Sustainable Drainage Systems (SuDS): A guide for architects, developers and contractors

Sustainable Drainage Systems (SuDS) are a core consideration for every new development in the UK. From bespoke homes to large residential schemes and commercial or industrial sites, planning authorities expect surface water to be managed in a way that is resilient, environmentally responsible and future-proof.

For architects, developers, and design & build contractors, understanding SuDS design early in the project lifecycle is key. Decisions made at RIBA Stages 0-2 can directly affect planning outcomes, site layouts, build costs and long-term maintenance responsibilities.

This guide provides an overview of Sustainable Drainage Systems and the UK Drainage Hierarchy.

If you are developing a Drainage Strategy for your project, download our ‘Resources Guide for SuDS Design’ here.

What is a Sustainable Drainage System (SuDS)?

Sustainable Drainage Systems (SuDS) are a sequence of water management practices designed to manage surface water runoff by mimicking natural drainage processes. Instead of collecting rainwater and relying on traditional underground pipes transporting it to sewers, SuDS drainage slows down the flow of rainwater before releasing it into systems nearby.

SuDS are structured around four ‘pillars’:

1. Water quantity
   Controlling the volume and speed of surface water runoff to reduce flood risk on the site.
2. Water quality
   Filtering pollutants and sediments before water enters groundwater or watercourses.
3. Amenity
   Creating attractive, usable spaces that add value to developments, such as landscaped swales, rain gardens and ponds.
4. Biodiversity
   Providing habitats for wildlife and improving ecological value.

By using features such as green roofs, permeable paving, swales and attenuation ponds, SuDS slow down the flow of rainwater before it reaches sewers or rivers, which helps prevent overwhelming the national infrastructure during heavy storms and replenishes groundwater levels naturally.

The Drainage Hierarchy / SuDS Hierarchy

When developing a SuDS strategy, planners expect you to follow the Drainage Hierarchy. This is embedded in UK planning policy and reinforced by the National standards for sustainable drainage systems (SuDS).

You must start at the top of the hierarchy and only move to a lower level if you can provide the higher options are not technically feasible.

The UK SuDS Drainage Hierarchy

Why the order of Drainage Hierarchy needs to be followed:

• Flood Prevention

By using Infiltration (Level 2) instead of Sewers (Level 4), you keep water out of the pipes, preventing the “bottleneck” effect that causes street flooding.

• Environmental Protection

Directing clean rainwater into a Combined Sewer (Level 5) is considered a waste of resources and contributes to sewage spills during heavy storms.

• The “Evidence” Rule:

Under current UK planning guidance (including the 2025 National SuDS Standards), a developer cannot simply pick Level 4 because it is cheaper. They must provide geotechnical evidence (like a BRE 365 soakaway test*) to prove why they cannot use Infiltration.

*A BRE365 soakaway test determines whether a new or substitute soakaway is suitable for managing and discharging surface water runoff.

Free resources for early-stage SuDS planning

Before engaging a civil engineer / drainage engineer, there are many free resources that can help you develop an informed strategy during early RIBA stages.

By providing as much of this information as possible to your civil engineer this will encourage project efficiencies during design and through planning.

You can find these within our Resources Guide for SuDS Design available to download for free here.

Frequently Asked Questions

• What does SuDS stand for?

  SuDS stands for Sustainable Drainage Systems. These are systems designed to manage surface water runoff in a way that mimics natural drainage processes, rather than relying solely on traditional underground pipes and sewers.

• What are SuDS drainage systems?

  SuDS drainage systems manage rainwater by slowing, storing, treating, infiltrating, or reusing surface water close to where it falls. This reduces flood risk, improves water quality, and limits pressure on public sewers during heavy rainfall events.

  Unlike conventional drainage, SuDS consider water quantity, water quality, amenity, and biodiversity as part of an integrated design approach.

• Who can help me with SuDS design?

  Early-stage SuDS design can be informed by architects, developers, and design teams using publicly available data such as geological mapping and flood risk information.

  However, a qualified drainage engineer or civil engineer is required to:

  1. Develop a compliant SuDS strategy

  2. Carry out drainage calculations

  3. Design infiltration systems, attenuation, and discharge arrangements

  4. Provide planning-ready technical evidence

  Engaging a drainage engineer with good early-stage site information helps reduce planning risk and redesign.

• Is SuDS a legal requirement?

  UK planning policy requires developers to manage surface water sustainably and to follow the SuDS drainage hierarchy. Local Planning Authorities and Lead Local Flood Authorities commonly mandate SuDS for:

  1. Major developments

  2. Sites within Flood Zones 2 or 3

  3. Developments over 1 hectare

  4. Sites in Critical Drainage Areas

  Where SuDS are not proposed, developers must provide clear technical evidence to justify why they are not feasible.

• What are some examples of SuDS?

  Common examples of Sustainable Drainage Systems include:

  1. Permeable paving
  2. Soakaways and infiltration trenches
  3. Swales and filter strips
  4. Attenuation basins and ponds
  5. Green roofs and blue roofs
  6. Rainwater harvesting systems

  The most appropriate SuDS solution depends on site constraints such as ground conditions, flood risk and available land.

• What is the drainage hierarchy?

  The drainage hierarchy is a UK planning framework that sets out the preferred order for managing surface water, from the most sustainable option to the least.

  The hierarchy prioritises:

  1. Rainwater reuse

  2. Infiltration into the ground

  3. Discharge to a watercourse

  4. Discharge to a surface water sewer

  5. Discharge to a combined sewer (last resort)

  Developers must demonstrate, with technical evidence such as BRE 365 infiltration testing, why higher-level options cannot be used before moving down the hierarchy.