Maximizing Sustainable Drainage on Constrained Sites: A Guide to Infiltration SUDS Near Buildings

As urban areas become increasingly dense and local sewer systems reach capacity, the pressure to manage surface water on-site continues to grow. One of the most effective ways to do this is through infiltration Sustainable Drainage Systems (SUDS). However, on infill or brownfield sites, designers often encounter limitations—particularly when buildings and foundations are close by. In this blog post, we will explore the benefits, risks, and regulatory requirements of using infiltration devices near buildings, with direct references to UK Building Regulations Part H and the BRE Digest on permeable paving close to buildings.

1. The Importance of Infiltration SUDS

1.1. Surface Water Management Hierarchy

Under the surface water management hierarchy, the first and foremost priority is to manage surface water at source. This involves preventing runoff from leaving the site and maximizing infiltration into the ground. By mimicking natural (greenfield) conditions, infiltration SUDS:

• Reduce the volume of water discharged to public sewers

• Recharge groundwater aquifers

• Improve local water quality

• Promote sustainable water management

1.2. Common Infiltration SUDS Options

1. Soakaways or Infiltration Trenches – Concentrate runoff into a relatively small footprint where it can infiltrate through permeable subsoil.

2. Permeable Paving – Allows surface water to pass through the pavement surface, dispersing the flow over a larger area before infiltration.

3. Swales and Basins – Vegetated channels or depressions that temporarily hold and filter runoff before it soaks into the ground.

   
   

2. Regulatory Constraints: UK Building Regulations Part H

UK Building Regulations Part H provides detailed guidance for the disposal of surface water. A critical rule states that there must be no concentrated infiltration of site runoff within five metres of a building foundation. This rule is designed to safeguard foundations against the risk of washout and potential structural instability.

Key Takeaway: If you plan to install a soakaway or infiltration device within five metres of a structure, Part H typically requires you to provide evidence that it will not compromise the building’s stability.

3. Utilizing Permeable Paving Closer to Buildings

3.1. Why Permeable Paving Is Different

The BRE Digest on permeable paving close to buildings recognizes that permeable pavement systems distribute water infiltration over a larger area than traditional point-source soakaways. This approach significantly lowers the risk of ground saturation and washout around foundations.

• Enhanced Dispersion: Instead of concentrating water in one spot, permeable paving spreads surface water infiltration, reducing peak flow and localized subsoil erosion.

• Reduced Risk of Foundation Damage: By minimizing the likelihood of water pooling, the risk of damage to structures and foundations is greatly diminished.

3.2. Geotechnical Assessment Requirements

Even if the design relies on permeable paving closer than five metres to a building, a site-specific geotechnical assessment is typically required to confirm:

• The soil’s infiltration rates

• The groundwater conditions

• That infiltration near the building will not lead to soil washout or structural movement

By securing a geotechnical engineer’s report, designers can demonstrate compliance with Building Regulations and allay any concerns about foundation stability.

4. Pile Foundations and Infiltration Risk

For sites that use pile foundations, the possibility of installing infiltration devices closer than five metres remains a consideration—provided the design is supported by thorough geotechnical analysis. While piles primarily rely on side friction for load support, there is still a risk of fines being washed away from around the pile shaft if infiltration is poorly managed. A robust geotechnical appraisal can:

• Determine whether the subsoil is prone to erosion

• Propose construction methods and infiltration system designs to minimize washout risk

• Demonstrate structural stability under wet conditions

  
  

5. Cost and Capacity Benefits of On-Site Infiltration

With growing urban development and climate change intensifying peak storm events, local authorities often impose strict discharge rate limitations for new developments. Meeting these lower discharge rates may require large and costly attenuation systems if infiltration options are dismissed prematurely. Engaging a geotechnical engineer to explore infiltration closer to buildings can result in:

1. Reduced Attenuation Tank Size – By handling a portion of runoff through infiltration, you free up capacity in attenuation systems.

2. Lower Construction Costs – Smaller or fewer underground tanks and reduced pipework can translate to significant savings.

3. Improved Sustainability – On-site infiltration lessens the burden on public sewers, recharges aquifers, and helps achieve local planning requirements for sustainable drainage.

   
   

6. Practical Steps for Designers and Developers

1. Review Ground Conditions Early

   Conduct a desk study and site investigation (including infiltration testing) at the earliest stages of design.

2. Consult with Specialists

   Engage a geotechnical engineer to assess the viability of infiltration near existing or proposed foundations.

3. Explore Permeable Paving Solutions

   Consider permeable paving as an alternative to traditional soakaways, especially on constrained sites.

4. Assess Building Regulations Part H Compliance

   Check the five-metre rule and gather necessary documentation (geotechnical reports, structural assessments) to ensure compliance.

5. Coordinate with Local Authorities

   Early dialogue with planning and drainage authorities can clarify permitted discharge rates and infiltration strategies.

6. Compare Costs vs. Benefits

   Weigh the cost of infiltration SUDS (plus geotechnical fees) against constructing large attenuation tanks and paying surcharges for high discharge rates.

   
   

7. Conclusion: Making Infiltration Work on Constrained Sites

As sewer capacities tighten and sustainability becomes a higher priority, infiltration SUDS are increasingly valuable for modern developments. While UK Building Regulations Part H generally discourages infiltration devices within five metres of a foundation, BRE Digest guidance and expert geotechnical assessments open pathways to safely implement solutions such as permeable paving closer to buildings.

By optimizing infiltration on constrained sites, developers and designers can:

• Comply with tighter discharge requirements

• Minimize on-site attenuation volumes

• Improve environmental outcomes through enhanced groundwater recharge

If you’re working on a site with limited space and tough drainage constraints, our civil engineering design consultancy can help guide you through the process. We specialize in infiltration SUDS strategies, geotechnical assessments, and drainage design to create cost-effective, regulation-compliant solutions.

Ready to Optimize Your Infiltration Strategy?

Contact our team today to discuss your next project. By integrating a thorough geotechnical approach with innovative SUDS designs, we’ll help you reduce costs, manage risks, and meet local planning requirements—all while enhancing sustainability and protecting the environment.

References

• UK Building Regulations Part H: Governs the design and installation of drainage and waste disposal systems.

• BRE Digest on Permeable Pavements: Outlines design considerations and best practices for infiltration close to buildings.

• Climate Change & Urban Sprawl Studies: Highlight the growing importance of on-site water management to mitigate flooding and preserve groundwater levels.

  
  

Optimize your site’s drainage strategy while ensuring robust foundation protection. Get in touch with our civil engineering design consultancy for expert guidance on infiltration SUDS and more.