Rainwater Harvesting for New Builds: Part G, the 2025 SuDS Standards and What Happens After Handover
TL;DR: No national law makes rainwater harvesting compulsory on new homes in England, but three rules now push it onto most schemes: the optional 110 litre water efficiency standard in Part G, the June 2025 national SuDS standards, which ask for it wherever there is demand for non-potable water, and water stress policies in local plans. Winning the planning condition is the easy part. Proving the system still works after handover is the hard part.
Last updated: 22 September 2026
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Key takeaways
- Building Regulations cap new homes at 125 litres per person per day, or 110 litres where the planning permission makes the tighter figure a condition.
- Approved Document G lets harvested rainwater be deducted in the water efficiency calculation.
- The national SuDS standards published by Defra in June 2025 say rainwater harvesting shall be considered wherever there is demand for non-potable water.
- A site where rainwater harvesting stops runoff in everyday storms is deemed to meet the SuDS interception standard.
- In July 2021, 15 water company areas in England were determined to be in serious water stress, and councils can use that to require 110 litres.
- A tank that sits full of mains water saves nothing and stores nothing, so the system has to be watched after handover.
Does a new build need rainwater harvesting?
No single law makes rainwater harvesting compulsory on a new home in England. Three separate rules push developers towards it instead: the optional 110 litre standard in Part G of the Building Regulations, the national SuDS standards published in June 2025, and local plan policies in areas of water stress. Most planning conditions that mention it trace back to one of these.
The first is the water efficiency limit. Regulation 36 of the Building Regulations says a new dwelling must not exceed a potential consumption of 125 litres of wholesome water per person per day. Where the planning permission specifies the optional requirement and makes it a condition, the limit drops to 110 litres. The wording is set out in Approved Document G, the government’s guidance on sanitation, hot water safety and water efficiency.
The second is drainage. On 19 June 2025 Defra published the national standards for sustainable drainage systems, updated on 30 July 2025. They are non-statutory, they apply to new development in England, and they put rainwater harvesting at the top of the list of places for runoff to go. Schedule 3 of the Flood and Water Management Act 2010, which would have made SuDS approval a legal step, is still not in force in England, so these standards reach developers through planning. Our guide to SuDS approval under Schedule 3 covers that gap.
The third is local policy. Councils in water-stressed areas write the 110 litre standard into their local plans and attach it to permissions as a condition, which is how rainwater harvesting ends up on the drawings.
How does rainwater harvesting help meet the 110 litre standard?
Rainwater harvesting helps because Approved Document G lets the savings be deducted in the water efficiency calculator. Water used for toilet flushing and washing machines can come from the tank instead of the mains, so the wholesome water figure per person falls. Efficient fittings do most of the work, and where they are not enough, harvested rainwater closes the gap.
Toilets are the obvious target. The Energy Saving Trust’s At Home with Water report, published in July 2013 and billed as the biggest ever review of domestic water use in Great Britain, found toilets account for 22% of household water use. The average household flushes about 4,600 times a year, which is roughly 28,000 litres of drinking-quality water sent down the pan.
Approved Document G describes when this matters. Its calculator guidance says rainwater harvesting and greywater recycling may be used where the efficiency of taps, WCs and other fittings has already been maximised and further savings are still needed. The rainwater calculation follows the method in BS 8515, using the collection area, a yield coefficient, the filter efficiency and local rainfall, then sets the savings against demand.
Two details catch people out. First, the simple fittings approach cannot be used once water re-use is provided; the full calculator has to be completed. Second, the guidance expects a record of any alternative water source supplied to the home, with enough information for the owner or occupier to maintain it, so the water efficiency is kept after handover.
How does rainwater harvesting count towards SuDS?
Rainwater harvesting counts twice under the 2025 national SuDS standards. Standard 1 puts it first among destinations for runoff where there is demand for non-potable water, irrigation need or water stress. Standard 2 treats a development as compliant with the everyday rainfall standard where harvesting means no runoff leaves the site in smaller storms.
Standard 1 ranks five final destinations for runoff, and priority 1 is collected for non-potable use, ahead of infiltration and every kind of discharge. It also says higher cost alone is not a reason to drop to a lower priority, so a drainage strategy that skips harvesting has to show why.
Standard 2 is the one that changes designs. It asks that at least the first 5 mm of rainfall in most events does not run off the site into surface waters or piped drainage, with 80% interception in summer (May to October) and 50% in winter (November to April). That is a volume target, and a rainwater tank is a volume.
The standards add a design reference. Rainwater harvesting systems intended to manage large rainfall events should be designed to BS EN 16941, the European standard for on-site rainwater systems published in the UK by the British Standards Institution. Its scope covers design, sizing, installation, commissioning and maintenance, and the SuDS standards ask for the interception calculations to be done to it.
There is a catch hidden in that double credit. A tank only intercepts rain if it has room when the rain arrives. After a wet fortnight, a full tank intercepts nothing, and every extra millimetre goes straight to the overflow. A system that counts towards SuDS on paper can deliver no storage at all on the day it is needed, which is why the way a tank empties matters as much as its size. Our SuDS monitoring guide covers the rest of the drainage train.
Which new developments feel the most pressure?
The pressure is highest where public water supply is already stretched. In July 2021 the Environment Agency’s advice led to 15 water company areas in England being determined to be in serious water stress, and the determination report says local authorities can use it to decide whether to require 110 litres per person per day in new development.
The long view is tighter still. The Environment Agency’s National Framework for Water Resources 2025, published on 17 June 2025, says England will need up to an additional 5 billion litres of water a day for public supply by 2055 if nothing is done. It names demand from household growth as the second biggest driver, adding 990 million litres a day.
