If your garden is small but you have a large roof area, you may consider a more advanced system to collect water for your toilet or washing machine.
However, it's worth looking into this quite carefully as such a system will not always be financially or environmentally beneficial due to impacts such as the manufacture & installation (e.g. with concrete backfill) of the tank and the energy use and periodic replacement of the pump.
Dr Judith Thornton, water treatment expert and author of CAT's 'Choosing Ecological Water Treatment' book, has researched this area for many years. She writes:
"By far the largest environmental impact relating to your water use is heating it, so measures relating to hot water efficiency and low carbon water heating should be your first concern. General water efficiency measures and collecting as much water for the garden as you can use are also priorities."
Various academic reviews and reports have found that the benefits of systems to use rainwater within a house are very often outweighed by the environmental impacts. Bear in mind that mains water actually has a relatively low carbon impact compared to other areas of water use. Here is more from the water treatment book:
"There is room for system improvement (such as using more efficient pumps and using different materials for the storage tanks), but it is also worth noting how small the actual numbers are: the carbon dioxide (CO2) impact of supplying water to your home is around 100g CO2 per household per day. For most people this is about 1/600th of your total daily CO2 impact, so if being environmentally friendly is your motivation for considering alternative water supplies, there will certainly be higher carbon areas of your life you could work on before thinking about RWH systems!"
"Water scarcity is often used as justification for RWH systems. However, as with CO2 impact, this claim does not stand up to scrutiny. Areas with a scarce water resource are also those with low rainfall. Consequently, at the times and places when water supplies are likely to be most stressed, rainwater systems are least likely to be able to meet any shortfall. This is reflected in the recommendation that storage volumes are smaller in low rainfall areas."
A rainwater harvesting system to provide toilet flushing for a 3/4 bedroom house is likely to cost at least £3,000 if professionally installed. Costs for retrofitting systems will generally be higher than for those built into new houses. It may be possible to put in a simpler gravity-fed system (e.g. for a ground-floor toilet) for much less. The payback time can therefore be very long, and a system may never recover its costs if parts need replacing before savings are realised, as outlined in more detail in the Choosing Ecological Water Treatment book:
"In rough terms, given an average UK water bill (metered users) of around £330 per year (for water and sewage), and likely water savings of 20-30%, the system might save you £30 per year - and that is before you account for any maintenance or capital costs. A comprehensive PhD study that modelled several thousand different configurations of system demonstrated that mains water would need to cost more than £10 per cubic metre before RWH systems made financial sense. Regional differences in rainfall and water bills mean that rainwater harvesting systems may be economically viable in some instances (for example in the south-west of England), but do your own calculations rather than relying on those from RWH system suppliers."
To minimise the environmental impact of one of these systems the book recommends to "use a polyethylene tank rather than fibreglass, and ensure it is of a class that does not need concrete backfilling of the excavation." It also says to look for a low energy system (e.g. using a 12 volt DC pump) and to think about metering both mains and rainwater supply to see how the system is performing.
Rainwater harvesting is most effective integrated into new developments, especially large buildings (schools or commercial premises) with big roofs, high non-potable water use, and a metered supply. Rainwater collection on this scale can also help reduce flash flooding in urban areas with poor drainage.
In rural areas, a composting toilet could be a more sustainable way to reduce water use.