A photograph of a SuDS (sustainabel drainage system) pond in Cambridge

May 2, 2019, by Blue-Green team

Government’s latest review of sustainable drainage: a box-ticking exercise reinforcing a suboptimal state of affairs?

The Ministry of Housing, Communities and Local Government’s (MHCLG) review of current planning policy for sustainable drainage (SuDS) was published in August 2018. Could this exercise have more to do with validating the Government’s current policy approach, rather than an accurate review of the application and effectiveness of planning policy for sustainable drainage systems? Tudor Vilcan (The Open University) explores in this blog.


In England, sustainable drainage systems (SuDS) are still implemented through the planning system, for which policy was strengthened in 2015 to make SuDS a requirement in all new major developments and encourage greater uptake. The MHCLG review provided the opportunity to gauge how the new policy is being implemented (p4). As the original intentions of Schedule 3 of the Flood and Water Management Act (FWMA) 2010 were to make the requirement for SuDS mandatory on new developments, complying with new National Standards on SuDS (which would have set out how drainage systems should be designed, constructed and maintained), then a comprehensive and critical assessment of the performance of current policy is vitally important in terms of policy design.

The National Planning Policy Framework (NPPF) (DCLG, 2012) expects local planning authorities, when determining planning applications, to ensure that sustainable drainage is prioritised in areas at risk of flooding, to conserve and enhance biodiversity, and, adopt strategies to mitigate and adapt to climate change. The review examined the extent to which national and local planning policy has been successful in encouraging the uptake of SuDS in new developments, examining how national planning policies for SuDS are reflected in local plans and the uptake of SuDS in major and minor new housing developments and commercial/mixed-use developments (MHCLG, 2018, p5).

The report suggests that SuDS are implemented at rates of 80 to 90% in England. It also states that more than 80% of the adopted local plans contained policies reflecting the requirements of the 2012 National Planning Policy Framework. As the report is very brief in describing its methodology, we sent a Freedom of Information (FOI) request to find out more. We find the report’s findings to be problematic in three key areas.

1. The nature of SuDS?

The FOI states that ‘a specific definition for SuDS was not employed in order to avoid ruling out any novel applications’. The report provides an indication as to what was actually counted as a SuDS feature, in that it makes an inventory of the range of SuDS technologies and features found in the planning applications: ‘ponds and attenuation basins, green roofs, permeable paving, tanks, swales and soakaways and frequently involved combinations of one or more of these components’ (MHCLG, 2018, p. 8, emphasis added).

Although DEFRA’s non-statutory standards (DEFRA, 2015) classify tanks as SuDS, in The SuDS Manual (CIRIA, 2015), attenuation storage tanks (which also include oversized pipes, geocellular storage and vortex flow control systems) are described as a ‘below-ground void space for the temporary storage of surface water before infiltration, controlled released and use’ (Ballard-Woods et. al, 2015, p. 437). They only have a flow control function (relating to water quantity), but any drainage system located below ground misses on all the other benefits of SuDS (such as water quality, amenity and biodiversity). Of further concern, only ‘generally’ were ‘‘traditional’ drains and sewers, gullies and catchment pits identified by LPA (Local Planning Authorities) and LLFA (Lead Local Flood Authorities) officials as falling outside the scope of acceptable SuDS’ (MHCLG, 2018, p. 10). In addition, ‘heavily engineered components involving such elements as excessive amounts of concrete, pumping systems, underground storage tanks and connections to main drains, or elements that were difficult to access or maintain were also viewed unfavourably’ (ibid) – hence we are concerned that in the absence of a specific definition, which components were counted as SuDS in the 80%.

The Big SuDS Survey

In The Big SuDS Survey, around 30% of all SuDS contained such systems (attenuation storage tanks and proprietary treatment systems that included vortex flow devices). The qualitative answers in the Big SuDS Survey suggested that ‘we would class oversized pipes, storage tanks and vortex flow control devices as surface water management tools rather than SuDS’ (Melville-Shreeve et al, 2018, p. 15). The Landscape Institute report on achieving sustainable drainage has also found that councils feel constrained by Defra’s Non-Statutory technical standards: ‘they do not see tanks and large pipes as sustainable but find them difficult to challenge as they cannot refuse them if they deal with quantity’ (Landscape Institute, 2019, p. 11). In brownfield development, where the natural hydrology of the site is compromised, tanks might get approved as a way of addressing flow control. But there is a risk that they will tick the SuDS box, when in reality, green SuDS can be implemented as well.

Multiple Benefits

The MHCLG report itself states that SuDS can also provide additional benefits such as removing pollutants from urban run-off and combining water management with green space that offers scope for recreation and wildlife’ (MHCLG, 2018, p 4). Hence, if we are referring to SuDS according to this definition to include multiple benefits, then SuDS would need to be counted differently and that is likely to impact the reported uptake rates. The question remaining unanswered here relates to the nature of SuDS and the emphasis we put through planning on the meaning of sustainability. Or as the Landscape Institute report asks; ‘is this sustainable drainage, or just drainage?’ (Landscape Institute, 2019, p. 4).

2. Methodology: representativeness and robustness

The MHCLG report uses what is called a nested design, where the researchers first selected a number of LPAs, and them sampled a series of planning applications from the selected LPAs.

