Drainage evidence for a single new dwelling

One new house does not need a modelled drainage design, and any consultant offering you one has misread the scheme. What it does need is a short written statement that works down the surface water hierarchy, gives a reason for rejecting each option above the one chosen, and shows where foul flows are going. Two pages, a sketch and a percolation test result will validate almost anywhere in England. Applications for single dwellings still fail validation regularly, and the reason is nearly always that the statement says "drainage to be agreed" instead of answering the question.

What a minor application has to establish

For a single dwelling outside Flood Zones 2 and 3 and under one hectare, the lead local flood authority is not a statutory consultee. The drainage judgement sits with the case officer, sometimes with informal advice from a drainage engineer, and the officer is working from the council's local validation list rather than from a technical standard.

Most local lists ask for the same four things: the proposed method of surface water disposal, evidence supporting it, the proposed method of foul disposal, and confirmation of where each discharge ends up. That is the whole brief. A Drainage Strategy at this scale is a statement, not a design package.

The one thing to avoid is deferring everything to a condition. Officers will often grant with a pre-commencement drainage condition anyway, but a scheme that arrives with no position at all invites an invalidation notice first and a delay of several weeks.

Working down the hierarchy

National policy and Part H of the Building Regulations both put surface water disposal in the same order, and the council checks it in that order.

1. Infiltration to the ground, by soakaway, permeable paving or an infiltration trench.
2. Discharge to a watercourse, ditch or other surface water body.
3. Discharge to a surface water sewer.
4. Discharge to a combined sewer, as a last resort and normally only with the undertaker's written agreement.

Each step down needs a stated reason. "Ground conditions unsuitable" is not a reason. "Infiltration rejected: percolation testing on 14 March recorded no measurable fall over three hours in two trial pits at 1.5 metres, with groundwater encountered at 1.2 metres" is a reason, and it takes one sentence.

Discharge rates matter even on a single plot where the site is greenfield. The default expectation is that post-development runoff should not exceed the greenfield rate, with attenuation storage sized for the difference. On a one-plot infill in a garden the practical answer is usually permeable driveway construction plus a modest attenuation crate, and saying so plainly is better than producing a calculation that implies a level of certainty the site data does not support.

Testing the ground before designing the soakaway

Infiltration testing is the single most useful thing you can do before the application, and it is inexpensive. The standard method is the BRE soakaway digest procedure: at least two trial pits, three fill and drain cycles in each, measuring the time taken for the water level to fall through the middle portion of the pit, at the depth and location the soakaway will occupy.

Three practical points decide whether the result is usable. Test at the proposed invert depth, because a permeable topsoil over clay gives a flattering answer near the surface. Record the groundwater level encountered, since a soakaway base has to sit clear of the winter water table. And site the test where the soakaway will actually go, which under Approved Document H guidance means at least five metres from building foundations and clear of the neighbouring boundary.

Where the plot sits over made ground, a former quarry or a coalfield referral area, infiltration may be ruled out on contamination grounds rather than permeability. Say that in the statement, because it pre-empts the obvious officer question.

Foul drainage and the right to connect

Section 106 of the Water Industry Act 1991 gives an owner the right to connect drains to a public sewer, subject to notice to the sewerage undertaker and to the undertaker's ability to refuse where the mode of construction or condition of the drain would prejudice the system. That right covers foul flows. It is not a right to send surface water into a foul or combined sewer.

State the connection point, the approximate route and whether the connection crosses third party land, because a connection that depends on a neighbour's consent is a genuine planning consideration for deliverability. Where public sewerage is unavailable, a package treatment plant or septic tank brings in the Environment Agency general binding rules for small sewage discharges, including the volume limits and the requirement that a discharge to a surface water body goes through a treatment plant rather than a septic tank.

Where the local sewer network is known to be under pressure, a Foul Sewage Assessment with written capacity confirmation from the undertaker removes a common reason for a holding objection.

When a flood risk assessment is still required

A single dwelling normally needs a Flood Risk Assessment in any of these situations:

• The site is wholly or partly in Flood Zone 2 or 3.
• The site is one hectare or more, even in Flood Zone 1.
• The site is in an area with critical drainage problems notified by the Environment Agency.
• The site is at risk from sources other than rivers and the sea, including surface water, groundwater and sewer flooding.

That last bullet catches more single plots than the first three combined. A garden plot in Flood Zone 1 with a recorded surface water flow path across it will attract a request, and the sensible response is a short assessment that maps the flow path and sets finished floor levels above it. Dwellings are classified as more vulnerable development, so a site in Flood Zone 3 also brings the sequential test into play before anything else is discussed.

The conditions to expect

Assume two. The first requires a detailed surface water drainage scheme to be submitted and approved before development starts on site, based on sustainable drainage principles. The second requires a verification report on completion, confirming the system was built as approved.

Both are dischargeable, both cost a fee, and both take up to eight weeks. Where the drainage answer really is simple, ask the officer to condition only the verification report, or to word the condition so that the submitted statement discharges it. That request is far more likely to succeed if the submitted statement was specific in the first place. It is worth knowing what a consultee looks for in drainage output even at this scale, because the same reviewer sees both.

Schedule 3 to the Flood and Water Management Act 2010, which would introduce a separate sustainable drainage approval process, has been the subject of announcements without full commencement. Check the current position before assuming a separate approval applies to a single dwelling.

Validating first time

Submit a single drainage note covering: the surface water method with the hierarchy reasoning, the percolation test result with date and depth, an outline soakaway or attenuation size, the foul connection point, finished floor level relative to the surrounding ground, and a statement on flood sources.

Put the percolation test certificate in as a separate document rather than burying it in an appendix, and name it so the validation officer can find it. Small thing. It is the difference between a two-day validation and a three-week one.

Enter your postcode on PlanScope to see which drainage and flood documents a single plot is likely to need.

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.