Guidance

Preparing a flood risk assessment: standing advice

Find out if you need to follow standing advice when completing a flood risk assessment and what to do.

Local planning authorities should use flood risk standing advice for local planning authorities to check when to consult the Environment Agency or use standing advice.

Local planning authorities should use this guide to check the:

  • flood risk assessments they receive contain the correct information
  • applicant has followed the standing advice

You need to follow standing advice for vulnerable developments (including change of use) with a flood risk vulnerability classification of:

  • ‘more vulnerable’ in flood zone 2 (except for landfills, waste facility sites, caravan or camping sites)
  • ‘less vulnerable’ in flood zone 2 (except for waste treatment sites, mineral processing sites, water treatment plants and sewage treatment plants)
  • ‘water compatible’ in flood zone 2

Use the advice for minor extensions to complete an assessment for a minor extension in flood zone 2 or 3. A minor extension is a household or non-domestic extension with a floor space of no more than 250 square metres.

Find out what flood zone your development is in on the Environment Agency’s flood map.

If your development is not covered by the standing advice in this guide:

Research your development site

For all developments covered by standing advice, you should do the following research before starting your flood risk assessment:

Check if you need to do a sequential test and exception test

Before you start a flood risk assessment, check if you need to carry out the sequential test. The sequential test compares your proposed site with other available sites to show which one has the lowest flood risk. If the sequential test has been satisfied you need to check if the exception test also needs to be done.

What to include in your assessment

For all developments covered by this standing advice, your flood risk assessment should include:

  • your site address
  • a description of your development
  • an assessment of the flood risk from all sources of flooding for your development, plus an allowance for climate change
  • the estimated flood level for your development
  • details of your flood resistance and resilience plans
  • any supporting plans and drawings
  • any information the relevant standing advice tells you to include

The estimated flood level is:

  • a river flood level with a 1 in 100 or greater annual probability
  • the tidal flood level with a 1 in 200 or greater annual probability
  • plus an allowance for climate change

You may be able to get the estimated flood level from the Environment Agency or your local planning authority. If not, you’ll need a flood risk specialist to calculate this for you.

Your written flood risk assessment can be in any format, but should include the relevant plans, surveys and assessments. Check with your local planning authority as it may use specific software, for example, for producing detailed hydraulic models.

Advice for minor extensions

You need to provide a plan showing the finished floor levels and the estimated flood levels.

Make sure the floor levels are either no lower than existing floor levels or 300 millimetres (mm) above the estimated flood level. If they are not, ask your local planning authority if you also need to consider extra flood resistance and resilience measures.

State in your assessment all levels in relation to Ordnance Datum (the height above average sea level). You may be able to get this information from the Ordnance Survey. If not, you’ll need to get a land survey carried out by a qualified surveyor.

Your plans need to show how you’re going to ensure the development is not flooded by surface water. An example of this could be to divert surface water away from the property or by using flood barriers.

If your minor extension is in an area with increased flood risk as a result of multiple minor extensions in the area, you need to include an assessment of the off-site flood risk. Check with your local planning authority if this applies to your development.

Make sure your flood resistance and resilience plans follow the guidance on improving the flood performance of new buildings.

Standing advice for vulnerable developments

For all relevant vulnerable developments (for example, more vulnerable, less vulnerable and water compatible), you should follow the advice for:

  • surface water management
  • access and evacuation
  • floor levels

Surface water management

Your plans for the management of surface water need to meet the requirements set out in either your local authority’s:

  • surface water management plan where available
  • strategic flood risk assessment

They also need to meet the requirements of the approved building regulations Part H: drainage and water disposal. Read section H3 rainwater drainage.

You need to get planning permission to use a material that cannot absorb water (for example impermeable concrete) that is larger than 5 square metres in a front garden.

Access and evacuation

You need to provide details of your emergency escape plans for any parts of a building that are below the estimated flood level.

Make sure your plans show:

  • single storey buildings or ground floors that do not have access to higher floors can access a space above the estimated flood level (for example higher ground nearby)
  • basement rooms have clear internal access to an upper level (for example a staircase)
  • occupants can leave the building if there’s a flood and there’s enough time for them to leave after flood warnings

Floor levels

You need to provide both the:

  • average ground level of your building
  • finished floor level of the lowest habitable room in your building

Ground floor levels should be a minimum of whichever is higher of:

  • 300 millimetres (mm) above the general ground level of the site
  • 600mm above the estimated river or sea flood level

State in your assessment all levels in relation to Ordnance Datum (also known as height above average sea level). You may be able to get this information from the Ordnance Survey. If not, you’ll need to get a land survey carried out by a qualified surveyor.

If you cannot raise floor levels above the estimated flood level, you need to consider extra flood resistance and resilience measures.

Extra flood resistance and resilience measures

Follow the extra flood resistance and resilience requirements for developments in flood risk areas where ground floor levels are lower than the estimated flood level for the site.

Water depth up to 300mm

The design of the building or development should keep water out as much as possible. You should use materials that have low permeability (materials that water cannot pass through, for example, impermeable concrete).

Water depth from 300mm to 600mm

The design of the building or development should keep water out (unless there are structural concerns) by:

  • using materials with low permeability to at least 300mm
  • using flood resilient materials (for example lime plaster) and design (for example raised electrical sockets)
  • making sure there’s access to all spaces to enable drying and cleaning

Water depth above 600mm

The design of the building or development should allow water to pass through the property to avoid structural damage by:

  • using materials with low permeability to at least 300mm
  • making it easy for water to drain away after flooding
  • making sure there’s access to all spaces to enable drying and cleaning

Submit your flood risk assessment

Submit your completed flood risk assessment with your planning application to your local planning authority.

The local planning authority will review your flood risk assessment and tell you if it’s satisfactory. Planning applications that do not have a satisfactory flood risk assessment may be refused.

Contact the Environment Agency

Environment Agency

PO Box 544


Rotherham
Yorkshire
S60 1BY

Published 1 April 2012
Last updated 1 March 2019 + show all updates
  1. Clarity improved and links updated. Clarified status of advice for minor extensions and advice on climate change. Added ‘extra resistance and resilience measures’ section from 'Flood risk standing advice for local planning authorities' guide. Included advice for areas of flood zone 1 that will be at future risk.

  2. Changed URL for Find out what flood zone your development is in, to the new Flood Map for Planning service. Changed URL for Check if your development is within 20 metres (m) of a main river, to the Main River map

  3. First published.