Introduction
Sustainable Drainage Systems (SuDS) are a critical part of planning applications — especially for developments in flood-sensitive areas or where impermeable surfaces are being added. But many applicants fall into the same traps, leading to unnecessary delays or even outright rejections from the planning authority.
At Nimbus Engineering, we’ve reviewed countless SuDS submissions and worked with local councils across the UK. Here are the top 5 mistakes that developers, architects and self-builders make — and how to avoid them.
1. No BRE 365 Infiltration Testing
Why it matters:
Local authorities need evidence that you’ve tested whether infiltration-based SuDS (like soakaways) are viable. Skipping this step or using desktop assumptions is a red flag.
What to do instead:
Always carry out an on-site BRE 365 soakaway test. It proves whether water can soak into the ground and informs your strategy. Nimbus Engineering offers fast, UK-compliant testing with full certification.
2. Generic or Copy-Paste Reports
Why it matters:
Planning officers can spot a templated report a mile away. If your SuDS assessment doesn’t refer to site-specific conditions, local policies, or the actual development layout, it risks being rejected.
What to do instead:
Every SuDS report should be tailored to:
- The topography and geology of your site
- Local Lead Flood Authority (LLFA) preferences
- Specific rainfall events and climate allowances
Nimbus ensures your report reflects the actual characteristics and risks of your development.
3. Missing or Inaccurate Runoff Calculations
Why it matters:
One of the primary functions of a SuDS report is to show how surface water will be managed. Without robust calculations for greenfield and post-development runoff, authorities can’t assess your design.
What to do instead:
Use hydraulic modelling tools (e.g. MicroDrainage, Causeway Flow) to calculate:
- Pre- and post-development discharge rates
- Attenuation storage volume needed
- Flow control settings for 1-in-30 and 1-in-100-year storm events (+40% climate change)
Our team handles these in-house for fast, accurate results.
4. No Maintenance Plan
Why it matters:
Councils increasingly want to know who will maintain your SuDS features — especially if they’re above-ground like swales, basins, or permeable paving.
What to do instead:
Include a simple Maintenance & Management Plan that covers:
- The responsible party (owner, management company, council)
- Inspection intervals and cleaning tasks
- Access requirements for future maintenance
Nimbus includes this as standard with all full SuDS assessments.
5. Failing to Follow the SuDS Hierarchy
Why it matters:
The SuDS hierarchy prioritises infiltration-based systems first, then attenuation, then discharge into sewers. Ignoring this order — or jumping straight to pipes and tanks — risks rejection.
What to do instead:
Your report should demonstrate that:
- Infiltration was considered first (and ruled out if not viable)
- Above-ground, nature-based features were explored
- Only then were underground tanks or sewer connections proposed
We ensure your report walks through this hierarchy clearly and logically.
Bonus Mistake: Submitting Too Late
Don’t leave your SuDS report to the last minute. Many planning applications are held up simply because drainage wasn’t addressed early enough.
Solution:
Speak to Nimbus Engineering early in your project. We’ll advise what you need and deliver reports quickly — helping you avoid rejections, delays, or costly redesigns.
Conclusion
SuDS reports aren’t just technical paperwork — they’re critical planning tools. By avoiding these 5 common mistakes, you’ll give your application the best chance of success.
Get in touch with Nimbus Engineering today for professional, site-specific SuDS reports that planning officers trust.
