A developer’s feasibility checklist for rainwater harvesting (aligned to the National SuDS Standards)

Rainwater harvesting is easiest to do well when it is considered early, before roof layouts, plant space, drainage routes, and SuDS features are fixed. Developers who leave it late often end up with a system that is hard to maintain, hard to justify, and easy to value engineer out. This article is a feasibility checklist written for developers and project teams. It is designed to help you decide, quickly but properly, whether rainwater harvesting should be a “yes”, a “yes with conditions”, or a “not on this site”.

The goal is not to force a system onto every project. The goal is to make sure you ask the right questions at the right time, and that your answers support a clear SuDS narrative.

Step 1: Confirm the role rainwater harvesting will play in your SuDS strategy

Start by being clear about what you want the system to do. On most developments, rainwater harvesting is not there to replace other SuDS measures. It is there to help with source control by capturing roof runoff for reuse, and to support water efficiency by reducing mains demand for non-potable uses.

At feasibility stage, agree a simple statement that can carry through design and approvals. For example, you might position rainwater harvesting as a source control measure that reduces the volume of roof runoff leaving the building during many rainfall events, while downstream SuDS features manage the remaining runoff, exceedance, and treatment across the site. If you cannot describe it in plain language, the design is probably not yet clear enough.

Step 2: Check whether the roof catchment is suitable

Feasibility starts with the roof because that is the most practical source for harvesting on most sites. You want to know the roof area, the roof material, and the likely debris load. A simple question helps here. Is this a roof you would be happy to collect water from, store, and reuse for WC flushing or irrigation?

If the roof has heavy plant, unusual finishes, or a high debris load from nearby trees, it does not automatically rule the system out, but it does change the filtration and maintenance approach. At feasibility stage, you are not designing the filter. You are identifying whether the roof is likely to be straightforward, or whether it needs extra attention.

You also want to look at how the roof drains. If the downpipes are easy to route to a plant room or tank location, integration is simpler. If the building form creates long runs, multiple discharge points, or awkward levels, you may still proceed, but you should expect higher coordination effort.

Step 3: Identify realistic end uses and demand profile

A rainwater harvesting system performs best when there is consistent non-potable demand. Without demand, the tank stays full and the system overflows frequently. That reduces the value of the system and can create maintenance issues.

At feasibility stage, you do not need perfect numbers, but you do need a realistic demand story. For apartment blocks, WC flushing is often the most reliable demand because it is regular and predictable. For commercial buildings, demand depends on occupancy and operating hours, so the story needs to be tied to the actual use class. For housing estates, demand is often more seasonal if the main use is irrigation, and that can mean the tank is full through wetter months.

The key question is whether the chosen end uses will create drawdown. Drawdown is what creates storage capacity between rainfall events. If you do not have drawdown, you do not have meaningful source control benefit.

Step 4: Decide early where the tank and controls will live

Many feasibility discussions fail because the team assumes space will be found later. In practice, space is the constraint.

For above-ground plant, you need a sensible location for controls, pumps, and any mains top-up arrangement, with safe access for maintenance. For below-ground storage, you need a tank location that does not clash with foundations, services, or future landscaping, and you need access points that will remain accessible after completion.

At this stage, you are not fixing the final layout. You are confirming that there is a credible place for the system to exist without creating conflicts that will surface late in the programme.

Step 5: Map the overflow pathway into the wider SuDS management train

Overflow is not a failure. It is normal operation during larger rainfall events, or when demand is low. Feasibility should confirm that overflow can be routed safely into the next SuDS feature in the sequence.

This is where many schemes become weak. A tank is added, but overflow is left as a vague note. That creates risk because overflow will happen, and if it is not designed it can cause nuisance flooding or undermine the approvals narrative.

At feasibility stage, you want to be able to answer three questions. Where does controlled overflow go during normal operation? Where does exceedance go if something blocks or the system is overwhelmed? How will these routes be inspected and maintained after handover?

Step 6: Consider water quality and treatment in a simple, honest way

Feasibility is not the time for detailed water quality modelling, but it is the time to avoid unrealistic claims.

Roof runoff is generally cleaner than runoff from trafficked areas, but it still carries debris and fine sediment. That is why filtration and maintenance matter. If you are proposing WC flushing, you need to be confident that the system design and maintenance plan will support that use without creating odour, staining, or reliability issues. If you are proposing irrigation, you need to consider clogging risk and the practicality of seasonal maintenance.

You should also be clear that rainwater harvesting does not remove the need for treatment of runoff from roads and parking areas. Those surfaces typically still need their own SuDS treatment measures.

Step 7: Maintenance feasibility is part of technical feasibility

A system that cannot be maintained is not feasible, even if it fits on a drawing.

At feasibility stage, confirm who will own and operate the system after handover. If it will be a management company or FM provider, you need an O&M plan that is clear and budgeted. If it will be individual homeowners, the system needs to be simpler and the maintenance needs to be realistic for non-technical users.

You also want to confirm access. Can someone safely reach filters and controls? Can inspection points be opened without specialist equipment? Is there a practical way to remove and clean debris? If the answer is unclear, the system may still be possible, but it should be flagged as a design risk that needs resolving early.

Step 8: Construction and commissioning risks should be flagged now

Developers often underestimate how much construction-phase debris can affect performance. If roof drainage is connected too early, filters can clog and tanks can fill with silt. If the system is not cleaned and commissioned properly, the operator inherits a problem from day one.

Feasibility should therefore include a simple construction plan assumption. You should assume temporary protection of inlets, a defined point in the programme when the system becomes live, and a clean-out and inspection before handover. This is not over-planning. It is the difference between a system that performs and a system that becomes a complaint.

Step 9: Build a simple approvals narrative that is easy to defend

A good feasibility outcome is not just a technical yes. It is a narrative that ties together SuDS, water efficiency, and long-term operation.

You want to be able to explain, in plain language, that roof runoff is captured and reused to reduce immediate runoff and reduce mains demand for non-potable uses. You then explain that controlled overflow is routed into the next SuDS feature, and that the wider drainage strategy manages the remaining runoff, treatment, and exceedance across the site.

If you can tell that story clearly, you are far more likely to have a smooth approvals process, and you are far less likely to see the system removed during value engineering.

Step 10: Make the feasibility decision and record it properly

Finally, decide whether rainwater harvesting is a yes, a yes with conditions, or a no for this site. A yes with conditions is common and sensible. It might mean the system is feasible if plant space is confirmed, if a specific end use is agreed, or if overflow can be integrated into a particular SuDS feature.

Whatever the decision, record it in a short feasibility note that sits alongside the drainage strategy. Include what the system is intended to achieve, what assumptions were made about demand and space, and what needs to be confirmed at the next design stage. This small step prevents the common problem where rainwater harvesting is “agreed” in principle, then quietly disappears when coordination gets difficult.

Closing thought

Rainwater harvesting is not a box-ticking item. For developers, it is a design and delivery decision that touches architecture, MEP, drainage, landscape, and long-term operation. A proper feasibility check brings those disciplines together early, so the system can be integrated into the SuDS management train and defended through approvals, construction, and handover.