Drainage for a single plot: what is proportionate

Single plot drainage requirements are far lighter than most self-builders are told. For one new dwelling the lead local flood authority is not a statutory consultee, there is no requirement for hydraulic modelling, and the information the planning authority needs is usually a drainage layout showing how foul and surface water leave the site, an infiltration test result if a soakaway is proposed, and a soakaway sized by a recognised method. That is it, unless the site is in a flood zone or has a specific local constraint.

What goes wrong is that the applicant is handed a validation checklist written for major development and assumes all of it applies.

What one dwelling has to show

The planning authority's interest is whether the drainage can work without causing flooding elsewhere and without an unacceptable discharge. Four things answer that.

The discharge route for surface water, following the hierarchy in Approved Document H of the Building Regulations: infiltration first, then a watercourse, then a surface water sewer, and a combined sewer only as a last resort.

Evidence that the chosen route is available. For infiltration that means a test. For a watercourse it means the riparian owner's position and a consent for any works affecting an ordinary watercourse. For a sewer it means the undertaker's position.

The size of the proposed soakaway or storage, with the calculation shown.

The foul water route, which is either a connection to the public sewer or an off-mains system.

Put those on one drawing with levels, pipe sizes and gradients. A drawing beats a report on a single plot, every time.

Why major standards get misapplied

Three habits cause it.

Validation checklists are written once and applied to everything. A checklist that asks for a drainage strategy including greenfield runoff calculations, attenuation design, exceedance routing and a maintenance plan is aimed at a ten-unit scheme, and an officer applying it literally to one house is following the document rather than the policy behind it.

The lead local flood authority's standards document is usually written around major development too, and it reads as though it applies universally. Below the major threshold of ten dwellings, 1,000 m² or 0.5 hectares, that authority has no statutory consultee role at all.

And applicants over-provide out of caution, which then sets a local precedent and raises expectations on the next application.

If you are asked for a full drainage strategy on a single plot, ask which policy requires it and which part of the validation list applies. A polite, specific question often resolves it. Where the site does have a real constraint, provide the information rather than arguing.

Sizing a soakaway for a small roof and drive

The standard method for small soakaways is the approach set out in the building research digest on soakaways, which balances the volume of water entering the soakaway in a design storm against the volume it can discharge into the ground over the same period.

You need three inputs. The impermeable area draining to it, which is the roof plan area plus any hard surfacing. The soil infiltration rate, measured by a trial pit test rather than estimated from a geological map. And a design rainfall event, conventionally the 1 in 10 annual probability event for a domestic soakaway, with a longer return period where the consequence of failure is higher.

Illustrative example. A house with 90 m² of roof plan area and 40 m² of drive gives 130 m² of impermeable area. On a sandy soil with a good infiltration rate, a soakaway of modest volume will empty between storms. On a clay soil the test will often show that infiltration is not viable at all, and no amount of extra volume fixes that. The figures here are illustrative and any real design needs a measured infiltration rate.

Two practical points. Site the soakaway at least five metres from the building, further on shrinkable clay, and clear of foundations, drains and boundaries. And carry out the infiltration test at the depth and position the soakaway will occupy, because the result at 0.5 metres tells you nothing about conditions at 2 metres.

Where infiltration fails, the alternatives are a lined attenuation feature with a controlled outfall, a connection to a watercourse, or a sewer connection. Each needs third-party agreement, which is why testing early matters.

Where a flood risk assessment is required anyway

Proportionality has limits, and this is the main one.

A site-specific flood risk assessment is required where the site is in Flood Zone 2 or Flood Zone 3, or where the site is one hectare or more even in Flood Zone 1. Minor development and changes of use in Zones 2 and 3 are not subject to the sequential and exception tests in the way larger proposals are, but they still need a site-specific assessment.

A flood risk assessment for a single plot is a modest document. It identifies the zone from the flood map for planning, obtains the Environment Agency's detailed flood level data where available, sets finished floor levels with freeboard above the design flood level including the relevant climate change allowance, shows safe access and egress, and sets out resilience measures.

Surface water flood risk is separate from river and sea flood risk, and a site in Flood Zone 1 can still sit in a surface water flow path. Check the long-term flood risk service as well as the planning flood map, and check whether the authority has designated a critical drainage area.

Keeping foul and surface water separate

Approved Document H requires separate systems where practicable, and planning authorities increasingly condition it.

Connecting a new roof to a combined sewer because it is the nearest pipe is the single most common drainage mistake on infill plots, and it is the one the water undertaker objects to. Check what the existing drainage is before you design the new.

Off-mains foul drainage needs its own attention. A package treatment plant or septic tank discharging to ground or to a watercourse is subject to the environmental regulator's rules, either under general binding rules or under a permit where the discharge exceeds the thresholds or sits in a sensitive location. Establish which applies before you submit, because a discharge that needs a permit and cannot get one is a scheme-stopping problem. A foul sewage assessment is the document that sets the position out where the connection is not straightforward.

What to put on the drainage layout

Nine items, on one sheet.

1. Existing and proposed levels to ordnance datum, including finished floor level.
2. All impermeable areas with their square metres marked.
3. Surface water pipe runs with sizes, gradients and invert levels.
4. Soakaway position, dimensions and construction, with the calculation in a note.
5. The infiltration test position, date and measured rate.
6. Foul drainage route, with the connection point or treatment plant position.
7. Manhole and inspection chamber positions with cover and invert levels.
8. The discharge point for any outfall, with the owner identified.
9. An exceedance note saying where water goes if the system is overwhelmed.

Item nine takes one sentence and answers a question officers often ask. Where the overland route is towards a neighbour's garden, say what the levels do about it.

Avoiding an unnecessary consultant appointment

For a straightforward plot on permeable ground in Flood Zone 1, a competent designer or a builder working with a drainage supplier's sizing tool can produce what the application needs, and a hole dug for an infiltration test costs very little.

Appoint a consultant when the site is in Flood Zone 2 or 3, when infiltration has failed and you need a controlled discharge, when the site is in a critical drainage area or on a surface water flow path, when the plot is on a steep slope where runoff concentrates, or when the authority has raised a specific drainage objection. Our note on managing runoff on a steeply sloping site covers that last case.

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This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.