12 LPAs were selected out of the total of 338 in England, ‘selected by listing all 338 LPAs by land value estimates, dividing the LPAs into twelve groups, then using the average land value for all LPAs within each group to identify the LPA with land value closest to the average in each group’ (MHCLG, 2018, p. 5). For the sampling strategy used for selecting the planning applications where SuDS were counted (in the 12 LPAs selected), ‘evidence was collected through analysis of a range of approved planning applications (minor residential, major residential and commercial/mixed-use developments) spanning the period 2012-16’.  From the FOI request, we also understood that the strategy selects one application for each type of development for four distinct periods of time (post 2012, in flood risk areas and non-flood risk areas and post 2015 (June to December 2015 and post 2016)). This means that there were 13 (for one category it requests 2 applications instead of 1) applications selected for each LPA, with a total of 156 applications.

From a statistical point of view, the design is therefore not probabilistic, meaning that none of the choices (of either LPAs or planning applications) have been random. The 156 applications also represents a very small sample size in comparison to the overall number of planning applications. The combination of a non-probabilistic design and small sample size means we do not have a mechanism in place to estimate uncertainty (using statistical probability theorems), that is, to provide an indication of the reliability of a measurement. This ultimately means that the report’s finding that 80% of planning applications have SuDS could have a variability of +5 % or -50%. Notably, The Big SuDS Survey put the uptake of SuDS at around 40% (Grant et al., 2017) and the more recent report from the Landscape Institute found that only ‘3% of authorities reported receiving adequate information to assess a planning application’ (Landscape Institute, 2019, p. 5). For these reasons we remain somewhat sceptical regarding the uptake rates of SuDS reported by MHCLG (2018).

3. Critical interpretation of data (or lack therefore of)

A major issue permeating the report relates to the manner in which the data is interpreted. We pull out just a few examples. The report states that when SuDS did not feature in an application, ‘drainage of surface water to a water body was often described in such a way that could be interpreted as sustainable’ (p. 8). This is clearly problematic, because simply discharging to a body of water does not necessarily bring with it the same benefits as SuDS, especially if this is through gullies or pipes and fails to provide flow attenuation and infiltration, not to mention the wider array of multiple benefits.

We feel the conclusions on post-construction checks and proper functioning of SuDS are also based on imperfect premises. Interviews with members of the LPAs highlight that they lack regimes and resources for ensuring that SuDS have been constructed as designed and agreed on. The majority of LPAs have to proceed via a reactive rather than proactive approach towards the issue, essentially reliant on receiving complaints about SuDS malfunctioning. Rather than reflecting and responding to the implications of such a major issue, the report states that they were not provided with an example of a failed scheme by the sampled LPAs, and in the two cases of unsatisfactory performance, the issue was resolved quickly (p11). This appears at odds with the findings of the interviews presented above; if there are few to zero post-construction checks performed, then how are we to know whether other schemes are faulty or not to date? These are only likely to manifest during a flood event and many SuDS components might take some time to exhibit symptoms of malfunctioning over their decades’ long lifespan. The performance of SuDS could remain a loaded gun issue.


The question of the effectiveness of the ‘strengthened’ planning system to implement SuDS in England is a vital one considering the importance of SuDS in the management of surface water flooding and adaptation to the effects of environmental change. MHCLG’s report represented an opportunity to provide a comprehensive and critical assessment of the performance of current policy. We do not conclude that the MHCLG review has ‘shown that current arrangements for SuDS in planning has been successful in encouraging the take-up of sustainable drainage systems in a cross-section of new developments with almost 90% of all approved planning applications sampled featuring SuDS’ (p12). However, firstly and fundamentally, we are concerned as to what is considered to constitute a SuDS feature. Second, the methodology raises questions in relation to the representativeness and robustness of the report’s findings. Although MHCLG does acknowledge that there is potential for industry bodies to address skills and knowledge gaps, and that more emphasis by applicants is required on SuDS adoption and maintenance arrangements (p12) there is an overarching tendency in the report towards a lack of genuine engagement and critical reflection in the interpretation of data. The recommendations of this report would appear to have influenced the latest version of the NPPF (2018), for which we are concerned will not offer the substantial improvement and support to stakeholders required when it comes to the future uptake of sustainable drainage and its maintenance in England.


Please contact Tudor Vilcan (tudorel.vilcan@open.ac.uk) for more information or to share your thoughts.



DCLG (2012) National Planning Policy Framework. Available at: http://webarchive.nationalarchives.gov.uk/20180608095821tf_/https://www.gov.uk/government/publications/national-planning-policy-framework–2. Accessed (15.01.18).

DCLG (2018) National Planning Policy Framework. Available at https://www.gov.uk/government/publications/national-planning-policy-framework–2. Accessed (15.01.18).

DEFRA (2015) Non-statutory Technical Standards for Sustainable Drainage Systems. March 2015. Report PB14308. Dept. Environment & Rural Affairs (DEFRA), London. UK.

Grant, L., Chisholm, A., and Benwell, R., 2017. A place for SuDS? Assessing the effectiveness of delivering multifunctional sustainable drainage.  The Chartered Institution of Water and Environmental Management. London, UK.

Landscape Institute, 2019. Achieving sustainable drainage. A review of delivery by Lead Local Flood Authorities, (online) available at https://www.landscapeinstitute.org/wp-content/uploads/2019/01/11689_LI_SuDS-Report_v4a-Web.pdf.

Melville-Shreeve, P., Cotterill, S., Grant, L., Arahuetes, A., Stovin, V., Farmani R., and Butler, D. (2018) State of SuDS delivery in the United Kingdom. Water and Environment Journal, 32, 9–16.

Posted in Urban Flood Resilience research