Flood Risk Assessment Guidance
Part one
The full Strategic Flood Risk Assessment document created by Metis Consultants Ltd is available to download from our website
6.1 Overview
Applicants and LPAs need to consider flood risk to and from a proposed development as part of any planning proposal. To assess the flood risk to and from a development, a site-specific Flood Risk Assessment (FRA) and / or Drainage Strategy should be carried out by the applicant. The site-specific FRA and / or Drainage Strategy should demonstrate that the proposed development will manage different sources of flood risk for the entirety of the development’s lifetime. Applicants should justify the timescale for the lifetime of the proposed development.
It must be demonstrated that the development will not be at risk of flooding or increase flood risk elsewhere. The site-specific information supplied for any development application should be proportionate to the identified flood risks and appropriate to the scale, nature and location of the development. Developments must be appropriately resilient to the potential impacts of climate change. Complying with the NPPF and policies from the London Plan (Policy SI 13) and Kingston’s Core Strategy (Policy DM 4).
Applicants must prioritise SuDS when proposing measures to reduce local flood risk. Measures that manage runoff as close to source as possible and contribute to the four pillars of SuDS (amenity, biodiversity, water quality and water quantity) should be proposed where possible. These key principles need to be applied at the strategic level for borough-wide planning and at the site level for development proposals and site allocations. Applicant (Section 6.5), LPA Development Management (Section 6.6) and Planning Policy (Section 6.7) specific guidance regarding FRAs is available in this section.
6.2 Sequential and Exception Tests
The NPPF requires that a sequential, risk-based approach to the location of development is taken to avoid, where possible, the risk of flooding to people and property. The approach needs to take current and future impacts of climate change into account. To demonstrate that efforts have been made to steer development to areas with the lowest risk of flooding, applicants may be required to carry out the Sequential and Exception Tests as part of their site-specific FRA.
The Sequential Test requires that proposed development sites are located within areas of lowest flood risk. Only if it can be demonstrated that there are no suitable sites within the wider search area can alternative sites (i.e. within areas that may potentially be at risk of flooding) be considered. For this SFRA, the ‘wider search area’ is defined as the entire borough extent, though there are locally defined search area exceptions depending on the location and type of the proposed development. Further information on search area exceptions can be found in Section 6.5.
The NPPF recognises that it may not always be possible to locate development in areas with a lower risk of flooding. These developments may be proposed in established communities that require continued development to grow. For these types of proposals, the NPPF provides the Exception Test. The Exception Test is a method to demonstrate and help ensure that flood risk to people and property will be managed satisfactorily, while allowing necessary development to go ahead in situations where suitable sites at lower risk of flooding are not available. To pass the Exception Test, the following two conditions need to be passed in line with paragraph 160 of the NPPF:
- The development would provide wider sustainability benefits to the community that outweigh the flood risk; and
- The development will be safe for its lifetime taking account of the vulnerability of its users, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall.
If a site passes the Exception Test, a site’s layout and design should follow a sequential approach. Areas of the site that are at a lower risk of flooding should be prioritised for development. Should a site lack suitable space for development within a low flood risk zone, then less vulnerable use classes should be located in the higher flood risk zones and more vulnerable use classes in the low flood risk zones. Additionally, a site with higher flood risk could prioritise ground flood development with low vulnerability uses and higher vulnerability uses on the first floor and above.
For further developer and applicant guidance on applying the Sequential and Exception Test, see Table 6‑1 and Section 6.5. LPA Development Management and Planning Policy guidance on the Sequential and Exception Tests can be found in Section 6.6.
6.3 Planning Application and Development Requirements
Planning permission is required if the work being carried out meets the Section 55 of the Town and Country Planning Act 1990 definition of a ‘development’. Section 57 of the Town and Country Planning Act 1990 states that all work falling under this statutory definition of 'development' requires planning permission unless it meets permitted development criteria. The following are planning application definitions for development types:
- Major Developments: For residential developments, 10+ dwellings or site area over 0.5 hectares. For non-residential developments, total building floorspace exceeds 1,000m2 or site area over 1 hectare.
- Minor Developments: For residential developments, 1-9 dwellings, site area under 0.5 hectares. For non-residential developments, total building floorspace is less than 1,000m2, site area under 1 hectare.
