Part B Development Management Policy - Draft Local Plan for Buckinghamshire (Reg 18)

2. Development Management Policies - Climate change

2.67 Floodrisk

Definition of Flood Risk

2.67.1 Flood is formally defined within the Flood and Water Management Act 2010, as occurring "where land not normally covered by water becomes covered by water". Flood risk is defined as the combination of the probability of a flood occurring and the consequences or harm should that flooding occur.

2.67.2 Development proposals must meet the Flood Risk Vulnerability and Flood Zone Compatibility Table set out in the NPPF. Inappropriate development in areas with a high or medium risk from all sources of flooding will not be supported.

2.67.3 For the purposes of this policy, 'Residual risk' comes in two main forms: Residual risk from flood risk management infrastructure; and Residual risk to a development once any site-specific flood mitigation measures are taken into account. Residual risk is the risk remaining after avoidance, control and mitigation have been utilised.

2.67.4 The term 'Inappropriate Development' in (2) this refers to the Flood Risk Compatibility in Planning Practice Guidance for Flood Risk and Coastal Change Table 2 and the NPPF Annexe 3 Flood Risk Vulnerability Classification.

Split of responsibility

2.67.5 The operational responsible bodies for flood risk management on Buckinghamshire's watercourses are:

Buckinghamshire Council are responsible for:

• surface water runoff (including snow and other precipitation)- water on the surface of the ground, which has not entered a watercourse, drainage system or public sewer;
• ordinary watercourses (all watercourses not designated as 'main rivers', typically smaller than main rivers);
• groundwater (all water which is below the surface of the ground and in direct contact with the ground or subsoil).
• fixing drainage and flooding issues on highways and roads across Buckinghamshire.

Environment Agency are responsible for taking a strategic overview of the management of all sources of flooding and coastal erosion. Also has operational responsibility for managing the risk of flooding from main rivers, reservoirs, estuaries and the sea (as well as being a coastal erosion risk management authority).

Canal River Trust are responsible for canals.

Riparian owners are responsible for maintaining the watercourse or ditch running through, underneath, or adjacent to the boundary of their land. If the watercourse forms the boundary of the land, the riparian ownership responsibilities are shared equally between the landowners on either side.

Water companies are responsiblefor public sewers and watermains.

Bedford Group of Internal Drainage Boards responsible for watercourses in their area (https://www.idbs.org.uk/about-us/boards-drainage-district/ )

Early development proposals

2.67.6 It is recommended that as part of the early discussions relating to development proposals, developers discuss requirements relating to site-specific flood risk assessment and drainage strategies with both the Local Planning Authority and the Lead Local Flood Authority (LLFA), to identify any potential issues that may arise from the development proposals.

Planning application stage

2.67.7 At the planning application stage, developers will need to undertake more detailed hydrological and hydraulic assessments of the watercourses where required, using channel, structure, and topographic survey. This should happen particularly where there are no detailed hydraulic models. The modelling should verify flood extents with the latest climate change allowances. Developers should engage at the earliest opportunity with the EA and LLFA on detailed modelling requirements.

Sites in Local Plan versus sites not in Local Plan

2.67.8 For clarity, the council uses the term 'Development Plan' as this policy applies to both this Local Plan, the Minerals and Waste Local Plan, and also Neighbourhood Development Plans. For sites allocated within the Local Plan, the Local Planning Authority should use the information in the SFRA produced for the Local Plan evidence base to inform the Exception Test.

2.67.9 For developments that have not been allocated in the Local Plan, developers must undertake the Sequential Test followed by the Exception Test (if required) and present this information to the Local Planning Authority for approval. In areas at risk of flood this will need to demonstrate that there are no sites available in a lower flood risk zone. This assessment will need to include allocated sites. The scope of the sequential test should relate to the scale and location of a development and should be agreed with the Council beforehand. The Council needs to be satisfied that the sequential test assessment justifies the site being developed before planning permission can be granted. Applicants should refer to tables 1 to 3 in the National Planning Practice Guidance. Depending on vulnerability of users, developments may need to demonstrate that they can meet the Exceptions test before being deemed appropriate.

