Appeal 3311897

The appeal is dismissed.

The main issue is whether the proposal would be acceptable with regards to flooding and flood risk.

The appeal site is located within [name] 2, with a medium probability of river flooding. In line with Policy I4 of the Vale of Aylesbury Local Plan (2021) (the Local Plan) and paragraph 168 of the National Planning Policy Framework (the Framework), the proposal is supported by a Flood Risk Assessment (FRA) undertaken by [name] dated May 2022.

The FRA confirms that the appeal site is also in an area with a high risk of surface water flooding, as well as an area with shallow groundwater and thus a high risk of groundwater flooding. The Local Flood Officer has confirmed flood depths on the appeal site during a surface water flood event at up to 600mm. Representations from those who live close by, refer to several flood events in the last 25 years, including an event that occurred in December 2020. The Lead Local Flood Authority confirm that during this most recent event a number of properties were flooded in the immediate vicinity of the proposed extension.

Paragraph 159 of the Framework seeks to avoid inappropriate development in areas at risk of flooding. Where development is necessary in such areas, it should be made safe for its lifetime without increasing flood risk elsewhere. The Planning Practice Guidance (PPG) advises, as a minimum, that [name] demonstrate compliance with these aims of the Framework. Building on this Policy I4 of the Local Plan requires a FRA to assess all sources and forms of flooding, adhere to the advice in the latest version of the SFRA, and to provide specific evidence and information to demonstrate that the impacts of and from all forms of flooding would be minimised.

To ensure that the proposed extension would be safe during a flood event, the FRA confirms that its finished floor level (FFL) would match that of the existing house. There would also be direct access to the front door of the appeal property that leads to a section of [site address] with no surface water flows. However, the FRA also recognises that flood water would be displaced by the structure of the proposed extension, which would potentially raise flood levels that would affect others. In order to compensate for the loss in flood storage the proposed extension would be constructed with a floodable underfloor void (hereon referred to as the sub floor void).

[name] understand that the height of the proposed sub floor void would likely contain the on-site flood depth of up to 600mm, the FRA does not demonstrate that the void would provide equivalent flood storage to the existing garden space. It may be that, with the sub floor void, the supporting structure for the proposed extension would be small compared to the extension as a whole. However, it would still result in an increase in impermeable area and, from the evidence before me and my observations on site, I am not satisfied that the sub floor void alone would ensure that there was no loss of flood storage.

Moreover, the [name] & Drainage Details1 at [name] of the FRA show that part of the proposed surface water drainage system would be located within the sub floor void, which would further reduce its flood storage capacity. While the system includes water butts, which could initially reduce the total volume of water reaching the ground, there are no details before me setting out how and when the water butts would be emptied or what the grey water would be used for. It is likely that this would require human intervention. Given this, although I am satisfied that the maintenance of the void, could be addressed by a maintenance and management plan secured by condition, I am not satisfied that the practicalities of emptying the water butts could be prescribed into that plan.

It would also appear that the FRA does not include a comprehensive assessment of the existing surface water flood risk. It is not clear from the description set out at paragraph 7.4 of the FRA whether the assessment has considered the impact of all the water flows shown on the Environment Agency Surface Water Flood [name]. In particular, those from further west along [site address] .

I recognise that the flood event of 23rd December 2020 did not result in the flooding of the appeal property. However, it did flood properties close to the appeal site, which would likely be impacted by any changes to the flood depths and flows that could result from the proposal. It should therefore be afforded proper consideration in the FRA. It has been put to me that during the 2020 flood event the flood depth peaked at a level around 600mm below the floor level of the appeal property. However, there is no detailed analysis supporting this and it is not clear where this data has been drawn from.

The FRA also fails to take into account climate change with respect of changing rainfall intensity and its impact on surface water flooding. Even if any increase in flood levels accounting for climate change would be no greater than the margin of error recognised in a detailed modelling exercise, it is important to understand where any additional flood water would flow as a result of the proposal and whether the sub floor void capacity would be able to 1 [name] 2179/01 accommodate it. The FRA should demonstrate how flood risk will be managed over the development’s lifetime taking climate change into account. This is particularly important in this case, given the highest flood risk is from surface water and groundwater flooding and the most recent flood event resulted from intense rainfall on saturated ground.

The FRA proposes the installation of a ‘bypass’ drainage system to deal with the roof drainage of the new structure. This is due to the shallow groundwater meaning conventional soakaways cannot be used. Roof water would be collected in conventional gutters and downpipes and then be directed to a distribution system below the suspended floor, above the sub floor void beneath the extension. While the system would seek to disperse the roof water evenly over the ground below the extension in a similar place to where it would currently land, I am not satisfied that this would replicate precisely the existing hydrological path of the rainwater and there is no substantive evidence to demonstrate this. There is also no evidence before me regarding the capacity of the system and whether it would function consistently in all rainfall conditions, as well as during a flood event. Given the lack of evidence, it is not possible to ascertain that the proposal would not result in increased flood risk elsewhere.

Although the PPG advises that preference should be given to solutions that allow surface water to be discharged into the ground through infiltration, it also recognises that there are other options available, which should be considered, and the most appropriate method of discharge selected. Nevertheless, no infiltration rates have been provided to demonstrate that infiltration is the most effective method of surface water disposal for the proposal, and the FRA does not consider any other drainage options.

It has been put to me that the detailed design of the surface water drainage system could also be addressed by condition. However, the layout and function of drainage systems need to be considered at the start of the design process as it could affect the fundamental design of the development. Also, given the high risk of surface water flooding and the demonstrable harm that has resulted from recent flood events, it is vital, specifically in this case, to ensure the adequacy of a sustainable drainage system prior to granting permission.

In conclusion, the PPG recognises that minor development can affect flood risk within or beyond a property, particularly in areas susceptible to flooding. While the scale of the proposal may be modest, the level of information within the FRA must be proportionate to the degree of risk now and in the future, which in this case is high. From the evidence before me, I am not satisfied that the submitted FRA demonstrates compliance with the requirements of Policy I4 of the Local Plan or the aims of the Framework in this regard.

Accordingly, for the reasons above, it has not been adequately demonstrated that the proposal would be acceptable with regards to flooding and flood risk. It would conflict with Policy I4 of the Local Plan, which seeks to minimise the impacts of and from all forms of flood risk. It would also be contrary to paragraph 159 of the Framework and thus would conflict with Policy I4 of the Buckingham Neighbourhood Development Plan (2015) (NDP), which seeks compliance with national policy and guidance on flooding and flood risk.

The proposal would conflict with the development plan, read as a whole. It has not been demonstrated that there are any material considerations of sufficient weight to indicate that a decision should be taken otherwise than in accordance with it. The appeal is therefore dismissed.