- Change of Use: Developments classified as a ‘Change of use’ if - (i) the application does not concern a major development; and (ii(a)) no building or engineering work is involved; or (ii(b)) the building or engineering work would be permitted development were it not for the fact that the development involved a change of use (such as the removal of internal dividing walls in a dwelling house to provide more spacious accommodation for office use).
General planning application guidance is available via the PPG Site-specific FRA Checklist and the EA’s Standing Advice. Table 6‑1 provides the local requirements that must be addressed as part of the flood risk and drainage strategy submission documents. The guidance is applicable for Major, Minor, Change of Use, and Changes Under Prior Approval Notifications developments. Development type-specific guidance are highlighted where applicable. Table 6‑2 provides the requirements for the assessment and management of flood risk from other sources where applicable.
The information presented in Table 6‑1 and Table 6‑2 are a mixture of legislative and best-practice requirements from various sources, including the NPPF, PPG, the London Plan, and the Core Strategy. In some instances, the SFRA guidance and recommendations go beyond existing adopted policies at the time of writing (May 2021). This includes policy guidance on where Drainage Strategies are required, where specific information is needed to address flood emergency, finished floor level requirements and flood plain compensation. This also includes the addition of the surface water 1 in 100 year extent (1% annual exceedance probability) being adopted into Flood Zone 3a (surface water) - refer Section 5.10.3.
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Flood Zone 3b (Fluvial) |
Flood Zone 3a (Fluvial and Surface Water) |
Flood Zone 2 (Fluvial) |
Flood Zone 1 |
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The Flood Risk Vulnerability and Flood Zone Compatibility table in the PPG highlights that only 'Essential Infrastructure' and 'Water Compatible' developments may be granted planning permission. As the functional floodplain, land in Flood Zone 3b will be protected by not permitting any form of development on undeveloped sites unless it is for ‘Water Compatible’ development or ‘Essential Infrastructure’. Redevelopment of existing developed sites will only be supported if there is no intensification of the land use and a net flood risk reduction is proposed; any restoration of the functional floodplain will be supported (see ‘Flood Compensation Storage’ section of this table). Proposals for the change of use or conversion to a use with a higher vulnerability classification will not be permitted. |
The Flood Risk Vulnerability and Flood Zone Compatibility table in the PPG highlights that land use is restricted to ‘Water Compatible’, ‘Less Vulnerable’ and ‘More Vulnerable’ development. ‘Highly Vulnerable’ developments will not be permitted as it is not a permitted development type in Flood Zone 3a. Self-contained residential basements and bedrooms at basement level will not be permitted (see ‘Basements’ section of this table). |
No land use restrictions. Self-contained residential basements and bedrooms at basement level will not be permitted (see ‘Basements’ section of this table). |
No land use restrictions |
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The Sequential and Exception Tests do not need to be applied if your site:
The Sequential and Exception Tests need to be applied for all major developments and minor developments as set out below. |
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Developments categorised as ‘Essential Infrastructure’ can only be considered following applications of the Sequential and Exception Tests. Paragraph 15 of the PPG states: "If an area is intended to flood, then this should be safeguarded from development and identified as functional floodplain, even though it might not flood very often. Development can only be permitted following application of the Sequential Test, and a successful application of the Exception Test.” |
The Sequential Test is required for all developments except for those categorised as ‘Highly Vulnerable. This use is not permitted (see ‘Land Uses and Development Restrictions’ section of this table). Developments categorised as ‘Essential Infrastructure’ and ‘More Vulnerable’ can only be considered following application of the Exception Test. |
The Sequential Test is required for all development types. Developments categorised as ‘Highly Vulnerable’ can only be considered following application of the Exception Test. |
The Sequential Test only needs to be applied for development proposals in Flood Zone 1 if the SFRA and accompanying Web Map indicates there may be existing flood issues from other sources (refer to Table 6-2) or flood issues in the future. |
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A site-specific FRA is required for all development proposals – sites in Flood Zone 3b must also demonstrate that:
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A site-specific FRA is required for all development proposals. These must demonstrate that there will be no net loss of floodplain storage (see the 'Flood Compensation Storage' section of this table). |
A site-specific FRA is required for all development proposals. Assessment needs to demonstrate the reduction of flood risk at the site through various mitigation techniques. |
A site-specific FRA is required for all development proposals where there is evidence of a risk from non-fluvial of flooding, including surface water, groundwater and sewer flooding. |
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A site-specific FRA is required for all developments within Kingston Town Centre area (see Section 6.5.2 for further information). |
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Flood risk from all sources should be assessed, including the potential impacts of climate change over the development’s lifetime. The EA’s 2016 climate change allowances (including subsequent updates) must be used when assessing peak river flows, sea level rises and peak rainfall intensities. |
