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Policy SI5 Water infrastructure


  1. In order to minimise the use of mains water, water supplies and resources should be protected and conserved in a sustainable manner.
  2. Development Plans should promote improvements to water supply infrastructure to ensure security of supply. This should be done in a timely, efficient and sustainable manner taking energy consumption into account.
  3. Development proposals should:
    1. minimise the use of mains water in line with the Optional Requirement of the Building Regulations (residential development), achieving mains water consumption of 105 litres or less per head per day (excluding allowance of up to five litres for external water consumption)
    2. achieve at least the BREEAM excellent standard (commercial development)
    3. be encouraged to incorporate measures such as smart metering, water saving and recycling measures, including retrofitting, to help to achieve lower water consumption rates and to maximise future-proofing.
  4. In terms of water quality Development Plans should:
    1. promote the protection and improvement of the water environment in line with the Thames River Basin Management Plan, and should take account of Catchment Plans
    2. support strategic wastewater treatment infrastructure investment to accommodate London’s growth and climate change impacts. Such infrastructure should be constructed in a timely and sustainable manner taking account of new, smart technologies, intensification opportunities on existing sites, and energy implications. Boroughs should work with Thames Water in relation to local wastewater infrastructure requirements.
  5. Development proposals should:
    1. seek to improve the water environment and ensure that adequate wastewater infrastructure capacity is provided
    2. be designed to ensure that misconnections between foul and surface water networks are eliminated and not easily created through future building alterations.

Londoners consume on average 156 litres of water per person per day – around 17 litres above the national average. All water companies that serve London are located in areas classified as seriously water-stressed. London is at risk of drought after two dry winters. During 2006 and 2012 water use restrictions affecting London were imposed. These restrictions were limited to sprinkler, hosepipe and non-essential user bans. A severe drought – with rota cuts, standpipes, reduced mains pressure or adding non-potable water to the mains supply – would have major implications for Londoners’ health and wellbeing, the environment and London’s economy. The Mayor will work with the water industry to prevent this level of water restriction being required for London in future.

An important aspect of avoiding the most severe water restrictions is to ensure that leakage is reduced and water used as efficiently as possible. The Optional Requirement set out in part G of the Building Regulations should be applied across London[125]. A fittings-based approach should be used to determine the water consumption of a development. This approach is transparent and compatible with developers’ procurement and the emerging Water Label[126], which Government and the water companies serving London are supporting.

[125] Planning Practice Guidance: Paragraph 014 Reference ID: 56-014-20150327: Where there is a clear local need, boroughs can set out Local Plan policies requiring new dwellings to meet the tighter Building Regulations optional requirement.


Even with increased water efficiency and reduced leakage, water companies are forecasting an increasing demand for water. Without additional sources of supply, the increased demand will increase the risk of requiring water restrictions during drought periods. Security of supply should be ensured. Demand forecasts need to continue to be monitored and based on the consistent use of demographic data across spatial and infrastructure planning regimes.

Variations of the following four strategic water supply options to serve London are under consideration through Thames Water’s Water Resource Management Plan process and one or a combination of some of these are expected to be proposed to serve parts of the Wider South East including London:

  • treatment / re-use of effluent from sewage treatment works – likely within London
  • desalination – potentially within London
  • transfer of river water from the River Severn to the River Thames catchment
  • a new reservoir – likely to be near the Upper Thames in Oxfordshire.

The Mayor is reviewing the available information on each of the supply options alongside evidence of their impacts on Londoners and Mayoral priorities. In preparing its Water Resource Management Plans, Thames Water is exploring coordinated supply options with the other water companies serving London and the South East of England through the Water Resource South East expert group. Water Resource East is undertaking similar work in the East of England area. All this will involve partnership working with key stakeholders within London and beyond its boundaries.

Infrastructure investment is constrained by the short-term nature of water companies’ investment plans. Similar to the approach to electricity supply (see also paragraph 9.3.8), in order to facilitate the delivery of development it is important that investment in water supply infrastructure is provided ahead of need. To minimise wastage, water supply infrastructure improvements should give consideration to the replacement of ageing trunk mains.

In the context of the significant investment needed, measures to protect and support vulnerable customers in particular from rising water bills are important.

In relation to wastewater, Water Framework Directive requirements should be maintained through the Thames River Basin Management Plan and the Catchment Plans prepared by the Catchment Partnerships, of which there are 12 in London. These Partnerships share lessons, experiences and best practice, and help achieve a coordinated approach to delivering the Thames River Basin Management Plan.

The Urban Wastewater Treatment Directive drives improvements in wastewater treatment infrastructure. Figure 9.4 provides a spatial illustration of the wastewater drainage capacity across London. Additional land may be required for upgrades or improvements at some wastewater treatment plants during the Plan period. Different wastewater treatment options may vary significantly in terms of their energy requirements, and there are significant opportunities for energy generation from wastewater treatment (sewage sludge).

The Thames Tideway Tunnel is under construction and will help to improve the water quality of the River Thames by significantly reducing the frequency of untreated sewage being discharged into the Thames (known as combined sewer overflows). Thames Water is also planning a major sewer tunnel in the Counters Creek catchment of west London. Sustainable drainage measures are of particular importance in areas with sewer capacity limitations and their widespread implementation over the coming decades will help the resilience of London and avoid the need for further major sewer tunnel projects.

London’s tributary rivers suffer significant pollution from misconnected sewers. This allows untreated sewage into what are often small streams, many of which flow through London’s parks and open spaces. Conversely, if surface water is misconnected to the foul system, sewer capacity issues are created within sewers and at sewage treatment works. Development proposals should therefore be designed to ensure that the potential for misconnections is eliminated.

Integrated Water Management Strategies should be considered for major development locations such as Opportunity Areas, where particular flood risk and water-related constraints such as limited sewer capacity require an integrated approach to the provision of infrastructure and management of risk.

A water advisory group has been established to advise the Mayor on strategic water and flood risk management issues.

Note for Figure 9.4: Thames Water has developed a model of its drains and sewers in London to assess waste water flows. The model compares the theoretical capacity of the drain or sewer pipe against how much waste water flow the pipe is currently receiving during a one in two-year rainfall event. The model’s outputs can be visualised as a ‘heat map’ which highlights at a strategic scale where there is a higher (green) or lower (red) ability to receive additional flows. ‘Green’ areas do not mean that no additional drainage infrastructure is required. The modelling does not consider how waste water is routed through the network, so it should be noted that some ‘green’ areas will flow into ‘red’ areas and hence increasing flows upstream will exacerbate performance in the downstream catchments.