Foul Drainage Strategy for Planning

A foul drainage strategy sets out where the foul water from your development will go, and shows the planning authority that the route is available, acceptable and buildable. On most sites that single question, mains or off-mains, is the whole job.

Unda has worked on flood risk and drainage across England and Wales since 2014, with more than 5,000 assessments delivered and chartered consultants on every project. Foul is usually the quieter half of a drainage submission, and it is also the half that quietly holds applications up, normally for one of two reasons: a discharge point nobody confirmed, or a water company capacity enquiry started six weeks too late to be any use to the programme.

For around nine in ten foul strategies, the work is identifying the discharge location. Almost everything else on this page follows from that one answer.

What a foul drainage strategy covers

The core question is easy to ask and often awkward to answer: where does the foul water go? A strategy answers it with evidence, then shows that the route it names can actually be built on the site you have, at the levels you have, without relying on land you do not control.

We establish the discharge route from a handful of things, checked together rather than in sequence, because any one of them can rule out the others.

• Sewer records from the sewerage undertaker, which show what is in the ground near the site, where it runs, and whether it is foul, surface water or combined.
• Site levels, because a sewer 20 metres away and 3 metres above your lowest floor is a very different offer from a sewer 20 metres away and below it.
• Site constraints: third-party land between you and the connection, a road crossing, protected trees, an existing building sitting in the only sensible line.
• Whether a mains connection is realistic as well as available. Those are two different tests, and councils increasingly ask about the second one.
• Ground conditions and groundwater, where the answer is going to be an off-mains system discharging to ground.

Out of that comes the evidence a planning application needs: a named discharge point, the reasoning that led to it, and enough supporting information for a case officer to accept the principle. Councils do not all ask for the same list, so the strategy is written against the validation requirements of the authority you are applying to rather than against a house template. National guidance on water supply, wastewater and water quality sets the policy frame the local requirements sit inside.

Is a foul drainage strategy the document you need?

Foul sits alongside three other things people arrive looking for, and because they are separate documents written for separate audiences, it is worth ten seconds to check you have landed on the right one.

• Surface water and SuDS. Rainwater is assessed by the Lead Local Flood Authority, which is the statutory surface water consultee. Foul is not its remit, and that work lives on our surface water drainage strategy page.
• A combined submission. Most applications need both, and councils often ask for a single report covering the two. Our drainage strategies hub explains how the pieces fit together and what triggers each one.
• A condition on a permission you already have. If you have consent and a drainage condition to clear, the condition wording sets the requirement. See discharge of drainage planning conditions.
• Flood risk. A flood risk assessment covers the risk to and from the site. It is a different document, though it is routinely bound together with the drainage work and quoted alongside it.

Not sure which of those you have been asked for? Send us the wording and we will tell you, at no cost.

Connecting to the mains sewer

Most schemes connect to the public foul sewer, and the first job is finding out whether that is genuinely available. Sewerage undertakers hold asset records of the public network, and while those records show you the main, they will not always show the lateral, the depth or the condition — and they will never tell you whether the connection can be made from your site without crossing land you do not own.

The 30 metre rule

There is a workable rule of thumb here, and for anyone hoping to go off-mains it is probably the most useful thing on this page.

A connection to the public foul sewer is expected where the sewer is within 30 metres, and that distance is multiplied by the number of dwellings. Ten dwellings, 300 metres.

So a single house 40 metres from a public sewer has an argument worth making, while a ten-unit scheme 200 metres from one almost certainly does not, and building an application around a package treatment plant at that distance is how people lose three months. The right to connect sits in section 106 of the Water Industry Act 1991, and the undertaker can specify the point and manner of the connection.

Capacity, and the water company enquiry

Available is not the same as adequate to take your flows. Where a network is already under pressure, the undertaker may need to confirm it can accept the additional load, and some councils want evidence that the conversation has happened before they will determine the application.

That evidence comes from a pre-development or pre-planning enquiry. Thames Water's version confirms capacity within about three weeks and issues a letter you can submit with the application; the other undertakers run equivalent services, with their own forms and fees. We run these enquiries on our clients' behalf as a matter of course.

The point worth taking from that is the lead time. A capacity enquiry is a procurement item rather than a desk study, which means you cannot do it the night before validation, and on a programme where the submission date is fixed it is one of the first things we start.

Our article on water authorities in flood risk and drainage planning sets out who does what, and sewer flooding and its planning implications covers why capacity attracts the scrutiny it does.