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If the site for a development proposal falls within the Flood Zone 3a (surface water) extent (based on the Policy Web Map), the applicant is required to submit an FRA as part of a planning application. This is a requirement for all major developments, minor developments, and change of use developments that have a bearing on a site’s existing drainage regime. |
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Where a site-specific FRA is required, predicted flood depths should be analysed and appropriately mitigated. Mitigation may include (but not be limited to) flood resistance measures (where predicted flood depths are less than 0.3m) or flood resilience measures (where predicted flood depths are greater than 0.6m). Predicted flood depths between 0.3m and 0.6m should be analysed on a case-by-case basis to determine if resistance measures are sufficient. Design plans should show floor levels (relative to Ordnance Datum) and predicted flood depths. |
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A Drainage Strategy is required for all major developments. Minor developments and change of use developments that have a bearing on a site’s existing drainage regime also need to provide a Drainage Strategy as part of the proposal. |
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The Drainage Strategy requires information on the proposed SuDS and surface water runoff discharge destination in line with the London Plan and Core Strategy. Each stage of the drainage hierarchy should be appropriately assessed with supporting information to demonstrate if measures could be implemented as high up the hierarchy as possible. The Drainage Strategy also requires supporting calculations on the greenfield and proposed development's peak discharge rates, and water storage volumes for different rainfall events with climate change allowances. These calculations need to ensure that proposed developments are designed to the Non-Statutory Technical Standards for Sustainable Drainage Systems. Maintenance and operation requirements must be designed into the proposals to ensure lifetime management of the SuDS features, in accordance with Written Ministerial Statement HCWS161. A Kingston SuDS Proforma needs to be provided for all planning applications in which a Drainage Strategy is required. Permission to connect to the local sewer network and pipes, including written confirmation that capacity exists in the network, should be sought from TWUL in line with Core Strategy Policy DM 4. Evidence demonstrating that an agreement in principle for any proposed new sewer connections has been reached must be provided as part of the Drainage Strategy. Failure to do so could impact the detailed design and overall Drainage Strategy for the site. The requirement to confirm local sewer network connections is for major developments only. |
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Basements should not be permitted in Flood Zone 3b. |
Basement dwellings are categorised as "Highly vulnerable" infrastructure by the PPG and should not be permitted in Flood Zone 3a. Other new basement developments are restricted to Less Vulnerable / Water Compatible uses only. All basement rooms must have internal access and egress to a higher floor above the design flood level (1 in 100 year plus an appropriate allowance for climate change) which can be utilised as part of emergency evacuation procedures. All basements, including lightwells and vents that could allow water inundation, must have access thresholds raised 300mm above the 1 in 100 year plus an appropriate climate change allowance flood level. As part of any assessment, evidence needs to be submitted to confirm the local water table level. |
Self-contained basement dwellings and bedrooms are not permitted in Flood Zone 2. The same rule applies to basement dwelling redevelopment works such as extensions and conversions. All basement rooms must have internal access and egress to a higher floor above the design flood level (1 in 100 year plus an appropriate allowance for climate change) which can be utilised as part of emergency evacuation procedures. All basements, including lightwells and vents that could allow water inundation, must have access thresholds raised 300mm above the 1 in 100 year plus an appropriate climate change allowance flood level. As part of any assessment, evidence needs to be submitted to confirm the local water table level |
Where there is evidence of flood risk from surface water, groundwater and / or sewer flooding in the area, a site-specific FRA is required for new and existing basement dwelling proposals (Refer Table 6-2). Flood mitigation measures for these sites are required to demonstrate that the development will not be impacted by flooding, or have any adverse impacts on flooding locally during a 1 in 100 year event. As part of any assessment, evidence needs to be submitted to confirm the local water table level. |
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Finished floor levels required for developments are based upon their Vulnerability Classifications:
The EA’s 2016 climate change allowances (including subsequent updates) must be used to incorporate the appropriate climate change allowances. |
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If permissible development decreases the volume of a fluvial or surface water floodplain, flood storage compensation should be addressed though the following step-approach. This is a requirement within Flood Zone 3b (fluvial), Flood Zone 3a (fluvial), Flood Zone 3a (surface water) and the fluvial flood risk extent for the 1 in 100 year plus 35% climate change allowance (which covers some parts of Flood Zone 2). Please refer to the glossary for definitions. Step 1 (below) must be followed unless sufficient evidence is provided that this is not reasonably practical. If Step 1 is proven to not be reasonably practical at that site, then Step 2 must be followed. This process repeats until Step 4, which is the bare minimum requirement (and only appropriate if sufficient evidence has been provided).
