SEPA position statement on elevated buildings in areas of flood risk

This position statement addresses only matters that are within SEPA’s flood risk remit for proposals for elevated buildings in flood risk areas. This is where structures such as pillars or stilts are used to raise a building above the expected flood water level. The ground below the building remains at risk of flooding and allows for the free flow of water.

Avoiding development in areas of flood risk is still the best approach to managing flood risk, both now and in the future – in most cases this means avoiding development in the functional floodplain completely. But we recognise that in managing climate change there may be a need to bring previously used urban land near our rivers and coasts back into positive use and to enable existing built-up areas to adapt to increasing flood risk. This must happen in a way that ensures there is no unacceptable risk from flooding to people and places, and that those places are safe and resilient in the long term.

Sites suitable for the development of elevated buildings are best identified through the development planning process, where the need to manage flood risk rather than avoid it can be considered alongside and balanced with other policy and placemaking considerations.

It is important to highlight that this guidance addresses all matters that are within SEPA’s flood risk remit only. Local authorities must ensure elevated buildings are structurally safe and capable of being maintained for their lifetime. SEPA not opposing elevated buildings in the clearly defined circumstances below does not mean such development is achievable. Decision makers must also determine development proposals in the context of other applicable policy and regulatory frameworks, and these are not covered in this position statement.

All underlined terms are defined in the glossary.

Summary

SEPA will not oppose elevated buildings on flood-risk grounds where we are certain that all the following exceptional circumstances are met:

  1. Sites must have been previously developed and be within a built-up area.
  2. The underside of the building must be above the height of the relevant design flood level, plus an allowance for freeboard, plus a separate allowance for climate change.
  3. The proposed development must have a neutral or better impact on floodplain capacity.
  4. Proposals must not create an island of development (i.e. development must adjoin developed areas outwith the future floodplain); and
  5. Safe, flood free pedestrian access and egress from the building to a place of safety outwith the future floodplain must be provided and secured in perpetuity.
  6. The development footprint and design as proposed cannot be located in lower risk areas elsewhere in the site where the use of elevated buildings would not be required.

A detailed flood risk assessment will be required to demonstrate how these criteria are met.

Detailed advice

There is a requirement for the first occupied or utilised floor of the building to be above the future floodplain, regardless of the proposed use of that floor. Elevated buildings designed to manage flood risk must be safe to use and resilient for the long term, so even where this level is proposed for occupation by a lower vulnerability use, it is capable of being changed later. This will be achieved because there is a requirement for the underside of the building to be above the flood level.

In the area below the first occupied or utilised level of the building, the building structure must be designed, used, and maintained to allow the free conveyance and/or storage of water up to and including the future flood level throughout the lifetime of the development. This can be achieved by ensuring openings are designed to be as large as technically feasible and not enclosed in any way, to reduce the risk of blockage by mobile debris during a flood, and ensuring this area is unused in perpetuity (including not for storage, refuse or parking uses), which may need to be secured via a planning mechanism (i.e., condition or legal agreement - for the planning authority to advise). The underside of the building should be higher than the design flood level, including allowances for both climate change and freeboard, to have a neutral impact on the capacity of the floodplain.

To ensure that elevated buildings do not create islands of development, at least one side of any building of this nature must directly adjoin land which is out with the future floodplain. The purpose of permitting a more adaptive approach to these developments is to create places which are safe and resilient for the long term, which requires a higher level of flood resilience than is required for some other types of development. Using a connecting walkway across land which would flood, now or in future, is not a suitable way to achieve connection to higher ground because it will mean the building is an island; it may also limit the future adaptability of the building. The diagrams below illustrate these points:

Elevated buildings may fail, as with any built structure that becomes subject to fast flowing water and mobile debris. Flood water is powerful, and ensuring structures continue to function through likely repeated inundations requires purposeful consideration and design. SEPA has no remit to advise on structural stability - building standards are set by the Scottish Government and administered by local authorities. It is the responsibility of the developer/building owner to ensure that buildings are structurally safe and compliant with the building warrant as approved by the local authority. Maintenance of the building is the responsibility of the building owner/operator.

A detailed flood risk assessment will be required to support applications for most developments of this type, unless sufficiently detailed flood risk assessment or study information already exists. The assessment must meet the usual requirements set out in the SEPA requirements for undertaking a Flood Risk Assessment and is needed to demonstrate:

  • The design future flood level (using the latest recommended climate change allowances)
  • The availability of land outwith the future floodplain to which the building will adjoin
  • The neutral impact of the development on the capacity (both conveyance and storage) of the floodplain.
  • Sensitivity and uncertainty analysis to support the freeboard allowance provided

Glossary

allowance for climate change

SEPA publish guidance on climate change allowances for use in land use planning: Climate change allowances for flood risk assessment in land use planning (sepa.org.uk). This ensures that the future flood level is calculated appropriately to include the expected increase in flooding we expect because of climate change, either from increased rainfall, increased river flows or rising sea levels, depending on the location of the site. It is separate to the allowance for freeboard – see below.

built-up area

A site would be considered to fall within a built-up area if: i). It is within the settlement boundary of a village, town, or city; and ii) the adjoining land uses are predominantly developed in nature i.e., most of the boundary is adjacent to existing development. A site would not be considered to fall within a built-up area if: i). It is in a sparsely developed area characterised by isolated developments or no other development at all; and ii). It is outwith a settlement boundary; or, iii). It is within a settlement boundary but is located on the periphery of the settlement and is predominantly or completely surrounded by undeveloped land.

floodplain capacity

The ability the floodplain has to store and convey (allow the free movement of) floodwater. If the volume of water that can be conveyed through, or stored within the floodplain is reduced, this can result in flooding being increased in other areas.

future floodplain

The functional floodplain plus an allowance for climate change but exclusive of a separate allowance for freeboard.

island of development

Buildings located above the design flood level but surrounded by lower ground at flood risk on all sides not directly connected to areas outwith the functional floodplain. To avoid doubt, the inclusion of a raised walkway does not stop a layout being considered an island of development.

previously developed

Land which has been previously developed. This can include vacant or derelict land and land occupied by existing, redundant, or unused buildings. Often referred to as brownfield sites. Undeveloped sites can also be termed greenfield.

relevant design flood level

The maximum level (height) that the flood water is expected to reach, during the design flood. The design flood is the hypothetical flood that has been used to design the development. For these types of developments, the design flood is the largest flood that would have a 0.5% probability of happening in any year. An allowance for future climate change must be added for all elevated buildings.

safe, flood free pedestrian access and egress

A route for the movement of people (not vehicles) of all abilities (on foot or with mobility assistance).

separate allowance for freeboard

In this context freeboard is the difference between the design flood level and the underside of the building. It is an important safety margin designed to allow for the uncertainties involved in flood estimation and physical factors that cannot be assessed and vary between sites e.g., post construction settlement and wave action. In many cases an adequate freeboard allowance is 600mm above the design flood level[1] (in some situations, a more detailed assessment of appropriate freeboard will need to be carried out). It is separate to the allowance for climate change – see above.

underside of building

The lowest level of the building structure that could impede the flow of water underneath it.

[1] In line with CIRIA Guidance C624 Development and Flood Risk – Guidance for the Construction Industry 2004.

Published: 5 January 2022 – Version 1
Last updated: 5 January 2022