National standards for sustainable drainage systems: what you need to know

In summer 2025, the Department for Environment, Food and Rural Affairs (Defra) released updated guidance for designing surface water drainage for new infrastructure and development schemes.

These revised standards set out clear expectations for designers, property developers, local authorities, and other stakeholders, such as sewerage undertakers and the Environment Agency.

Direct runoff from the development should go to the listed destinations following the order below as much as possible:

• Priority 1: collected for non-potable use
• Priority 2: infiltrated to ground
• Priority 3: discharged to an above ground surface water body
• Priority 4: discharged to a surface water sewer, or another piped surface water drainage system
• Priority 5: discharged to a combined sewer

What’s changed?

The first principle of sustainable drainage systems (SuDS) is simple but transformative: “surface water drainage systems shall be designed, constructed, maintained and operated following a natural approach to managing water.” Gone are the days when traditional roof, pipe, sewer was considered acceptable. Under the new hierarchy, discharging to a sewer is now the fourth priority, after exploring:

1. Collection for non-potable use
2. Infiltration to ground
3. Discharge to an above ground surface water body

Previously, collection for non-potable use was not included within the hierarchy, albeit this has been introduced already by some local planning authorities (LPA’s) within local policy.

To move down this hierarchy, clear evidence and investigation must be provided to demonstrate why higher priority options are not feasible. This demonstrates the importance of ground investigations (read our blog on mitigating one of the largest potential risks to your construction project here) including infiltration testing and groundwater monitoring.  Timing matters too; winter testing may be required for results to meet statutory authority requirements.

Discharge rates: stricter rules for brownfield sites  

Another key consideration is the rate of surface water discharge from the site if a sewer connection is necessary. New developments are typically required to discharge at the QBar greenfield runoff rate, or 3l/s/ha (whichever is greater).  

Previously, LPA’s and water companies often allowed exceptions for brownfield sites, permitting discharge at a reduced percentage of the existing brownfield runoff rate. Under the new standards, this has been tightened, and previously developed sites are now expected to discharge at no more than five times the greenfield runoff rate.

Designing SuDs for amenity and biodiversity

The revised standards emphasise a site wide, integrated approach to SuDS design. High quality solutions should be planned from the earliest design stages, working hand in hand with landscape design to maximise amenity value and biodiversity.

Long-term maintenance and decommissioning should be considered alongside the scheme’s pre-construction design to ensure that amenity and biodiversity benefits are retained throughout the project lifecycle.

Is early engagement critical?

Principle 8 of the new standards encourages early consultation with Lead Local Flood Authorities (LLFAs) and LPAs to agree on design, construction, operation, and maintenance considerations. This proactive approach helps avoid delays, ensures an efficient planning process, and is especially important because these standards are already influencing consultation responses for previously submitted applications.

Does it have to be done?

Until the long-awaited enactment of Schedule 3 of the Flood and Water Management Act 2010, the revised standards and SuDS remain non-statutory.  

However, this is not an excuse to avoid them. The majority of LPAs already require SuDS through local policy, as set out in the National Planning Policy Framework, and LLFAs are well within their rights to challenge developments that fail to incorporate them. Ignoring SuDS can lead to planning delays and costly re-design or retrofit later.

The revised National standards for SuDS mark a clear shift towards further prioritising sustainable, nature-based water management. Developers who embrace these changes early, and accompany design work with a robust ground investigation, coordinated design, and proactive engagement, will avoid delays and deliver projects that enhance both amenity and biodiversity as part of an effective surface water drainage system.