Competence for a foul sewage assessment

Who writes a foul sewage assessment matters less than whether the person doing it has carried out the percolation test correctly and read the undertaker's response properly. There is no licence and no competent person scheme for the document. What there is, on a rural plot, is a chain of technical facts that decide whether the scheme works: the distance to a public sewer, the infiltration rate of the subsoil, the depth to groundwater, the size of drainage field the ground can accommodate, and whether the discharge falls within the general binding rules or needs an environmental permit.

Any of those can stop a single plot scheme, and all of them are findable before an application is submitted.

What the assessment has to establish

Four answers, evidenced rather than asserted.

Whether a public foul sewer is available, where it is, and what the undertaker says about connecting to it. The right to communicate with a public sewer under section 106 of the Water Industry Act 1991 is subject to the undertaker's conditions on the manner of the connection, so a written response is the evidence, not a map.

If no sewer is available, which option in the hierarchy applies and why the preferred options were discounted.

If the discharge goes to ground, the percolation rate from a properly conducted test, the depth to the highest groundwater level, and the drainage field area that follows from them, shown to fit on the land available at the required separation distances.

Whether the discharge complies with the general binding rules or requires a permit, with the reasoning stated.

The foul drainage hierarchy and justifying a departure

National guidance sets a hierarchy and expects it to be followed in order: connection to a public sewer first, then a package sewage treatment plant, then a septic tank with a drainage field as the last resort.

Authorities and building control bodies read the hierarchy strictly for new development. A public sewer within a reasonable distance of the site is treated as available, and in practice the conventional threshold used is around 30 metres, with the figures and the test set out in the drainage Approved Document. Connection cost on its own is rarely accepted as a reason to depart, because the alternative imposes a long-term environmental cost on everybody else.

So the departure case has to be real. Genuine reasons include no sewer within reach, levels that make gravity connection impossible and pumping disproportionate, a sewer that crosses third party land with no prospect of an easement, or an undertaker response confirming there is no capacity and no reinforcement programmed.

Where the answer is a treatment plant rather than a septic tank, say so clearly in the assessment. New discharges to a surface water body cannot rely on a septic tank at all, which is the single most common misunderstanding on rural plots.

Percolation testing and who carries it out

The test method is prescribed, and a test carried out any other way is worthless.

In outline, trial holes are dug to the depth of the proposed drainage field distribution pipes, with a square section hole at the base. Water is added to a stated depth and the time taken for the level to fall between stated marks is recorded, repeated so that an average can be taken, across more than one hole. The result is converted into a percolation value expressed in seconds per millimetre.

Two conditions are routinely ignored and both invalidate the result. The test should not be carried out during heavy rain, frost or after prolonged dry weather, because the soil condition is unrepresentative. And a separate trial hole has to be dug deeper to establish the highest level of groundwater, which means testing in the wettest part of the year or interpreting evidence of seasonal water levels. A field designed on a July test in clay can sit in standing water by February.

The fieldwork itself is simple enough that a competent contractor can carry it out. The interpretation is not. Whoever signs the assessment has to be able to defend the number, explain the conditions on the day and justify the design that follows, which is why a drainage engineer or an environmental consultant is the usual appointment rather than a groundworker with a stopwatch.

Drainage field design competence

The field area follows from the percolation value and the number of occupants, through the formulae in the drainage Approved Document, with a different factor applied depending on whether the effluent has been through secondary treatment.

Design competence means getting six things right at once.

• The area, calculated from the percolation value and population rather than from a rule of thumb
• Separation from buildings, boundaries, watercourses, springs, wells, boreholes and any road, to the stated distances
• At least a metre of unsaturated soil between the base of the field and the highest groundwater level
• Distribution that spreads the flow evenly across the field rather than loading one end
• Land that will never be built over, surfaced or trafficked, and is shown as such on the layout
• A realistic plan for desludging access, which means a tanker route and a hose run within reach

The commonest failure on small plots is simply space. A high percolation value in clay produces a field larger than the garden, and the answer is then a treatment plant discharging to a watercourse with the appropriate consent, or a different site layout, or occasionally a shared system. Better to find that before the plot is bought.

Keep the foul drainage design and the surface water design in step, because they compete for the same land. The attenuation feature and the drainage field cannot occupy the same corner, and the drainage strategy should show both.

General binding rules and when a permit is needed

Small sewage discharges in England are regulated either by general binding rules, which require no permit provided every rule is met, or by an environmental permit.

The rules differentiate between discharges to ground and discharges to surface water, set volume limits for each, require the system to be of an appropriate type and standard, and impose maintenance and record keeping obligations on the operator. Discharges above the volume limits, or in sensitive locations such as a groundwater source protection zone or close to a well, spring or borehole, or affecting a protected site, fall outside the rules and need a permit.

Read the current rules rather than a summary, because they have been changed more than once and the detail of the distance and volume thresholds is exactly what decides the answer. Where a permit is needed, factor in the determination period before assuming the scheme can start on site.

Register the discharge where the rules require it, and tell the client that the obligation transfers with the property.

Where the undertaker's answer decides it

The sewerage undertaker's written response is the single most useful document in the assessment, and it is free.

It will confirm the location and size of public sewers, whether there is capacity, whether a connection is acceptable at the point proposed, whether a requisition for a new sewer would be needed, and whether a build-over agreement is required where the proposal sits near an existing asset. On a site where the answer is positive, that response ends the hierarchy argument in one page.

Get it early and attach it. An assessment that reasons its way to a septic tank without a written undertaker response invites the officer to ask for one, and that costs a month. The same discipline applies to water supply, where the water supply assessment rests on an equivalent enquiry.

Make sure the report names its author and carries a date, with the test date and weather conditions recorded, because the percolation result is only meaningful alongside the conditions it was obtained in.

Building control after the permission

Planning permission is not the end of it. The installation is a building control matter under the drainage Approved Document, and the body inspecting it will want to see the field as constructed, the invert levels, the distribution arrangement and the commissioning of any treatment plant.

Advise clients to keep the installation record, the commissioning certificate and the maintenance schedule together. Desludging intervals, servicing records and the discharge registration are what a buyer's solicitor asks for years later, and a system with no paperwork is treated as a defect.

Where the draft needs a qualified reviewer, the professional directory is the route to a review request and sign-off, and a foul sewage assessment drafted in advance is a starting point for that reviewer rather than a replacement for them.

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.