2.67.10 The policy makes provision for exceptions, including in situations where a site-specific flood risk assessment demonstrates that no built development within the site boundary, including access or escape routes, land raising or other potentially vulnerable elements, would be located on an area that would be at risk of flooding from any source, now and in the future (having regard to potential changes in flood risk).

2.67.11 Developers should consider flood resilience measures for new developments. New development and re-development of land should wherever possible seek opportunities to reduce overall level of flood risk at the site, for example by:

• Reducing volume and rate of runoff through the use of SuDS.
• Relocating development to areas with lower flood risk.
• Creating space for flooding.

2.67.12 For the purposes of this policy, 'Minor development' means:

• Minor non-residential extensions: industrial/commercial/leisure etc. extensions with a footprint less than 250 square metres.
• Alterations: development that does not increase the size of buildings e.g. alterations to external appearance.
• Householder development: For example, sheds, garages, games rooms etc. within the curtilage of the existing dwelling, in addition to physical extensions to the existing dwelling itself. This definition excludes any proposed development that would create a separate dwelling within the curtilage of the existing dwelling e.g. subdivision of houses into flats.

2.67.13 Green and blue Infrastructure should be considered within the mitigation measures for surface water runoff from potential development and consider using areas at risk of flooding as public open space. Consideration must be given to the potential cumulative impact of development on flood risk. This requires catchment-based modelling or developer contributions toward the outputs of such modelling where this has been done on another site. When designing a building, the finished floor levels should be a minimum of either 600mm or 300mm above the 1 % Annual Exceeded Probability (AEP), plus climate change peak flood level, depending on the development vulnerability classification.

2.67.14 Developments should look for opportunities for betterment where surface water flooding issues are present, which could be implemented through masterplans for individual strategic sites or new settlements. Proposals should integrate water conservation through rainwater harvesting and water butts and promote land management practices to attenuate runoff and alleviate potential issues downstream. Finally, development proposals should identify opportunities to help fund future flood risk management through developer contributions to reduce risk for surrounding areas and identify opportunities to make space for water to accommodate climate change.

2.67.15 Culverting open watercourses will not be acceptable, except where essential to allow highways and/or other infrastructure to cross, and where there are no reasonably practicable alternatives to culverting. In any case, the LLFA will not support development above existing culverts. Where new culverts are necessary, their design must comply with Buckinghamshire Council's Culvert Policy, secure Land Drainage Consent from the Lead Local Flood Authority and be designed in line with CIRIA's Culvert, Screen and Outfall Manual (C786). Planning permission does not negate the requirement for land drainage consent, nor does it guarantee that consent will be given.

Access and Egress

2.67.16 Planning applications must ensure development is 'safe', dry pedestrian egress from the floodplain and emergency vehicular access should be possible for all residential development. If at risk, then an assessment should be made to detail the flood duration, depth, velocity and flood hazard rating in the 1% AEP plus climate change flood event, in line with Flood Risk Guidance for New Development FD2320 (as replaced).

2.67.17 Safe refuge areas should be provided wherever there are significant residual risks to developments associated with extreme flood events and/or rapid inundation.

2.67.18 Access and egress should be considered at the site, but also in the vicinity of the site, for example, a site may have low surface water risk, but in the immediate locality, access/ egress to and from the site could be restricted for vehicles and/ or people.

Floodplains

2.67.19 The extent of the functional floodplain (Flood Zone 3b) is defined through the local plan's latest Strategic Flood Risk Assessment.

2.67.20 Development proposals should identify long-term opportunities to remove development from the floodplain and to make space for water. Development concerning brownfield sites in the functional floodplain should seek to reduce risk to the wider floodplain and provide flood risk betterment.

Other Important Considerations

2.67.21 If an ordinary watercourse is within or immediately adjacent to the site area, consultation with the Lead Local Flood Authority should be undertaken. If alterations or discharges are proposed to the watercourse, a land drainage consent will be required. Where necessary, blockages of nearby culverts may need to be simulated in a hydraulic model to confirm residual risk to the site.

