Drainage Rates and Scheme Viability: Financial Impacts of New Planning Policy

by Jamie Lynch

In the wake of the unprecedented flooding across England and Wales in late 2015 and early 2016, the Government and the Environment Agency (EA) have released new guidance for developers on taking climate change into account in the assessment of flood risk. With previous standards being seen as insufficient to deal with the regular frequency of ‘extreme’ events, the objective is to increase the amount of surface water storage capacity in new developments depending on their location.

The Old Guidance and the New

Under the previous guidance, developers were required to design surface water attenuation based on local flood risk conditions plus an industry standard 20% allowance for climate change. The new guidance promotes allowances for climate change that depend on which river basin catchment the site is in, where the site is within that catchment and the likely lifespan of the development. This is designed to allow a more bespoke approach to be taken to each development that more closely reflects the type of development and the current and future flood risk at each site.

This change will lead to variable outcomes for developers depending on type and location of development. For example, temporary or short term developments in lower risk areas will see the requirement for attenuation reduced from that prescribed under the current standard. However, for long term development, such as housing, in some parts of the country, this may require developers to make provision for up to a 105% increase in drainage to allow for climate change events.

The guidance came into force on 19th February 2016. Any projects which have not been submitted or validated by the Local Planning Authority are subject to the new measures set out by the EA although preliminary contact with LPAs has indicated that the standards may be negotiable in the short term as the industry gets to grips with the changes.

Industry Impacts

Firstly, the effect of the new guidance is that it renders much of the existing publicly available data held by the Environment Agency out of date. This means that sites that were previously outside of the flood zone may now find themselves constrained by flooding – although the remodelling of the EA’s data will in itself be a vast undertaking. This gap in the evidence base is likely to result in much greater need for hydraulic modelling at the planning stage for sites that are near to rivers that could cause flooding in the future.

Developers should also be aware that there is the real potential that the net developable area of sites may reduce if significant attenuation features are required to allow for these new standards. This may have the effect of making previously viable schemes unviable and developers should exercise caution when purchasing or optioning land in areas with a risk of flooding.

How to Tackle the New Guidance

Going forward, flood risk and drainage will be a significant determining factor for site acquisition, planning and development. Obtaining advice from a drainage engineer at a very early stage is strongly recommended to avoid costly oversights.

For further information, please contact one of HOW Planning’s Environmental Team in Manchester.


Published: 31/03/2016

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