Rainwater harvesting collects water from your roof, filters it and stores it in a tank so your garden can be watered from stored rainwater rather than the mains. Paired with an automatic irrigation system, it keeps lawns, borders and pots healthy through London's dry spells and hosepipe restrictions while cutting mains water use — and the water itself is naturally soft, chlorine-free and better for plants.
Why harvest rainwater in a London garden
London's rainfall is steady and reliable year-round, yet dry summer spells and hosepipe bans are becoming more common across South East England. A rainwater tank turns every shower into a free reserve for your garden, reduces your mains water bill, and provides naturally soft, chlorine-free water that most plants, lawns and acid-loving shrubs prefer. It's also one of the simplest sustainability measures you can add to a garden, and it pairs perfectly with an automatic irrigation system.
How rainwater harvesting works
Rainwater runs off the roof into gutters and downpipes, passes through a filter that removes leaves and debris, and is stored in a tank. A pump then feeds your irrigation system, hose or garden tap whenever water is needed. The system is sealed, low-maintenance and quietly collects water every time it rains — a mains top-up valve can be fitted so the tank never runs dry during extended dry periods.
Underground vs above-ground tanks
Underground tanks keep water cool and out of sight (which helps limit algae) and free up garden space, but need excavation and cost more to install. Above-ground tanks are simpler and cheaper to fit and easier to maintain, but take up space and benefit from a shaded position. We advise on the right option for your garden, access and budget, and handle the excavation, pipework and commissioning.
Tank sizing
The right tank size depends on your roof catchment area, local rainfall and how much water your garden uses. As a rough guide, every square metre of roof collects about one litre for every millimetre of rainfall — so a typical 100m² London roof can collect a large volume over a year. The tank is sized to balance collection against demand and space. See our tank-sizing guide or ask us to calculate it on a site visit.
Pumps, filters and controllers
A filter protects the pump and irrigation lines from debris, and the pump provides steady pressure to your sprinklers and drip lines. We match the pump to your system's flow and pressure requirements, fit a calming inlet to avoid disturbing sediment, and can include a level indicator so you can see how much water is stored. A mains top-up valve keeps the system running if the tank runs low.
Integration with irrigation
Rainwater storage is most useful when it's plumbed into an automatic irrigation system. We design both together so your pop-up sprinklers and drip lines draw from the tank first, switching to mains only if the tank is empty. A single controller runs the whole garden, and rain and moisture sensors stop watering when the soil already has enough.
Maintenance
Maintenance is light: keep gutters clear, check and clean the filter, inspect the pump periodically and keep the tank lid sealed. We can include this with your irrigation servicing visits, along with winter checks on the pump and exposed pipework.
Winter considerations
Above-ground tanks and exposed pipework should be protected from freezing. Underground tanks are naturally insulated. We winterise exposed components — lagging pipework, draining or protecting the pump — and advise on frost protection as part of installation.
What we provide
- Free site visit, roof catchment assessment and tank sizing calculation
- Underground and above-ground rainwater tanks
- Filters, pumps, level indicators and mains top-up valves
- Full integration with new or existing irrigation systems
- Calming inlets and sediment management
- Servicing, winterisation and pump checks
Collection
Rainwater runs off the roof into gutters and downpipes.
Filtration
A filter removes leaves and debris before water enters the tank.
Storage
Water is stored in an underground or above-ground tank.
Distribution
A pump feeds your irrigation system, hose or tap points on demand.
Underground vs above-ground tanks
Both options store the same rainwater — the choice comes down to space, access for excavation, budget and how visible you want the tank to be. We advise on the right option at the free site visit.
Underground tank
- • Out of sight, frees up garden space
- • Cool, dark storage limits algae growth
- • Naturally insulated against frost
- • Requires excavation and access for plant
Above-ground tank
- • Simpler and cheaper to install
- • Easy to inspect and maintain
- • Ideal for retrofits with no excavation
- • Takes up garden space, benefits from shade
Feed it straight into your irrigation
Rainwater storage is most useful when it's plumbed into an automatic irrigation system. We design both together so your sprinklers and drip lines draw from the tank first, switching to mains only if needed.
See irrigation systems →Where we work
We design and install rainwater harvesting systems across West and South West London, including Ealing, Chiswick, Richmond, Kew, Barnes, East Sheen, Hammersmith, Twickenham, Putney, Kingston Upon Thames and Wimbledon.
Frequently asked questions
Practical answers from the Cutting Hedge team.
How does rainwater harvesting work for a garden?
Rainwater runs off your roof into gutters and downpipes, passes through a filter to remove leaves and debris, and is stored in a tank. A pump then sends that water to your irrigation system, hose or tap points whenever it's needed. The system is sealed, low-maintenance and quietly collects water every time it rains.
What size rainwater tank do I need?
Tank size depends on your roof catchment area, local rainfall and how much water your garden uses. As a rough guide, every 1m² of roof collects roughly 1 litre per 1mm of rainfall — so a 100m² London roof can collect around 600 litres from a single 6mm shower, and tens of thousands of litres over a year. Our tank-sizing guide walks through the calculation, or we can size it for you on a site visit.
Can rainwater be used for garden irrigation?
Yes — rainwater is ideal for irrigation and is naturally soft, slightly acidic and free of chlorine, which most plants prefer. We connect the tank to your irrigation system so lawns, borders and pots are watered automatically from stored rainwater, reducing mains use and keeping the garden going through hosepipe restrictions.
Underground or above-ground tank?
Underground tanks save space, keep water cool (which limits algae) and are out of sight, but cost more and need excavation. Above-ground tanks are simpler and cheaper to fit and easier to maintain, but take up garden space and benefit from a shaded position. We advise based on your garden, access and budget.
Does a rainwater harvesting system need maintenance?
Light maintenance only — checking and cleaning the filter, inspecting the pump, keeping gutters clear and an occasional tank check. We offer servicing and can include it with irrigation servicing visits, along with winter checks.
Can you retrofit rainwater harvesting to an existing garden?
Yes. We can add a tank to an existing downpipe and connect it to a new or existing irrigation system. Underground tanks require excavation, so the easiest retrofits use above-ground tanks where space allows.
Is rainwater harvesting worth it in London?
London receives less annual rainfall than cities like Rome or Sydney, but it falls steadily year-round, making collection reliable. Paired with irrigation, a rainwater tank materially cuts mains water use and keeps your garden watered during the summer dry spells and hosepipe bans that are becoming more common across South East England.
Can rainwater harvesting feed indoor toilets and washing machines too?
Whole-house rainwater systems that feed WCs and washing machines are possible but require a larger tank, dual plumbing, a mains top-up and Building Regulations compliance. Our focus is garden rainwater harvesting integrated with irrigation — simpler, lower cost and exempt from most regulation.
