Infiltration is one way of managing rainwater as part of a Sustainable Drainage System (SuDS). It allows water to soak into the ground instead of flowing into drains or sewers.
Ground conditions in Cardiff are complex, so infiltration is not suitable on every site. This page explains how we assess infiltration and what this means for development.
What are infiltration and soakaways
Infiltration systems collect rainwater and allow it to drain slowly into the ground.
A soakaway is a structure below ground that temporarily stores rainwater before releasing it into the surrounding soil. When designed and located correctly, soakaways can help reduce flood risk and pressure on sewers.
Where infiltration may not be suitable
Infiltration does not work everywhere.
Large parts of Cardiff are underlain by clay and glacial deposits. These can prevent water from draining safely into the ground.
Infiltration may be unsuitable where it could:
- increase flood risk
- affect underground water supplies
- damage buildings or infrastructure
- cause ground instability
Each site must be assessed individually.
Why testing and evidence are needed
Where infiltration is proposed, proper ground testing is essential.
Testing must show:
- how quickly water can drain into the ground
- whether the ground conditions are suitable across the wider area
- that infiltration will not cause flooding or environmental harm
Without this evidence, infiltration proposals cannot be approved.
Planning for drainage from the start
All new developments must allow enough space for sustainable drainage.
Drainage needs must be considered at the start of the design process. Development layouts should not prevent the use of SuDS, including infiltration where it is suitable.
If there is not enough space for drainage features, the proposal may need to be redesigned.
Where soakaways work best
Soakaways are more likely to be acceptable when they:
- are located away from buildings, boundaries and highways
- can be easily accessed for inspection and maintenance
- form part of a shared or communal drainage system
Communal areas, such as the front of properties or shared open spaces, are usually more suitable than rear gardens.
Types of soakaway we accept
We only accept geocellular crate soakaways.
These are preferred because they:
- are easier to maintain
- are less likely to block with silt
- provide more storage in less space
Stone-filled soakaways and deep borehole soakaways are not accepted because they are difficult to maintain and repair if they fail.
Using soakaways for individual homes
Soakaways serving individual properties are generally discouraged.
They are often difficult to access, maintain or replace, particularly in rear gardens. Over time, home extensions or landscaping can block access and increase flood risk.
Where individual soakaways are proposed, strong justification and early discussion with us are essential.
Using permeable paving
Permeable paving can form part of a sustainable drainage system, but surface water must flow across the paving to allow treatment.
We do not support underground distribution boxes beneath permeable paving, as they bypass treatment and add unnecessary complexity.
Permeable tarmac is not permitted due to long‑term maintenance issues.
Keeping buildings safe
All infiltration systems must comply with:
- the Statutory Standards for Sustainable Drainage
- Building Regulations
For example, soakaways must be kept a safe distance from buildings to avoid structural damage.
If drainage standards and building regulations cannot both be met, the design must be changed.
If infiltration cannot be used
Infiltration is encouraged where it is safe and suitable.
Where it cannot be demonstrated to work safely, other drainage options must be considered in line with national standards. This ensures development does not increase flood risk.
Talk to us early
Ground conditions in Cardiff can be unpredictable.
Early discussion helps to:
- understand whether infiltration is realistic
- avoid delays and redesign
- reduce the risk of refusal
Engaging with us early gives the best chance of achieving an acceptable drainage solution.
