Estimate your project costs instantly with Build It's interactive self-build cost calculator
Calculate NowGet an accurate, detailed cost breakdown of your project
Submit plans
Carbon emissions have grabbed the headlines for many years when it comes to sustainability, but water is just as important, if not more. Human settlement can only take place where there is a water supply, after all. Fortunately, we are a well-watered isle. But in parts of the country – especially the Southeast, where rainfall is lowest – there is pressure on water supply, which may in turn limit development.
Unfortunately, the Southeast has also experienced the largest population growth in recent times, and it’s expected to continue. Perhaps the obvious solution is to concentrate new house building in the Northwest and other areas where land is much cheaper, employment is needed and building costs are lower.
The other factor in this equation, which is completely outside of the control of developers, is the water supply companies. While population has grown, storage capacity and the distribution system in general has not kept pace. Public utilities have failed us. It’s convenient to blame climate change but, if you look at the hard numbers, almost no new storage capacity has been added to the network over the last 35 years. During that period, the UK population has risen by over 20% and, as has been widely noted, water companies have been in private ownership. Leakage in the supply system, initially reduced after privatisation, also continues to be a major issue, running at around 20% according to Ofwat.
Over the years, housing density has increased – so people have smaller gardens (if at all) and thus there has been a reduction in demand for outside water use. Perhaps more importantly, WC flush volumes have decreased. As 20-30% of domestic water use is for WC flushing, a reduction from the old 9L to newer dual 4-6L delivers a really significant saving – with existing WCs being gradually replaced and water efficient versions installed in all new builds. There is also the fact that appliances such as washing machines and dishwashers have also become more water-efficient, driven in part by a desire to save energy.
However, against that we have to recognise that there is increased demand for water in some areas of domestic consumption – we shower and bathe more regularly than we used to, for instance, and we wash our clothes and bedding more frequently. It’s often claimed that having a shower rather than a bath saves water, but I’m not convinced – for one thing it depends on how long you spend in the shower. Four minutes is optimal but rare.
This table shows an average water consumption, broken down into the different uses.
| Source | Percentage |
|---|---|
| Shower | 25% |
| Toilet | 22% |
| Cold taps | 22% |
| Hot taps | 11% |
| Washing machine | 9% |
| Bath | 8% |
| dishwasher | 1% |
| Garden | 1% |
| Car | 1% |
Demand per capita has fallen slightly from the oft-quoted 150 litres per person per day (lpd) to just below 140 lpd. A 5-10% decline over 30 years is normally accepted as a fair summary of what has taken place.
It’s hard to pin this down, but much of the saving can be attributed to the adoption of efficient WCs. The other important factor is the widespread introduction of water metering. This means that your bill is directly linked to your water use, and you are more likely to be careful if you know it’s costing you money.
In recent years, regulations on water efficiency for new buildings (and conversions) have been tightened significantly. Last updated in 2024, Part G of the Building Regulations covers water efficiency, via a methodology known as the Water Efficiency Calculator for New Dwellings. The headline from the regs is that potable water consumption in new dwellings must not exceed 120 litres per person per day (internal use) and 5 l/p/d (external use).
The Villeroy & Boch Subway 3.0 WC offers just a 4.5 litre full flush (as opposed to the usual 6 litres) and 3 litres for the small flush – potentially saving a huge volume of water in a family household. Available from Sanctuary Bathrooms
In some water-stressed areas, local authorities may impose even stricter standards – often 110 lpd, usually enforced as a condition of planning approval. The government has also expressed a desire to drive water use down further, possibly to 100 lpd. This of course only addresses demand, and as a society to get to the required balance we should also be regulating supply more carefully, increasing storage capacity and addressing leakage.
To achieve these demanding standards, it’s vital to select water-efficient goods (think bath sizes, shower heads, tap heads, dual flush WCs, and efficient appliances) and plug their details into the Water Efficiency Calculator – which is freely available online. To get through Building Regulations you will have to submit a compliant calculation below the required threshold, taking into account any harvested water.
Given that appliances are unlikely to get much more efficient and that meters are now standard in new builds, what can self builders do to reduce water demand even more?
Rainwater can be harvested, stored safely and used to flush WCs – and sometimes to feed washing machines. Even though demand for water for WCs has fallen, this still represents a significant proportion of daily use, so harvesting rainwater continues to make economic sense. Normally the primary collection system is the roof, as it’s existing or planned gutters and downpipes can be conveniently used to convey rainfall to a storage tank (usually located underground).
Tank sizing is subject to the law of diminishing returns. The larger the tank, the fewer the occasions when the mains backup will be required – but the effect decreases as tank size is increased. A tank size of 1,500 to 2,000 litres (1m3 = 1,000 litres) will supply most of the WC and washing machine requirements in the average house. As a rule of thumb, 5% of the roof’s available annual rainfall is a good starting point for storage capacity.
This image illustrates the basics of how a UK rainwater harvesting system can be set up for non-potable uses, such as WC flushing, car washing and irrigating the garden – with overflow to a soakaway or similar
Installing a rainwater harvesting system is much easier and cheaper when building a new house, rather than when retrofitting an existing building. For one thing, you have a digger on site to assist with construction of the foundations, so the additional cost of excavating a hole for an underground storage tank is minimal.