Standards for new homes are expected to move with it. The framework’s chapter on public water supplies notes that the government signalled it would review Part G to increase water efficiency requirements for new homes, and describes water credit schemes that let developers offset the water their homes need. A scheme designed to 110 litres today may be a scheme designed to the minimum tomorrow.
What goes wrong with rainwater harvesting after handover?
Most failures are silent because the home still has water. A rainwater harvesting system is backed up by a mains top-up, so when a pump, filter or valve fails, the toilets keep flushing on mains water and nobody notices. The planning condition was met, the calculation was signed off, and the saving quietly stops.
Four faults do most of the damage:
- The pump fails. The mains backup supplies every outlet, the tank fills and overflows, and the home runs at the old water use without anyone knowing.
- The filter blocks. Rain bypasses the tank through the overflow, the level never recovers and the mains top-up runs every day.
- The top-up valve sticks open. The tank fills with mains water, so it holds no room for rain and gives no storm storage at all.
- The overflow blocks. A full tank backs up into gutters and downpipes, and the water finds its own way out.
Responsibility is often unclear by the time these faults appear. Standard 7 of the national SuDS standards asks designers for a management and maintenance plan naming who maintains the drainage system for the lifetime of the development. Where a system serves more than one property, an appropriate body has to adopt it and look after it. On a single home, the new owner inherits a tank they may not know exists.
How do you monitor rainwater harvesting on a new development?
Monitor three things: the water level in each tank or butt, the flow on the mains top-up feed, and the condition of any pump chamber. Level shows whether the system is collecting and emptying. Top-up flow shows whether mains water is doing the work instead. Together they turn a silent failure into an alert with a timestamp.
Tank level. A level trace that rises with rain and falls with use is a working system. A flat line at the top means the tank is not being drawn down or is filling from the mains. A flat line at the bottom means the rain is not reaching it.
Top-up flow. The mains feed into a rainwater tank should run occasionally, after dry spells. If it runs every day, the rain side has failed. A clamp-on meter on that pipe answers the question without cutting into it.
Pump chambers and plant rooms. A pump that fails wet can flood the chamber around it. A leak sensor on the floor gives warning before the water reaches anything expensive.
Evidence. Level logs, release records and alerts, kept per property, are the record Approved Document G expects owners to have, and the evidence a SuDS maintenance plan needs to show it was followed.
The full AQUAIOT rainwater harvesting capability stack
Each layer below answers one part of the handover problem, from the butt on a single plot to the evidence a management company needs across a whole estate.
Plot-level storage that empties before storms
The AQUAIOT Smart Water Butt adds a level sensor and a release valve to a standard rainwater collector. It runs in three modes: manual release from the app or dashboard, threshold rules, or weather-forecast control that opens the valve when significant rain is predicted in the next 24 to 48 hours. That keeps room in the vessel for the storm, which is the point of the interception standard. The assembly fits 100 to 500 litre collectors, connects over WiFi, 4G or LoRaWAN and runs for up to 7 years on battery with LoRaWAN.
Communal tanks and attenuation chambers
Larger schemes use shared underground tanks or attenuation storage. The AQUAIOT Radar measures their level without touching the water: 60 GHz radar, accurate to within 2 mm, reliable in enclosed chambers where condensation defeats other sensors.
The mains top-up feed
The Clamp-on Ultrasonic Flow Meter fits outside the pipe on sizes from DN8 to DN100, with no cutting and no shutdown. On the backup feed it shows how often mains water replaces rain, which is the clearest single sign that a system has stopped working.
Pump chambers
A leak sensor on the chamber floor triggers the moment water touches its sensing membrane, runs on LoRaWAN and lasts up to 10 years on battery.
One record for the whole estate
The AQUAIOT Cloud brings level, flow and leak data together with alerts by SMS, email or app, and exports the timestamped history that planning and maintenance reports ask for.
Frequently asked questions
Is rainwater harvesting mandatory for new builds in England?
No national law requires rainwater harvesting on new homes in England. It becomes a requirement through planning: a condition applying the optional 110 litre water efficiency standard, a drainage strategy built on the 2025 national SuDS standards, or a local plan policy in a water-stressed area. The planning permission for the site is the document to check.
What is the 110 litre water efficiency standard?
It is the optional requirement in regulation 36 of the Building Regulations. New homes must not exceed a potential consumption of 125 litres of wholesome water per person per day, or 110 litres where the planning permission specifies the optional requirement and makes it a condition. Both figures include a fixed 5 litres per person per day for outdoor use.
Does rainwater harvesting count towards Part G?
Yes. Approved Document G’s water efficiency calculator deducts savings from harvested rainwater, calculated using the collection area, a yield coefficient, the filter efficiency and rainfall, following BS 8515. Where water re-use is provided, the simple fittings approach cannot be used and the full calculator must be completed.
Can rainwater harvesting meet the SuDS interception standard?
Yes. Under Standard 2 of the 2025 national SuDS standards, a development where rainwater harvesting means no runoff leaves the site in lower order storms is deemed compliant. The tank still needs spare room when rain arrives, so controlled emptying matters as much as capacity.
Who maintains rainwater harvesting on a new estate?
The SuDS management and maintenance plan should name who is responsible for the lifetime of the development. Systems serving more than one property need an appropriate body to adopt and maintain them, often a management company. A system on a single plot passes to the homeowner, with the records Approved Document G expects them to be given.
Where to start
Start with the planning permission and the drainage strategy: they say which standard applies and what the rainwater system is expected to do. Then decide how you will know it is still doing it a year after the last plot is sold.
If you are planning rainwater storage on a housing scheme, tell us about the site and a UK engineer will come back within one working day. For water security on existing sites, see our guide to drought resilience for UK estates, and for work still on site, construction site water monitoring.