Appeal 3367651

The appeal is dismissed.

The application form description of development cites “the conversion of garage and single storey side and rear extension.” The decision notice description provides more clarity including reference to the raised rear patio, so I have used that as the description in the banner above.

The main issue is the effect of the proposed development on flood risk.

The appeal proposal is for single storey extensions and raised rear patio to the semi-detached dwelling at [site address] . There are no interested party objections, and the Council has raised no issue with the proposal’s visual appearance. Given its modest size and scale overall in comparison to its host, and the lack of any terracing effect, I see no reason to conclude differently.

Residential buildings are categorised as a ‘more vulnerable’ use. The site lies within flood zones 2 and 3, with a 1 in 100 +17% climate change fluvial flood risk, and a medium to low risk of surface water flooding. The proposal’s total proposed impermeable area would be 32.6sqm, such that the Council’s Sustainable Drainage Proforma identifies a target attenuation of 4.17 cubic metres, and a minimum required attenuation of 1.47 cubic metres.

The National Planning Policy Framework ('the Framework') (2024) paragraph 170 requires development which is necessary in areas at highest risk of flooding to be made safe for its lifetime without increasing flood risk elsewhere. Paragraph 181 states that applications should ensure that flood risk is not increased elsewhere, and includes the criteria (c) that development should only be allowed in areas at risk of flooding where it can be demonstrated that it incorporates sustainable drainage systems, unless there is clear evidence that this would be inappropriate. Paragraph 182 also reiterates the need to incorporate proportionate sustainable drainage systems to control flow rates and reduce runoff volumes.

Policies DMD59 and DMD60 of the Enfield Development Management Document (DMD) similarly require new development to avoid and reduce the risk of flooding, not increase the risks elsewhere, and provide a sustainable drainage strategy and management of surface water. Policy DMD61 requires all developments to follow the drainage hierarchy in the London Plan, and demonstrate how proposed measures manage surface water as close to its source as possible. The appeal development must maximise the use of [name] (SuDS), including at least one 'at source' SuDS measure resulting in a net improvement in water quantity or quality discharging to sewers, with clear ownership, management, and maintenance arrangements. The Enfield Core Strategy 2010- 2025 (CS) Policies CP21 and CP28, and the London Plan policies SI12 and SI13 incorporate similar water management requirements.

The appellant’s [name] Assessment1 (FRA) identifies that in order to not increase flood risk, the site’s surface runoff would be improved by implementing SuDS measures of 4 water butts to harvest a total of 4 cubic metres of rainwater. The landowners would be responsible for their maintenance and repair throughout the development’s lifetime.

However, water butts are only intended to temporarily store rainwater. Despite reference to throttle attenuation, the proposal does not address where that water would then be directed. There is a distinct possibility that it would end in the mains drainage, including if the butts were subject to overflow such as if the occupiers were away for a period of time. There is no appropriate or reasonable control I could impose via planning conditions to guarantee how the site’s occupiers should use the water.

As such, water butts would not therefore provide appropriate long-term flood risk attenuation in line with the above policy requirements for SuDS provision, unless connected to an overflow system leading to further SuDS features. The FRA identifies that the underlying soil conditions of clay and fine silt has very low permeability, but this does not outweigh the need to consider other options in detail. The proposal has identified no other elements of at source measures across the site, such as permeable paving, cellular crate systems, rain gardens, and/or green roofs. For these reasons, I therefore find that the proposed water butts would not provide sufficient runoff attenuation.

The Council also objects because the development has not ensured safety from flooding, including equivalent retention or compensation of flood storage on site. Although capacity would remain under the proposed new decking, overall there would be less capacity than at present resulting from the additional massing of the new extensions. Flood storage has therefore not been sufficiently demonstrated.

The Council also refers that the finished floor level must be at least 300mm above the 1 in 100 year (plus climate change) fluvial flood depth. However, the FRA highlights that the finished floor level complies with Environment Agency Standing Advice, whereby it can be set at existing levels if flood proofing has been incorporated. The proposal would incorporate various flood resistance and 1 [name] ref QFRA 2618 V1.0, 03/01/2025 resilience measures in its construction and finishes, including for water exclusion and entry. As an extension to an existing dwelling, I find this would be appropriate.

The FRA also provides detailed information and a Flood Evacuation Plan for how the site’s occupants would be made aware of the flood risk, and flood warning and evacuation during an extreme event. The access route to a safe area is the same as at present, and the first floor of the existing dwelling would provide a safe haven if necessary if the evacuation route is under water.. Overall, I find these measures acceptable.

Nonetheless, the development would not appropriately manage the risk of flooding, due to insufficient use of SuDS in not following the drainage hierarchy, and insufficient flood storage compensation. It would thus conflict with numerous development plan policies as outlined above; SI12 and SI13 of the London Plan; CP21 and CP28 of the CS; and DMD59, DMD60, and DMD61 of the DMD.

In conclusion, the proposed development would conflict with the development plan as a whole. With no other material considerations outweighing this conflict, for the reasons given above I conclude that the appeal is dismissed.