Water efficiency standards for new homes in London

The water efficiency standard that applies to new homes in London is 105 litres per person per day of mains water, excluding an allowance of up to five litres for external use. That figure sits in Policy SI 5 of the London Plan 2021, it applies borough wide, and it is tighter than anything the Building Regulations impose on their own. Get it wrong at design stage and you are either changing every tap and shower in the scheme or arguing with a case officer about a number you cannot reach.

The regulatory baseline is softer. Requirement G2 of the Building Regulations sets a water consumption limit of 125 litres per person per day for new dwellings, with an optional requirement of 110 litres that a local authority can apply through a planning condition where there is a clear local need. London went further through its plan rather than through the optional requirement, which is why a London scheme has to hit 105 while an identical scheme in a non water stressed district may only have to hit 125.

Where the 105 litre figure bites hardest

The standard is a per person per day calculation, not a per dwelling one, so it does not get easier on larger flats. What drives it is fittings. A conventional specification with an eight litre per minute shower, a six litre dual flush WC and a five litre per minute basin tap will usually land somewhere in the high 90s or low 100s once the external allowance is counted, which leaves very little headroom.

The pressure points in practice are showers and baths. A single generous bath volume can move the calculated figure by several litres per person per day on a small flat, and a rain head shower at twelve litres per minute will take a scheme over on its own. Where a client wants both, the arithmetic has to be found elsewhere, usually in tap flow rates and WC cistern volumes.

Water neutrality is a different test, and it is not a London one

Architects working across the south east conflate the two, and the distinction matters. Water neutrality means that total water use across a defined area after development is no greater than before it, which usually forces offsetting through retrofit of existing stock or through a strategic scheme. It became a live planning constraint in the Sussex North water supply zone following Natural England advice, affecting parts of Horsham, Crawley and Chichester districts rather than anywhere in Greater London.

London is a seriously water stressed area, and water resource pressure is the reason for the 105 litre standard. But there is no London wide water neutrality requirement, and no London borough can refuse a scheme for failing a neutrality test that does not apply to it. If a consultant tells you otherwise, ask which published document imposes it.

The calculator, the fittings approach, and which one to use

Approved Document G gives two ways to show compliance for new dwellings. The water efficiency calculator takes the flow rate or volume of every fitting, applies fixed use factors and produces a litres per person per day figure. The fittings approach instead sets a maximum value for each fitting type, and if every fitting is at or below its value, compliance follows without a calculation.

The fittings approach is calibrated to the 125 litre baseline. It cannot get you to 105. So for any London residential scheme you are running the calculator, and the output per dwelling type is what the planning officer will want to see.

Three things make the calculator easier to live with:

1. Run it at concept stage on the worst case unit type, not at technical design on every unit. If the smallest one bedroom flat passes with headroom, the rest will follow.
2. Fix the shower flow rate first and treat it as a design constraint rather than a specification choice. It is the single largest variable.
3. Record the assumed bath capacity to the rim, measured as the calculator requires, because a procurement substitution late in the job is the commonest reason a scheme fails on handover.

Non-domestic buildings and the BREEAM route

For commercial and other non-residential development the London Plan looks to the water category within BREEAM rather than to a litres per person figure, because consumption in an office or a school depends on occupancy patterns rather than on fittings alone. Check the current wording of Policy SI 5 and your borough's local plan for the exact expectation, because boroughs have added their own thresholds and some tie the requirement to floorspace.

Where a scheme is going through a BREEAM Pre-Assessment at planning stage, the water credits are among the least expensive to secure: flow restricted taps, leak detection, a sub-metered supply and sanitary shut off. The assessment itself has to come from a licensed assessor, and councils expect a suitably qualified author for it.

Rainwater and greywater when the numbers do not add up

Reuse systems appear in the calculator as a deduction, and both routes have a British Standard behind them: BS 8515 for rainwater harvesting and BS 8525 for greywater reuse. On paper they close a gap of several litres per person per day. In practice I would reach for them last on a residential scheme.

The reasons are maintenance and liability rather than engineering. A communal rainwater system needs a management company, a service contract, a filter regime and a mains top up that cannot allow backflow, and on a block of fifteen flats the annual cost per unit is hard to justify against a tighter shower head. Greywater is harder again, because the treatment has to be reliable enough to serve WCs without odour complaints. Where reuse does earn its place is on larger schemes with irrigated landscape and a non potable demand that exists anyway, which is also where a Green Infrastructure Plan and the drainage design can share the same storage.

And if you do claim a reuse deduction in the calculator, expect a condition requiring the system to be installed and maintained, which binds the management company for the life of the building.

Evidencing the standard and the condition that secures it

At application stage the water standard usually rides inside the Sustainability Statement rather than in a document of its own, sitting alongside the Energy Statement that deals with the carbon hierarchy. What a borough validation list typically wants is narrow:

• The calculated litres per person per day figure for each dwelling type, with the calculator output attached.
• A schedule of fittings giving flow rates in litres per minute, WC flush volumes and bath capacity to overflow.
• A statement of whether any reuse system is relied on, and if so which standard it follows.
• For non-residential floorspace, the targeted water credits and the assessor's pre-assessment score.

The condition that follows is normally in two parts. One requires the approved fittings schedule to be implemented, the other requires written evidence before first occupation, which in practice means the building control water calculation or a post completion statement from the designer. Boroughs vary in wording, so read the draft conditions on a comparable consent in the same borough before you commit to a specification.

The sequencing that goes wrong most often is procurement. Planning fixes a fittings schedule at RIBA Stage 3, the contractor values engineers the sanitaryware at Stage 5, and the as built calculation lands at 112 litres. Raising the gap then means a section 96A or section 73 application to change the approved schedule, or a discharge of conditions submission that the borough refuses. Writing the flow rates into the employer's requirements, with no substitution without a recalculation, is a two line clause that avoids all of it. The same discipline applies to the way scheme capacity is set early, which is covered in the piece on optimising site capacity through a design led approach.

Figures in the London Plan and in Approved Document G change on their own cycles, so verify both against the current published versions before you rely on a number in a submission.

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