Greywater and rainwater reuse in a sustainability statement

Write rainwater harvesting into a planning statement for the right reason: the Water Efficiency Calculator for New Dwellings allows a fixed deduction of 5 litres per person per day where a rainwater or greywater reuse system is installed and meets the stated conditions. That deduction is what gets a scheme from a fittings-only result down to a tighter policy target. It is not a general sustainability flourish, and officers who read a lot of statements can tell the difference.

So the question for an architect is narrow. Does the scheme need those 5 litres, and is there a simpler way of finding them in the fittings specification first?

Where reuse helps a water efficiency target

Requirement G2 of the Building Regulations sets 125 litres per person per day for new dwellings. Planning policy can require the optional technical standard of 110 litres per person per day where the local plan has justified a clear local need, and the Planning Practice Guidance on optional technical standards explains how that condition works.

In London, policy SI 5 of the London Plan takes residential development to 105 litres per person per day excluding external water use, which is where reuse starts to earn its place rather than being a nice extra.

The arithmetic decides it. A well-chosen fittings specification, meaning a 4 or 4.5 litre dual-flush WC, a 6 litre per minute shower, aerated taps and a sub-10 litre dishwasher, will usually reach 110 without any reuse at all. Getting to 105 with fittings alone is possible but leaves nothing in reserve, and that is the point at which the 5 litre deduction becomes worth designing in.

Set out the calculation in the Sustainability Statement rather than asserting the result. Show the fittings schedule, the litres per person per day it produces, and then the deduction, so the officer can follow the line.

Rainwater harvesting sizing basics

BS 8515:2009+A1:2013 is the code of practice, and it offers a simplified sizing approach that is enough for a planning submission. You need the collection area, the local annual rainfall, a yield coefficient for the roof finish, a filter efficiency and the non-potable demand.

The simplified method takes 5 per cent of the lower of annual yield and annual non-potable demand as the storage volume. A worked illustrative example for a four-bedroom house with 70 m² of pitched tile roof, 700 mm of annual rainfall, a yield coefficient of 0.8 and filter efficiency of 0.9 gives an annual yield of roughly 35 m³. Where the non-potable demand for WC flushing and garden use is lower than that, the demand figure drives the tank, and a tank in the low thousands of litres follows. Treat those numbers as illustrative only and run the calculation on your own site data.

Two practical points the drawings need to show. Where the tank goes, because a below-ground tank interacts with foundations, services and any tree root protection area. And how the overflow discharges, because an overflowing rainwater tank is a surface water connection and belongs in the Drainage Strategy as well as the water calculation.

Attenuation and reuse are different jobs. A tank sized for reuse sits full most of the year and offers little attenuation, so do not claim a surface water benefit you have not designed for. Combined attenuation and reuse tanks exist, with a controlled discharge below the reuse volume, and they need to be drawn that way to be credible.

Greywater systems and their maintenance burden

Greywater reuse collects bath, shower and basin water, treats it and reuses it for WC flushing. BS 8525-1:2010 covers the code of practice and BS 8525-2 the water quality risk assessment.

The attraction is that supply follows occupancy rather than weather, so yield is predictable in a flat with no roof area of its own. The difficulty is that greywater is biologically active. Systems need filtration, disinfection or a short retention time, and they need servicing. Direct reuse systems with minimal storage are simpler than treatment systems, and in single dwellings they fail less often.

Be honest about this in the statement. A greywater system in an owner-occupied house with no service contract is a maintenance liability, and officers in authorities that have chased non-functioning systems know it. Where the scheme is a block with a managing agent and a planned maintenance schedule, the position is quite different.

Regulatory requirements for non-potable supply

Reuse pipework is not a planning matter, but getting it wrong undermines the planning commitment, so the statement should show awareness of it.

• Non-potable supplies must be physically separate from the wholesome supply, with backflow protection appropriate to the fluid category under the Water Supply (Water Fittings) Regulations 1999.
• Pipework, valves and outlets need marking as non-potable, and outlets accessible to occupants need labelling.
• Any top-up from the mains must be by an air gap arrangement rather than a direct connection.
• The water undertaker should be notified of the installation.
• Stored water carries a legionella risk that needs assessing where the system serves a shared building.

Evidence a council will accept

Validation lists differ, so check the local one. What a competent submission contains:

1. The water efficiency calculation in the published calculator format, with the fittings schedule it is based on.
2. A statement of the target being met, and the policy requiring it, cited by policy number from the adopted plan.
3. A sketch layout or schematic showing the tank, the plant space, the pipework route and the overflow.
4. Confirmation of the standard the system will be designed to, meaning BS 8515 or BS 8525.
5. The maintenance arrangement, named rather than implied.

Where the scheme is non-residential and the authority asks for BREEAM credits in the water category, the BREEAM Pre-Assessment is the place for that, and the credits are earned on metering and leak detection as much as on reuse.

Long term management arrangements

A condition requiring a reuse system to be installed is easy to discharge and hard to enforce afterwards. Authorities have learned this, and some now condition a maintenance plan and a handover pack rather than the hardware alone.

Offer that rather than resisting it. A short management schedule setting out service intervals, the responsible party, and what happens on sale of the dwelling, attached to the statement, answers the officer's real worry. For flats, the arrangement goes into the service charge and the managing agent's planned maintenance programme, and that can be said in a sentence.

Where the site falls in a water neutrality area, the position is stricter again. Natural England has advised on water neutrality in Sussex North, and schemes there need the offsetting arithmetic shown, which is the territory of a Water Supply Assessment rather than a general sustainability narrative.

When simpler fittings achieve the same result

Often they do, and saying so is better practice than specifying a system that will be value-engineered out at tender.

Run the calculator twice. Once with your preferred fittings and no reuse, once with the reuse deduction. If the fittings-only result clears the policy target with a litre or two to spare, specify the fittings and leave reuse out. If it does not, you have a reason for the system that will survive a cost review, because removing it breaks a planning commitment.

External water use is the other lever. Where the policy target excludes external use, as London's does, a water butt and a planting scheme that does not need irrigation do more for the scheme than an undersized harvesting tank. The biosolar and green roof interactions matter here too, and our note on biosolar roofs covering energy, drainage and biodiversity covers how those roof-level claims hold together.

This is general guidance rather than legal advice, and water efficiency policy varies by authority, so check the position with your local planning authority or a qualified professional before relying on it.

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