Who can design and certify SuDS on a small site

No licence exists for sustainable drainage design in England. Anyone can put a soakaway on a drawing. What decides whether the council accepts it is the method behind the numbers, the ground investigation that supports the infiltration rate, and whether the person who signed the drawing carries professional indemnity insurance that would respond if the system floods a neighbour.

For a small site, one or two plots up to nine dwellings, you are usually appointing a civil or drainage engineer rather than a large multidisciplinary firm. The question is what to ask them for.

What the drainage design has to show

Whatever it is called locally, the submitted document has to answer four things. Where does surface water go now, where will it go after development, how much will there be in the design storm including an allowance for climate change, and who keeps the system working for the next sixty years.

The order of preference is fixed. Discharge to ground first, then to a surface water body, then to a surface water sewer or highway drain, and only as a last resort to a combined sewer. Each step down the hierarchy has to be justified by evidence rather than convenience, which is why infiltration testing usually has to happen before the drainage layout is drawn, not after.

A Drainage Strategy that jumps straight to a connection into the public sewer, with no infiltration testing and no explanation, is the single most common reason a small site drainage submission comes back.

Competence for infiltration testing and design

There is no statutory competent person scheme here, unlike biodiversity net gain or protected species work. Councils judge competence by proxy, and so should you.

Look for chartered civil engineer status through the Institution of Civil Engineers, or membership of the Chartered Institution of Water and Environmental Management, together with named experience of schemes in the same lead local flood authority area. Local standards vary more than most people expect, and an engineer who has been through the process in your county will know whether the authority insists on a 1 in 100 year plus 40 per cent climate change allowance, what discharge rate it accepts for a greenfield site, and whether it will look at a scheme without a maintenance plan attached.

For infiltration testing, ask which method will be used. BRE Digest 365 sets out the soakaway design method and the trial pit test that supports it, with three separate fills of the pit and the rate taken from the slowest. A single test in one trial pit, at a depth unrelated to the proposed soakaway, is not evidence. The tests also need someone who can recognise groundwater when they see it, which is where a Geotechnical Assessment and the drainage design begin to overlap.

The lead local flood authority as a consultee

Lead local flood authorities became statutory consultees on surface water drainage for major development in April 2015. Major means ten or more dwellings, 1,000 m2 or more of floorspace, or a site of 0.5 hectares or more.

Below that threshold your scheme is minor development, and the authority is not obliged to comment. Many still do, either because the council refers everything in a critical drainage area or because the case officer asks. Some publish local standards that apply to all development regardless of size, and those standards are the document to find before design starts.

The practical consequence for a self-builder is that on a single plot you may never hear from a drainage engineer at the council at all. Your design still has to work, and a condition requiring details to be approved before commencement will bring the scrutiny later, when changing the layout is expensive.

Adoption and long term maintenance

Ask this question at the start, because it shapes the design.

1. Adoption by the sewerage undertaker under section 104 of the Water Industry Act 1991, which requires the drainage to be built to the sector's design and construction requirements and generally rules out features the undertaker will not adopt.
2. Adoption by the highway authority as part of a section 38 agreement, limited to drainage serving the adopted highway.
3. A management company funded by estate charges, common on larger schemes and disproportionate on small ones.
4. Private maintenance by the householder, with the obligations secured by a planning condition and recorded in a maintenance plan.

On a one to three plot site, option four is the realistic answer, and the council will want to see a plan naming every component, the inspection frequency and the replacement interval. Permeable paving that nobody sweeps silts up within a decade. Say who sweeps it.

Schedule 3 of the Flood and Water Management Act 2010, which would create SuDS approving bodies and a separate approval route, has not been brought into force in England as at September 2026. Government has stated an intention to implement it. Check the current position before you assume a scheme will or will not need separate approval.

Certification and as-built evidence

Discharging the drainage condition is where small sites come unstuck. The typical wording asks for the system to be constructed in accordance with the approved details and for written confirmation to be submitted before occupation.

What satisfies it varies, but expect to need as-built drawings, photographs taken during construction showing the geotextile, the sub-base and the pipework before they were covered, a signed statement from the engineer who inspected the works, and the maintenance plan. Photographs after backfill prove nothing. Tell your groundworker on day one that the drainage build will be photographed at each stage, and put it in their scope.

If the built system deviates from the approved drawing, deal with it through a non-material amendment or a revised condition discharge rather than hoping the difference goes unnoticed. Building control and planning look at different things, and a building control sign-off is not a planning discharge.

Where a specialist is unavoidable

Infiltration into ground that may be contaminated needs input from a contaminated land specialist before the design is fixed, because driving surface water through contaminated soil can mobilise pollutants into groundwater. The related question of who is qualified for intrusive ground investigation is covered in the article on who can carry out a Phase 2 ground investigation.

Sites in a groundwater source protection zone, sites over a principal aquifer and sites with a history of industrial use all fall into this category. So do sites where the water table sits close to the base of the proposed soakaway, since infiltration into saturated ground does not work no matter what the test said in August.

Where flood risk is also in play, the drainage design has to sit consistently with the Flood Risk Assessment rather than contradicting it, and both should be produced by people who have spoken to each other.

Briefing the design

Give the engineer the topographical survey, the proposed site layout with levels, the ground investigation report, the local flood authority standards and the council's validation checklist. Ask for the infiltration test results as raw data as well as a headline rate. Ask what happens in an event larger than the design storm and where the water goes when it exceeds the system, because officers increasingly ask that question and a blank look is a delay.

PlanScope can draft the narrative document that explains the strategy, with AI assistance and policy structure, ready for your engineer to check the calculations and sign off. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and the professional review route exists for exactly that. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.