About the project
Ambiental were appointed by an Independent Town Planning Consultancy to support with providing two National Planning Policy Framework (NPPF) compliant Flood Risk Assessments for two individual Change of Use applications in East Peckham. More specifically, one application was for a Class Q (change of use from an existing agricultural building to create a residential C3 use, whilst the second application was for the conversion of a redundant garage building to create an assisted-living annexe.
The existing buildings were both located on the same site. Both units were located within 15m of a nearby drain that falls within the ownership of the Upper Medway Internal Drainage Board.
The existing condition of the redundant garage featured a single skin brick structure with a timber and metal cladding lean-to at its western façade that was partly open. Similarly, the agricultural building was pictured to be open at its eastern façade. Subsequently neither buildings could be considered as truly water resilient, given that floodwater would be able to occupy the footprint during a flood event.
Ambiental were first appointed to assist with the conversion of the redundant garage building. After Maidstone Borough Council (MBC) permitted the application, the developer later requested an FRA to be produced in support of the Class Q application to convert the existing agricultural barn. More recently, MBC have since granted Prior Approval for this too.
Flood Map for Planning showing the Flood Zones of the site
The existing units are classified as “Less Vulnerable” developments under the NPPF vulnerability classification guidelines. Once converted to residential units, there will be an increase in vulnerability classification, as both buildings will be considered “More Vulnerable”.
The redundant garage building was located closer to the nearby watercourses and subsequently falls partly within Flood Zone 3 (high risk of fluvial flooding for this project), whilst the agricultural unit located further away was located wholly in Flood Zone 2 (medium risk of fluvial flooding).
Both buildings were to be converted to solid enclosed structures and therefore there was concerns that this could be challenging to achieve without displacing floodwater off site when comparing the existing situation against the proposed. Chapter 14 of the NPPF, states that new development should not increase flood risk elsewhere.
Furthermore, the floor to ceiling height of the redundant garage was not large enough to create two floors and so bedrooms were proposed at ground floor level. The Environment Agency’s required mitigation is typically more stringent where sleeping is proposed at ground floor level.
Ambiental assessed detailed hydraulic model data at the site, obtained from the Environment Agency. The client also provided Ambiental with a topographic survey. Subsequently, Ambiental were able to determine the predicted flood risk at the site for the critical return period. This was the 1 in 100 year +35% climate change – higher central allowance in accordance with the EA guidance introduced February 2016 and updated March 2020.
The agricultural unit is shown to remain wholly unaffected for the critical return period. Ambiental therefore argued that floodwater displacement could be deemed negligible. It was also recommended that Floor Levels were raised 300mm above surrounding ground levels.
A comparison of surveyed levels with modelled flood levels at the redundant garage building for the critical return period demonstrated that depths of up to 240mm could enter the building. The converted garage is to feature bedrooms at ground floor level. To demonstrate that the proposed development can be considered safe for its lifetime, Ambiental recommended sufficient floor level raising, in accordance with guidelines set out by the Environment Agency.
Given the depths experienced at the redundant garage, it was necessary that appropriate mitigation measures were recommended to prevent a displacement of floodwater post-development. Finished Floor Levels were recommended to be raised atop of undercroft voids. Floodwater would subsequently remain capable of flowing beneath the structure during the critical return period.
As existing, neither building could be considered a flood resilient structure given that both structures featured an open façade. Furthermore, detailed modelling demonstrated that the site has had a history of flooding and was predicted to continue to flood for larger events.
Regardless of this, following assessing the modelled flood levels and extents for the required flood events at the site, the Flood Risk Assessments suitably demonstrated that the developments could be designed to remain safe for their lifetime without increasing flood risk elsewhere. Whilst according to the guidance of the Exception Test.
Following submitting both FRA’s, along with relevant additional planning documents, MBC permitted the planning application for the conversion of the redundant garage in June 2019. Prior approval was later granted in December 2019 for the Class Q application.