Do You Need Infiltration Testing? When It’s Required for Planning

Do you need infiltration testing? You need it when your drainage strategy relies on getting surface water into the ground and your planning authority wants proof the ground can actually take it. If you are proposing soakaways, infiltration trenches or permeable paving over an infiltrating sub-base, expect to be asked for a test. Whether your particular site needs one comes down to three things: your ground conditions, the stage your application has reached, and the drainage method you are proposing.

That last point matters. Testing at the wrong time, or skipping it when it was needed, is one of the more common reasons a drainage condition stalls a planning application.

A soakaway test turns an assumption into evidence: the single figure it produces decides whether infiltration is viable and, if it is, how big the soakaway needs to be.

What infiltration testing tells you

Infiltration testing measures how quickly water drains into the ground at your site. That single figure, the soil infiltration rate, decides whether soakaways are viable and, if they are, how big they need to be. Defra's national standards for sustainable drainage systems place infiltration second in a five-tier discharge hierarchy, behind collecting runoff for non-potable use and ahead of any watercourse or sewer, so on most sites it is the first destination a planner expects you to have properly tested. A test gives you site-specific evidence instead of an assumption. For the full method and how the result feeds soakaway sizing, see our guide to BRE365 infiltration testing and soakaway design, and our explainer on the SuDS hierarchy for the five tiers in order.

One distinction before going further. This page is about surface water infiltration for soakaways, tested to BRE Digest 365. If your project is an off-mains foul drainage field serving a septic tank or treatment plant, that uses a different test, BS 6297, which we cover in our BS 6297 infiltration testing guide. The two are not interchangeable, and confusing them is an easy way to have an application held up.

Do you need a test? A quick decision guide

In short: you need a test if you are proposing infiltration drainage and your application has reached the stage where the authority wants it evidenced, unless the ground clearly rules infiltration out. The flow below walks through the same three questions we ask on every site.

When infiltration testing is required

In most cases your Lead Local Flood Authority (LLFA) will want infiltration testing wherever infiltration is proposed as the main way of managing surface water. The national standards are fairly blunt about the evidence they expect. Requirement 1.14 makes the use of infiltration drainage dependent on "the ground conditions being suitable and adequate infiltration rates being identified", and Requirement 3.14 states that geotechnical investigations "shall be undertaken" to confirm the ground can take the runoff. In practice that means a test. It tends to be asked for in a few recurring situations:

• Your strategy proposes infiltration. Soakaways, infiltration trenches, or permeable paving over an infiltrating sub-base.
• A drainage condition asks for it. Your permission carries a condition requiring infiltration data before construction can start.
• You are moving to detailed design. Outline has become detailed design, and you need to confirm the strategy actually works.

Some authorities have raised the bar further. Arun District Council tightened its drainage checklists in October 2025 so that assumed rates without test data no longer pass, and it now expects winter groundwater monitoring alongside the BRE 365 results. Our explainer on the 2025 national standards for SuDS covers what else changed. Requirements still vary by authority, but the safe assumption is that real test data will be asked for at some point if infiltration is on the table.

Schedule 3 of the Flood and Water Management Act 2010 has still not been commenced in England, so the national standards bite through planning policy rather than law. The draft NPPF would tie SuDS design to them directly.

That distinction is worth understanding if you are preparing a submission for later this year. A January 2026 ministerial answer said only that a final decision on commencing Schedule 3 "will be made in due course", so the standards remain guidance. Planning policy is where the movement is. The draft NPPF published in December 2025 carries a policy on sustainable drainage systems and watercourses requiring SuDS to be designed in accordance with the national standards, and the government's Chief Planner indicated in March 2026 that the revised framework should appear in summer 2026. The Environmental Improvement Plan published on 1 December 2025 goes further still, committing the government to consult on requiring standardised SuDS in all new developments with drainage impacts by 2029. None of that makes an infiltration assessment easier to avoid.

When you might not need it

Testing is not always necessary, or not necessary yet. You can often hold off in three situations. At outline planning stage, an indicative drainage strategy is frequently enough, with testing carried out later to support detailed design or a pre-commencement condition; our guide to what drainage information is needed for planning explains where that line usually sits. Where ground conditions clearly rule infiltration out, such as heavy clay or a high water table, you can justify discounting it without digging a pit. And on a small enough development, a desk-based infiltration assessment can show whether the option is feasible before anyone breaks ground.

A desk study on soil and geology can screen the site first: if it shows infiltration is plausible, a test confirms it; if it shows the ground almost certainly won't drain, you save the cost of a test that was only ever going to fail.

You can get an early read on the geology beneath your postcode with our ground conditions for a soakaway checker before committing to a trial pit.

How your ground conditions change the answer

The result you get depends heavily on what is under the site. Clay drains slowly or not at all; sand and gravel usually drain well; and groundwater can rule infiltration out even where the soil itself would drain. Both BRE Digest 365 and Requirement 1.15 of the national standards want the base of an infiltration feature to sit at least a metre clear of the maximum likely groundwater level on the site.

If groundwater is a known issue on your site, it is worth reading our note on groundwater monitoring for planning, because that can become a requirement in its own right. On former industrial sites, contamination can restrict infiltration whatever the soil does, since pushing surface water into contaminated ground risks mobilising pollutants.

What happens if you skip it or get it wrong

Relying on assumed infiltration rates is where applications come unstuck. If the assumption turns out to be optimistic, the drainage design fails and you are redesigning after the fact. The usual consequences are an LLFA objection, a request for the data you should have submitted, and weeks added to the programme. Poorly executed testing causes the same problems: testing in the wrong location, at the wrong depth, or in very dry or frozen conditions produces data that does not represent the site.

The fix is the same as the prevention: test in the right place, at the right depth, more than once, and let the lowest rate drive the design.

What if infiltration is not viable?

A failed test is not the end of your application. It is evidence, and planners accept it. Standard 1 of the national standards sets out the order to work down:

1. Collected for non-potable use. Rainwater harvesting, where there is genuine demand for the water.
2. Infiltrated to ground. Soakaways or other infiltration SuDS, evidenced by a test.
3. Discharged to an above-ground surface water body. A river, stream, ditch or pond, at a controlled rate.
4. Discharged to a surface water sewer, or another piped surface water system, again rate-limited.
5. Discharged to a combined sewer. Only as a last resort, where nothing above is available.

Those four words, "to the maximum extent practicable", are what an LLFA leans on. Cost alone is not a reason to move down a tier; you need evidence that each higher option was tested and found impracticable. If the ground will not infiltrate, you drop to the next viable destination and design attenuation to control the rate of discharge. Our overview of foul, surface and combined sewers explains how those discharge routes differ. Working down the hierarchy honestly, rather than skipping to the easy answer, is what discharges a drainage condition cleanly, and it is the logic behind our discharge of drainage planning conditions work.

How to find out whether your site needs it

The quickest way to settle this is to ask before you commit to a drainage method. Send us your site location and what you are proposing, and we will tell you whether testing is likely to be required, whether the ground is a realistic candidate for infiltration, and what the LLFA will expect to see. If you have already run a trial pit, our free BRE 365 infiltration rate calculator turns the timings into a design rate in seconds.

We can arrange BRE 365 testing through our network of geotechnical contractors and fold the results straight into your surface water drainage strategy, so the test data and the design arrive together rather than in two separate rounds. Start a quote and one of our flood risk consultants will get back to you within 60 minutes.

Frequently asked questions