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Drainage Assessment Guidance

Every planning application needs a drainage assessment. Here we identify what drainage details are needed for different types of planning applications.

The guidance sets out what applicants need to do to complete a drainage assessment which deals with drainage issues in their planning applications. Drainage assessments must include foul drainage and surface water drainage assessments. 

Foul Drainage

Any type of development that connects to the mains or a non-mains system will need a drainage assessment or statement to confirm the proposed arrangement. This includes householder applications for additional bedrooms / bathrooms.

Surface Water Drainage

The rural nature of the area means there is limited drainage infrastructure and leaves many areas vulnerable to surface water flooding. Continued development reduces the natural flood routes and increases the run off volume. 

To manage the flood risk and meet our responsibilities under the Flood and Water Management Act 2010, we will be taking a practical approach for replacement dwellings and developments of two units or fewer providing certain criteria can be met. The final drainage detail can then be conditioned, if permission is granted. For developments larger than this, we will need to be satisfied that there is a realistic drainage solution and full testing and calculations will be more than likely required.

How to complete a drainage assessment

Step One: Scale of Development

The first step is to identify the type of application:

Householder ApplicationsDevelopment within the curtilage of a dwelling house and are not a change of use.
Change of Use

No operational development

No building or engineering work is involved

Small Scale Minor Development1-2 dwellings (unless floor space exceeds 1000m²)


Site area less than 0.5 hectare
Minor Development

3-9 dwellings (unless floor space exceeds 1000m²)


Non-residential floor space up to 999 m²


Site area less than 1 hectare

Major Development

10+ dwellings (over 0.5 hectare if number of dwellings is unspecified)


Non-residential floor space greater than 1000 m²


Site area exceeds 1 hectare

Step Two: Flood Risk

Once the level of development has been established, the next step is see which flood risk category applies:

Low Risk

Large, level sites not in a flood zone or Critical Drainage Area

Medium Risk

Small, steep sites not in a flood zone or Critical Drainage Area

High risk

Site is within a Flood zone or Critical Drainage Area


How to find out if your development is in a flood zone of critical drainage area 

Flood Zones

Find out which flood zone your development is in by visiting the Government's flood map.

Critical Drainage Areas (CDAs)

You can find information and maps of CDAs on our Critical Drainage Areas page.

Step Three: Drainage requirements

Once the application type and level of risk are known then look at the Drainage Guidance to find the requirements for a drainage assessment.

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