No mains connection: off-mains foul drainage

Where a mains connection is not available or not reasonable, the scheme goes off-mains. Three systems are in play, they behave differently, and they are not interchangeable — which is why picking the wrong one is the commonest error we see on self-build and rural applications.

A drainage field is the ground outlet for a septic tank or a package treatment plant. It is not a fourth system type, and the distinction matters, because the field rather than the tank is usually what decides whether a site works at all.

Where the discharge goes to a watercourse rather than to ground, the outfall brings its own consents with it. Ordinary watercourse consent explains that route, and our rivers index covers the watercourses themselves.

The general binding rules

Small sewage discharges are governed by the general binding rules, and a discharge that meets them does not need a permit. Where it does not, whether because of the volume, the location, or proximity to a sensitive area, a permit from the Environment Agency is needed instead, and that is a separate application on its own timescale. Our article on the general binding rules in England carries the detail; the government's septic tank and treatment plant guidance and the Environment Agency's guidance on discharges to surface water and groundwater are the primary sources.

BS 6297 is not BRE Digest 365

This one catches people out often enough to deserve its own paragraph. BS 6297 is the percolation test for a foul drainage field, BRE Digest 365 is the infiltration test for a surface water soakaway, and they are different tests run to different methods producing numbers that are not transferable between the two. A BRE 365 result will not size a drainage field, and offering one in place of the other is a reliable way to earn a request for further information.

Where a site needs both, they are commissioned together and the site work happens on one visit. Our explainer on BS 6297 testing for foul drainage fields covers what the test involves.

The Foul Drainage Assessment form

Some councils ask for a Foul Drainage Assessment, the FDA1, where an applicant does not intend to connect to the mains. We complete it alongside the strategy where it is asked for, though in our experience it is not commonly required.

Pumped foul drainage

Sometimes the levels do not work. Where the site sits below the invert of the connection point, or the fall over the distance simply is not there, foul has to be pumped to reach the sewer or the treatment system.

We flag whether a pump is likely to be needed at strategy stage, because it changes the cost and creates a maintenance obligation that runs for the life of the building, and both of those are better raised early than discovered by a contractor.

One nuance is worth having, because it saves a lot of unnecessary worry: pumping foul does not attract the pushback that pumped surface water does. Councils and LLFAs resist pumped surface water because the alternative is usually gravity to a natural destination, so a pump reads as a shortcut. Foul has no such alternative. If the levels are against you a pump is the answer, and it is treated as ordinary practice rather than as a weakness in the scheme.

How we work with you

Our aim on every strategy is the same: find the foul solution that gives the development the strongest possible route through planning while staying practical and deliverable. Those two things pull against each other less often than people expect, and where they do we say so, rather than writing a document that satisfies a case officer and then cannot be built.

In practice that means three habits.

1. Engaging early enough to matter. The discharge route has a claim on the layout. Where the pumping station sits, where the drainage field goes, which corner of the site the connection runs from. Settle it before the layout is fixed and it costs you nothing. Settle it after and it costs you a redesign.
2. Being straight about the mains question. If a connection is realistic, we will say so, even where the client had hoped for a treatment plant. If it is not, we will build the off-mains case properly rather than test the council's patience with a thin one.
3. Flagging the lead times as soon as they bite. The capacity enquiry, the percolation testing window, the permit application if one is needed. None is difficult, and all of them take longer than people budget for.

We write to the requirements of the authority you are applying to rather than to a standard template, and we are happy to work within a layout that is already fixed. That is the normal starting position, not a problem. Where a scheme's requirements are set by a particular council, our local planning authority pages and area pages are a useful place to start.

Work it out yourself

A good share of foul enquiries are from people who need a number rather than a consultant, so our drainage calculators are free to use and need no sign-up. Several of them cover exactly this ground.

• Foul drainage flow rate calculator. Flows from population or unit numbers.
• Drainage fall calculator. Fall across a run at a given gradient.
• Foul pumping and receiving chamber calculator. Pump station and receiving chamber sizing.
• Septic tank size calculator. Tank capacity for a given occupancy.
• Drainage field size calculator. Field area from percolation results.
• Cesspool size calculator. Capacity and emptying frequency.
• Manhole and inspection chamber calculator. Chamber sizing and spacing.

Frequently asked questions

Talk to us about your site

Send us the site address, a layout, and whatever the council has asked for, and we will tell you what your foul strategy needs to cover and what it will cost. Our experienced consultants work across England and Wales, and every quote comes back within 60 minutes on a working day.

Call +44 (0) 1293 214444 or email enquiries@unda.co.uk.