2.67.22 Surface water risk should be considered in terms of the proportion of the site at risk in the 3.3% AEP (30-year), 1% AEP (100-year) or 0.1% AEP (1,000-year) events, whether the risk is due to a wider overland flow route. Surface water risk and mitigation should be considered as part of a detailed site-specific Flood Risk Assessment and Surface Water Drainage Strategy.

2.67.23 Sites where there is a canal within or immediately adjacent to the site area, developers should consult the Canals and Rivers Trust. Any proposed alterations to the canal or discharges must be agreed with the Canals and Rivers Trust. If a site is located within 250m of a landfill site, there could be amenity, dirt, and contamination issues. Sites could be sensitive from the perspective of controlled waters and therefore any redevelopment must ensure there is no pollution risk to the water environment.

2.67.24 Please see the SFRA and any other technical guide from the council for advice on development affected by Chalk streams including the sensitive management of run off and drainage. The SFRA and Water cycle studies have been produced to inform this local plan and utilise the best and latest available data. However, developers preparing planning applications will need to use the most up to date flood risk and water resources information available, in consultation with the Environment Agency and Lead Local Flood Authority.

2.67.25 Users of the Local Plan must check for the latest information on flood risk from the Environment Agency and other responsible bodies. It is important to note that the information relating to flood risk in this Local Plan and the latest SFRA is based upon the best data available at the time of writing. Mapping of flood risk, however, always involves a level of uncertainty and can never be an exact science. It should be recognised that areas that are highlighted as having high risk of flooding now or in the future in this document may not flood and areas with low risk are not guaranteed to be safe.

Strategic drainage strategies

2.67.26 Given the scale of strategic sites, developers will be required to submit a site-wide drainage strategy ahead of individual plot applications, to ensure the potential amenity, biodiversity and water quantity and quality improvements are maximised. If such measures are implemented at plot scale, there is the risk that while they would be implemented their benefits would not be maximised as they would be developed in isolation to other plots. For the purposes of this policy, strategic is defined as 50 dwellings and above or 1 hectare or more site area of non-residential development.

2.67.27 A strategic site-wide Drainage Strategy should consider local sources of flooding including ordinary watercourses, surface water and groundwater flooding across the expansion area. This would identify drainage issues, make allowance for extreme weather events caused by climate change and inform a strategic Sustainable Drainage scheme, making it clear to developers what is required from individual development parcels as these come forward.

2.67.28 Further guidance is available in the latest version of the Strategic Flood Risk Assessment.

2.68 Sustainable Drainage Systems

Definition of SuDS

2.68.1 Sustainable drainage systems (SuDS) aim to reduce the impact of development by replicating the natural processes through which rainwater is captured, stored, and transported within a development. Traditional piped drainage systems remove runoff from a site as quickly as possible, however SuDS slow flow and store runoff onsite before infiltrating into the ground or slowly releasing it offsite. When designed correctly, SuDS provide multifunctional benefits: water quantity, water quality, biodiversity and amenity.

2.68.2 All SuDS should be designed in accordance with the most recents National Non-Statutory SuDS Standards and CIRIA SuDS Manual. The National SuDS Standards (2025) were developed collaboratively by DEFRA, the Environment Agency, and industry experts. They reflect the latest understanding of climate resilience, water quality, and multifunctional SuDS design. Their use ensures that developments meet the highest standards of sustainable water management. While non-statutory, they are considered material to the assessment of planning applications under this policy and represent nationally endorsed best practice.

2.68.3 Applicants should also refer to Buckinghamshire Council's SuDS Guidance, which aligns with the National Standards and provides local interpretation and implementation advice. Examples of SuDS are also set out in the Strategic Flood Risk Assessment.

2.68.4 SuDS must be designed early in the planning process to influence site layout and not be retrofitted. Drainage information must be submitted at the application stage.

SuDS considerations

2.68.5 A detailed assessment of site constraints (as set out in CIRIA) must be undertaken at the outset of the planning process to determine the suitability of SuDS features. Site layouts must be informed by this assessment at an early stage to ensure that sufficient space is provided for above-ground multifunctional SuDS components.