You also have better access to the whole building as it is being constructed. This is important because harvested rainwater must be directed to WCs by separate and clearly marked pipes (purple in colour and labelled at regular intervals) so that later plumbers do not accidentally connect them to drinking water supply or other potable uses. It’s much easier to install these pipes before finishes are applied, while retrofitting a complete pipe run through an existing building can be logistically challenging.
People often confuse rainwater harvesting with greywater harvesting. Greywater is the waste coming out of sinks, baths and showers. As such, it contains a wide variety of contaminants, many of them pathogenic – ie potentially harmful to human health. It takes more complex systems to neutralise this health risk, so it may be safer or more cost-effective to get rid of this grey water asap and concentrate on rainwater instead.
your guide to rainwater harvesting
Case study Award-winning eco house with a rainwater harvesting system
When Nathan Davis first stood on the overgrown plot that would host his family home, he envisaged a building that would support multigenerational living, embrace the site’s irregular footprint and tread lightly on the environment. “We wanted something true to the landscape; a home that curved and flowed with it” says Nathan, co-founder of sustainable architecture practice ARCO2 and a certified Passivhaus Designer.
Photo: Chris Hewitt
This project won the 2025 Build It Award for Best Eco Home. The judges felt every aspect of this hands-on build was meticulously planned and realised – from natural insulation to ventilation, solar gain, airtightness, biodiversity and waste management. Partially sunken into the landscape, and set across four storeys, the curved design is organised as two wings: one housing utility spaces and services; the others the social core of the house.
From the beginning, environmental performance was as important as aesthetics. The timber house has been super-insulated using sheep wool and recycled newspaper insulation, with airtight detailing to minimise heat loss. Aluminium-clad triple-glazed timber windows, the air source heat pump, solar PV panels and rainwater harvesting system all contribute to low running costs.
Photo: Chris Hewitt
“We wanted a house that didn’t just look green but truly performed and would be future-proof: not only in terms of energy efficiency and adaptability, but also with the potential to generate income when the time comes to retire,” says Nathan. “Our heating bills are minimal as the passive solar design does so much of the work.”
The sedum roof established on the storage container/workshop not only enhances biodiversity but also reduces rainwater run-off and helps to moderate internal temperatures for the equipment and gym space.
| Location | Cornwall |
| Construction method | Timber frame & blockwork |
| Plot size | 0.235 acres |
| House size | 314m² |
| Build cost | £456,194 (£1,452 per m²) |
| Building work took | 8 years |
Power showers generally operate in the range 10-15 litres per minute (l/m). Efficient modern showers use 6-9 l/m and super-low-flow showers 4-6 l/m. On this basis, you can see how quickly a power shower would use up the entire daily water use for one person. If you’re trying to achieve a water-efficient home, then it’s a no-no.
Some people, however, find super-low-flow showers don’t give then the same feeling hen showering; so, there’s clearly a trade-off between performance and comfort. Efficient shower heads introduce air just before the nozzle, so you get the feeling of a more powerful shower with less water use.
The Quantum dual function bar valve shower system from Tavistock is designed to deliver a flow rate of 12 litres per minute at a pressure of 1 bar – with consumption increasing at higher pressures
You can make some savings with aerated tap heads, but they’re much more modest, as water use for handwashing tends to be relatively low. It might be more effective to reduce the lag time between hot water leaving the cylinder or boiler and arriving at the tap (eg by lagging hot water pipes if you’re renovating) as this generally translates to wasted water.
Many people immediately think of water butts when considering how to improve water efficiency in the garden. They can certainly help and should perhaps be standard in all new homes, regardless of garden size. However, the majority of outside use is for watering lawns – and it’s when a hosepipe is left on for a long time that the highest wastage occurs.
This Titan 110w mains-powered pump delivers water from your garden butt at higher pressure for jobs that need a hose. Available from Screwfix
What many don’t realise is that, although lawns turn brown in a drought, they haven’t died – they will soon green up again when it rains. In fact, once established in suitable locations, most plants survive without artificial watering; the vegetable beds or hanging baskets will generally be watered by hand, so the actual use should be very low.
The supply of water is only half the story; it also needs to be taken away from our houses. Most systems that connect to the public drainage system are combined, so sewage (including waste from baths and sinks) and rainfall (surface water) get discharged into the same pipework.
Surface water drainage is a growing issue for two principal reasons. Firstly, the frequency of heavy downpours appear to be increasing as a result of climate change. Secondly, the development of housing and commercial sites reduce natural percolation of rainfall into the ground.
Instead, it is conveyed directly into the drainage system, which can then become congested. Ultimately, these factors can increase the risk of flooding. The best way to address this is through the use of sustainable drainage systems (SuDS).
When it comes to drainage systems, the two main techniques are:
In terms of waste water, if you’re self building and your site can’t be cost-effectively connected to mains drainage, then you’ll probably be looking at septic tanks or a reed bed system to handle sewerage.
Properly designed, reed beds can be so effective that the eventual run-off is drinking quality water; while the ponds themselves are an ecological asset. Composting toilets are another solution for sites that have no foul sewer access, especially if there are drainage problems such as heavy clay soils. Although, this approach is perhaps still the preserve of the deep green developer.
Login/register to save Article for later