The EA’s 2016 climate change allowances (including subsequent updates) must also be incorporated to assess and calculate floodplain storage compensation. For further information on flood storage compensation, see Section 6.5.5. |
N/A |
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Voids may be applicable for major development and minor development proposals only, please refer to the SFRA glossary for development definitions. Voids will only be considered if an applicant has followed the flood storage compensation stepped approach and provided sufficient justification within an FRA. If permissible development decreases the volume of a fluvial floodplain and flood compensation storage cannot be provided, introducing voids may be a suitable alternative. Voids are not considered floodplain compensation but flood mitigation and are to be used as a last resort for flood storage mitigation. Voids may be suitable where it is not possible to achieve all the direct compensation required or for small scale development where it can be difficult to achieve full compensation. Ideally, void openings should be a minimum of 1m long and open from existing ground levels to at least the 1 in 100 year fluvial event, plus climate change flood level. By setting finished floor levels at 300mm above the design flood level (1 in 100 year plus an allowance for climate change), there is usually enough space for the provision of voids below. The following voids mitigation specification must be adhered to if considering voids: 1. The openings to the void should extend from the existing ground level and the underside of the proposed void should be set to a minimum of the 1 in 100 year event with an appropriate allowance for climate change flood level. 2. There should be a minimum of 1m of open void length per 5m length of wall. Void openings should be provided along all external walls. 3. If security is an issue, 10mm diameter vertical bars set at 100mm centres can be incorporated into the void openings. The use of under-floor voids will typically require a legal agreement or planning condition and maintenance plan to ensure they remain open for the lifetime of the development. For small scale development different design criteria may be acceptable. Sole reliance on the use of under-floor voids to address the loss of floodplain storage capacity is not acceptable on undeveloped sites. |
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Any feature that may obstruct flood flows or surface water overland flow routes (embankments, walls, fencing, walls, raised land etc.) should minimise or remove the obstruction to ensure flood risk is not increased on or off site. This could be achieved by relocating these obstructions or providing openings to allow water to flow through structures e.g. permeable fencing. |
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All Major Developments will be expected to incorporate measures that effectively manage actual and residual flood risk. |
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PPG defined ‘Essential Infrastructure’ and ‘Water Compatible’ use development needs to remain operational and safe in times of flood. Emergency Plans need to reflect this as these structures may assist in flooding evacuations. |
PPG defined ‘Essential Infrastructure’ use development needs to remain operational and safe in times of flood. Emergency Plans need to reflect this as these structures may assist in flooding evacuations. |
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As part of the second criteria of the Exception Test, there is a requirement to show that proposed developments are safe and that any residual risks can be satisfactorily overcome. Residual risk should be mitigated through flood resilient / resistant designs and emergency planning to make sure suitable measures are in place to offer protection. |
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Developments should be set back from main rivers including culverts and existing flood defence infrastructure (8m for main rivers). Developments sites within specified distances of main rivers may require a flood risk activity permit in addition to planning permissions. For non-tidal main rivers, flood risk activity permits may be required if development sites are within 8m of a river, flood defence structure or culvert. Further details on flood risk activity permits are available from the Environment Agency. |
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Development sites within 5m of ordinary watercourses require an approved ordinary watercourse consent in addition to planning permissions. |
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