2.68.6 While the National SuDS Standards are non-statutory, they represent the most up-to-date and nationally endorsed technical guidance on sustainable drainage. As such, they will be treated as a material consideration in the assessment of planning applications under Policy CC2. Applicants should also refer to Buckinghamshire Council's SuDS Guidance, which aligns with the National Standards and provides local interpretation and implementation advice.

2.68.7 The National SuDS Standards (2025) were developed collaboratively by DEFRA, the Environment Agency, and industry experts. They reflect the latest understanding of climate resilience, water quality, and multifunctional SuDS design. Their use ensures that developments meet the highest standards of sustainable water management.

2.68.8 SuDS strategies must be informed by site-specific ground investigations, including infiltration rate testing. Where sites are at risk of high groundwater, ground-water level monitoring over the winter period may be requested to ensure groundwater does not ingress into infiltration components and groundwater is not contaminated through infiltration (through the provision of a suitable freeboard).

2.68.9 Exemplar source control SuDS should be prioritised. This may include the use of permeable paving for hardstanding areas such as access roads, parking bays and driveways, and patios. The use of SuDS such as basins, ponds, swales, rain gardens/planters and tree pits are also strongly encouraged. Where infiltration is viable, these features can be used to provide storage prior to infiltrating into the underlying geology. Where infiltration is not viable, above-ground SuDS must be prioritised for attenuation prior to discharge offsite.

2.68.10 Brownfield sites typically have higher existing runoff rates compared to greenfield sites due to their existing impermeable area, therefore, the proposed discharge rate may be higher (the proposed discharge rate must not exceed the brownfield rate). However, through the use of well-designed SuDS strategies, it is possible to reduce discharge rates close to the greenfield rate. This approach must be adopted wherever feasible.

2.68.11 Where the final discharge point is the public sewerage network, the runoff rate agreed by the LLFA will also need to be agreed with the sewerage undertaker (through the provision of a Pre-Planning Enquiry).

2.68.12 It must be demonstrated that the designed SuDS strategy does not flood up to the 1 in 30-year storm event, and any flooding up to the 1 in 100-year storm event plus climate change must be safely contained onsite. Where appropriate, 10% urban creep allowance must be included.

2.68.13 Green above SuDS components are encouraged as they are easier to maintain than below ground SuDS components such as attenuation tank. A maintenance schedule is required to set out who will maintain the system, how the maintenance will be funded and should be supported by an appropriately detailed maintenance and operation manual.

2.68.14 The council supports making existing hard surface areas to be permeable under this policy.

2.68.15 Areas for parking and servicing should be planned for in a comprehensive manner as part of surface water management but the policy criteria adds detailed requirements necessary to address contamination risks.

2.68.16 Planning applications must demonstrate how the proposed SuDS strategy complies with the National SuDS Standards through submission of a compliance checklist or equivalent summary. More detailed information on how to demonstrate compliance with SuDS requirements can be found in Buckinghamshire Council's guidance on the website.

2.69 Water efficiency standards

2.69.1 The South-East region is an area classed as under serious water stress by the Environment Agency^[22]. Water supply services are provided by Affinity Water, Anglian Water and Thames Water. Water companies are required to undertake measures to reduce and to minimise the use of portable water and are working with the Environment Agency to reduce the abstraction of water from rivers and chalk aquifers.

2.69.2 Planning can also help to mitigate the effects of climate change on water scarcity by setting ambitious standards for water efficiency to minimise water demand from development. To achieve this, new development which results in the creation of one or more dwellings is required to be built to high standards of water efficiency using water efficient fixtures and fittings, or rainwater harvesting and greywater recycling.

2.69.3 The Environment Agency, Natural England and several water companies (in our area, Anglian Water and Affinity Water) have produced guidance on shared standards^[23] as part of Water Resource East. These standards for the whole of Buckinghamshire are set out in the policy.

2.69.4 Water efficiency improvements to the existing building stock where planning permission is required including changes of use to residential can also help offset the demand from new homes.

2.69.5 The council supports rainwater harvesting as a contribution to water efficiency through building design, landscape and parking areas.

2.69.6 These water efficiency standards are set at a point in time, based on current evidence. Developers will need to use the most up to date information available, in consultation with the Council and the water companies.