per year (m³)
The projected area is the footprint of the roof seen from above, overhangs included. Rain falls vertically: a 45° roof with 140 m² of tiles collects no more than a flat roof covering the same 100 m² footprint. So you measure it straight off a plan seen from above.
Annual rainfall depends on where you live. In London, the Met Office average is around 600 mm a year. Much of the north and west gets well over 1,000 mm, and the uplands far more. The averages for your nearest weather station, on the Met Office website, are the figure to use.
The coefficient allows for water lost to evaporation, splashing and the first flush rejected by the filter.
| Type of roof | Usual coefficient |
|---|---|
| Tiles, slates, metal sheet | 0.8 to 0.9 |
| Flat roof with gravel | about 0.6 |
| Smooth flat roof (EPDM, felt) | 0.7 to 0.8 |
| Green roof | 0.3 to 0.5 |
Example. A 10 × 9 m house with a 50 cm overhang all round has a roof footprint of 11 × 10 m, so 110 m². With 600 mm of rain and a tiled roof (0.8): 110 × 0.6 × 0.8 ≈ 53 m³ a year, or 53,000 litres. That's an average: summer gives less, autumn and winter more. The same house in Manchester or Cardiff collects noticeably more.
If only some of the gutters can reach the tank (the front slope drains to the road, say), count only the area of the connected slopes.
How much you really use
Rainwater is for uses that don't need drinking water. For a family of four, realistic orders of magnitude:
| Use | Assumption | Per year |
|---|---|---|
| WC flushing | about 25 L per person per day | about 36 m³ |
| Washing machine | 4 loads a week at 50 L | about 10 m³ |
| Watering the veg patch and beds | 50 m² watered in summer | 5 to 10 m³ |
| Cleaning, car, patio | a few buckets and a pressure washer | 1 to 2 m³ |
| Total | about 50 to 60 m³ |
In this London example, the roof collects roughly what the house uses. The tank overflows in winter, and in a dry summer you'll top up from the mains. With a wetter climate or a bigger roof, it's rarely empty.
Choose the size of the tank
The tank doesn't have to store a whole year. It has to last through dry spells, when it doesn't rain for two or three weeks. So the size comes from daily use:
- Without watering, WCs and washing only: about 46 m³ a year, so 125 L a day. For three weeks without rain: 125 × 21 ≈ 2,600 L. A common British rule of thumb, about 5% of the annual yield, gives the same order: 5% of 53 m³ is about 2,650 L.
- With summer watering, use can double or triple in the dry weeks, exactly when the tank isn't filling. At 350 L a day, three weeks need about 7,400 L.
That's why most underground tanks for a family home sit somewhere between 3,000 and 7,500 litres. Beyond 10,000 L the gain is small for a household, unless the roof is large and the garden thirsty.
For a shed or a carport of 20 to 30 m², a water butt or an above-ground tank of 200 to 1,000 L at the foot of the downpipe is enough.
Where to put it on the plot
Decide the spot on the plan, with the house, the downpipes, the services and the trees in front of you:
- Close to the downpipes. Every metre of pipe means more trench and more fall lost.
- Close to the house, if the pump supplies the WCs and the washing machine.
- Accessible for maintenance. The lid has to stay clear: not under a patio of bedded slabs, not under a flower bed.
- Away from driveways, unless the tank is designed for vehicle loads (or has a load-spreading slab above it).
- Far enough from the foundations. A common practical rule: don't dig below a 45° line running down from the bottom of the foundation. The builder will tell you what suits your ground.
- Away from the roots of large trees, which go looking for water and lift lids.
- With the water table in mind. In wet ground, an empty plastic tank can float up like a cork. Tell the groundworker.
Finally, check what already runs under the chosen spot: drain, cable, water main. The guide mapping underground pipes and cables explains how to find them. If the house has a septic tank or a treatment plant, keep the two clearly apart: the guide septic tanks and sewage treatment plants covers what to check.
Falls and connections
Water arrives and leaves by gravity: the fall decides how deep the tank goes.
- The downpipes join a buried collector pipe laid at a steady fall, often 1 to 2% (1 to 2 cm per metre). Over 12 m of pipe, the inlet into the tank is 12 to 24 cm lower than the foot of the furthest downpipe, on top of the depth needed to stay clear of frost.
- A filter before the tank holds back leaves and moss. Make sure you can open it.
- The overflow gets rid of water when the tank is full. It leaves lower than the inlet, with its own fall, towards a soakaway, a ditch or a drain, depending on what's allowed where you live. Into a drain, a non-return valve and a trap stop backflow and smells.
- The pump, in the tank or in the house, supplies a separate network: WCs, washing machine, outside tap. Label those taps and pipes as non-drinking water.
The overflow: soakaway or drain
More and more councils and water companies want rainwater kept on the plot. The overflow then goes to a soakaway: a gravel trench, a pit, buried crates, or a swale (a shallow planted dip).
Its size depends on how fast your ground drains: sand soaks water up quickly, clay hardly at all. A percolation test saves you digging a soakaway that overflows at the first downpour. Put it downhill from the house, away from foundations and basements.
What the rules may require
For new building work in England and Wales, the Building Regulations set an order of priority for rainwater from roofs: a soakaway or other infiltration first, then a watercourse, and a sewer only if neither is reasonably practicable. A planning condition on a new house or a large extension can also ask for sustainable drainage. Some water companies reduce the surface water part of your bill if no roof water reaches the public sewer. Before you order a tank, ask your council's building control and your water company what applies to you.
Draw it in Architektor
The tank and its pipes go on the underground level, the one for utilities. The house, the patio and the trees stay visible, faded: you see at once if the tank lands under a future path.
- The tank: Tank / manhole tool (K key), type “Rainwater tank”. Click to place it, then set its shape (rectangular or round), width, length, depth and volume in litres in the properties.
- The downpipes and the collector: Pipe tool (U key), service “Stormwater”. Start from the foot of each downpipe and run to the tank, clicking each change of direction. Enter the diameter and the depth, at least at both ends.
- The overflow: a second “Stormwater” pipe leaving the tank towards its outfall.
- The soakaway: Tank / manhole tool, type “Soakaway”, with its dimensions. Inspection chambers go in with the same tool.
- The pump line to the house or the garden tap: don't draw it as “Drinking water”, it would mislead the next plumber. Keep “Stormwater”, or “Irrigation” for a line that only serves the garden, and write “pumped rainwater, not drinkable” in the Notes field.
A planned tank is drawn with the status “To build”, in red, like on planning drawings. The Notes field takes what you measured (“bottom 2.40 m below the lid”): these notes stay private, even in a share.
With your own API key, the assistant can also do a first draft: “Place a 5,000 litre rainwater tank near the back wall and connect it to the downpipes.”
Before you call the groundworker
For quotes you can compare, the plan should show the connected roof area and the target volume, the position of the tank dimensioned from two corners of the house, the route of the downpipes and the overflow, the outfall, the access for the digger and the services that cross the trench.
Share it with the “Underground utilities” link: the groundworker sees only the services and pins questions in the exact spot. For the site, export the underground level as a PDF to scale.
A tank is part of the garden too: the water from a well-designed patio can run into it, and the guide draw a garden plan to scale covers watering and the vegetable patch.