Drainage strategy or a SuDS maintenance plan

A drainage strategy and a SuDS maintenance plan answer different questions, and the choice between a drainage strategy or SuDS maintenance plan is usually not a choice at all: a major scheme needs both, at different points. The strategy proves that the surface water design works hydraulically, with discharge rates, attenuation volumes and exceedance routes. The maintenance plan proves that the system will still work in thirty years, naming who inspects it, how often, and who pays.

Lead local flood authorities reject far more submissions for the second failing than the first.

What a drainage strategy sets out

A Drainage Strategy is design evidence. Its job is to demonstrate that surface water leaving the site after development is managed to a rate and volume the receiving system can take, through a drainage hierarchy that prefers infiltration, then a watercourse, then a surface water sewer, with a combined sewer as the last resort.

The content a lead local flood authority expects is fairly consistent:

• Existing site conditions, including topography, soils, groundwater levels and any existing positive drainage.
• Infiltration testing results where infiltration is proposed, carried out to a recognised soakaway method rather than a trial pit observation.
• The proposed discharge rate with the calculation method stated, and the consent or agreement in principle from whoever receives the flow.
• Attenuation volumes for the design events, including the climate change allowance applied and its source.
• The SuDS management train, showing treatment stages appropriate to the pollution risk of the contributing surfaces.
• Exceedance routes for events beyond the design standard, drawn on a plan rather than described.
• Half drain times and the position on groundwater and any contaminated ground.

The strategy also has to agree with the rest of the application. A layout showing an attenuation basin where the drawings show plots, or a strategy assuming infiltration on a site where the geotechnical work found clay at 600 mm, is the sort of internal inconsistency that generates a holding objection in a fortnight.

Discharge rates and attenuation volumes

Two numbers dominate the strategy, and they are where the argument happens.

The discharge rate for a greenfield site is normally limited to the greenfield run-off rate, derived by a published method and stated with the method named. For brownfield, authorities usually require betterment against the existing rate, with the required reduction set locally rather than nationally, so take the figure from the authority's own SuDS guidance or drainage policy rather than from a general rule.

The attenuation volume follows from the rate, the impermeable area and the design rainfall, including the climate change uplift. The allowances are revised periodically, so take the current peak rainfall intensity allowance from the Environment Agency guidance at the point of design and state the version used. A strategy built on a superseded allowance is the easiest objection a consultee will ever raise.

Three habits make the numbers reviewable. Show the impermeable area assumed, broken down by surface type. State the design events modelled and the model used. And give the sensitivity of the storage volume to the main assumptions, because a scheme that needs every assumption to go its way is a scheme that will need a section 73 application later.

What a maintenance plan commits to

A maintenance plan is a management document, not a hydraulic one. It is the answer to the question a flood risk officer asks after approval: who will clear the silt out of this, and with what money.

It should name each drainage component on a plan with a reference, and for each one set out the inspection frequency, the maintenance tasks with their frequency, the design life and the replacement expectation, the access arrangements including vehicle access for jetting or desilting, and the party responsible. Then it should set out the funding mechanism and the arrangements for handing the information to whoever ends up owning the system.

The details that get missed are mundane and expensive. Access to a basin behind rear gardens that no vehicle can reach. Permeable paving on a private drive with no maintenance obligation on the householder. A silt trap in an adopted highway that the highway authority has not agreed to adopt. A proprietary treatment device whose replacement cartridges carry a cost nobody has budgeted.

Write it as something a managing agent could actually follow, with task sheets rather than prose. Officers read maintenance plans quickly and look for the responsible party and the frequencies.

Adoption routes and who maintains

Four routes exist in England and most schemes use more than one.

Adoption of sewers and some SuDS components by the sewerage undertaker, under section 104 of the Water Industry Act 1991 and the sector's design and construction guidance, which sets out what will and will not be adopted and to what standard. Adoption of highway drainage by the highway authority as part of a section 38 agreement. Retention by a management company funded by an estate charge, which is the default for attenuation basins, swales and permeable surfacing in private areas. And retention by individual owners, which works only for the simplest components and should be avoided for anything that needs machinery.

Decide the route early, because it drives the design. Undertakers will not adopt components outside their published criteria, so a scheme designed on the assumption of adoption and then refused it leaves the developer with a management company it did not price.

Note for Wales, which differs: Schedule 3 to the Flood and Water Management Act 2010 is in force in Wales, so construction drainage there needs approval from a SuDS Approving Body alongside planning permission, against the Welsh statutory standards. In England, as at October 2026, Schedule 3 is not in force and surface water drainage is secured through the planning system, so check the current position before relying on either arrangement.

Conditions securing each document

On a typical major permission the two documents are secured by two different conditions with two different triggers.

The strategy is usually required before development begins, through a condition requiring a detailed surface water drainage scheme to be submitted to and approved in writing by the authority. Because that is a pre-commencement condition, the written agreement requirement in section 100ZA of the Town and Country Planning Act 1990 applies, so the authority has to seek your agreement to the wording.

The maintenance plan is usually required before first occupation, through a condition requiring a management and maintenance plan to be approved and then implemented. The implementation limb matters, because it is what makes the obligation continue beyond the discharge of the condition.

Read both triggers carefully at decision stage. A condition requiring the maintenance plan before commencement is common in standard condition libraries and is almost always wrong, because the plan depends on the detailed design that the first condition is meant to settle.

Evidence the lead local flood authority wants

The lead local flood authority is a statutory consultee on surface water drainage for major development, and its comments are the ones that decide drainage conditions. What it wants beyond the calculations is evidence that the design is deliverable.

That means the third party agreements in principle: the undertaker's confirmation on discharge to a public sewer, the riparian owner's or authority's position on a watercourse connection, and any ordinary watercourse consent for works affecting a watercourse. It means the land take for the drainage shown on the layout rather than assumed. And it means a clear statement of whether the drainage sits in the same ownership as the development, because a strategy relying on land outside the application site needs either a Grampian condition or that land inside the red line.

Where a basement or a flood zone is involved the evidence base widens, and the distinction between a Basement Impact Assessment and a Flood Risk Assessment is worth getting right before commissioning either. Foul drainage runs on a separate track again, which is why a Foul Sewage Assessment sits alongside the surface water work rather than inside it.

Sequencing the two submissions

The order that works is this.

1. At application stage, submit the drainage strategy with the flood risk work, at a level of detail sufficient for the lead local flood authority to accept the principle, including rates, volumes, the management train and the land take.
2. At discharge of the pre-commencement condition, submit the detailed scheme with the final calculations, the model output, construction details and the third party agreements.
3. Before occupation, submit the maintenance plan built on the as-designed scheme, with the adoption or management route confirmed and the funding mechanism in place.
4. On completion, hand the as-built record, the model and the maintenance schedule to whoever will own the system.

Doing the maintenance plan at step one is wasted work, because the detail changes. Leaving it to step three without deciding the adoption route at step one is worse, because the design will have assumed something the adopting body never agreed to.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Working out which drainage and flood documents your scheme needs? Browse all 66 report types and start a draft.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.