Living the (rural) dream? Don’t make a stink

Howes Percival

14th May 2021

Over a million properties in rural areas are unable to connect to the main sewerage infrastructure and rely instead on off-mains drainage systems. The individual property owner is responsible for the system and liable for any pollution arising from failures in the system.

So before you rush into buying that idyllic country cottage of your dreams, ask the seller whether the property’s drainage system complies with current environmental legislation.

All too often, the answer is ‘No’ (although the seller may well be unaware of this), opening up a can of worms that may delay a sale by several weeks or months, and involve considerable expense and inconvenience.

If you are planning to sell a house with an off-mains drainage system, check the rules and take any necessary remedial steps now to save yourself frustration and delays once you have found a buyer.

Off-mains drainage specialists have been particularly busy since the Environment Agency upgraded the rules last year and you may have to wait several months before they can assist you.


All homeowners need to be aware of the General Binding Rules: we have heard that the Environment Agency – the body responsible for enforcing the rules – is starting to impose heavy fines on householders for non-compliance.

We have previously reported on changes to the Rules prohibiting existing septic tank discharges into surface waters and you should refer to these and to our publication.

As a reminder, two sets of Rules regulate the discharge of small amounts of domestic waste, depending on where the sewage is discharged –

Small sewage discharges to the ground

Small sewage discharge to a surface water

There have not been any further changes to the Rules but in this article we also look at the practicalities of complying with them.


Since January 2020, septic tanks can only discharge to the ground via a drainage field: they can no longer discharge directly to surface water. This term includes a ditch, which may only occasionally contain water but is nonetheless a surface water for the purpose of the Rules.

Discharges to surface water may only continue where an existing environmental permit is in place and the Environment Agency will only grant new permits in very limited circumstances. On a sale of a property, an existing permit will need to be transferred to the new owner.


The Rules differentiate between ‘old discharges and ‘new’ discharges, imposing additional rules for new discharges.

If a householder –

i. started a new discharge from a septic tank or small sewage treatment plant on or after 1 January 2015, or

ii. had a discharge to a surface water before 1 January 2015 which they now want to change to discharge to the ground, or the other way round, or

iii. had a discharge to the ground before 1 January 2015 and now wants to install a new drainage field more than 10 metres away from the existing one, then the discharge is a ‘new’ discharge and additional rules apply.

Additional Rules

1. Where there is a public sewer within 30 metres of the property, you should apply to the relevant water company for a connection. A public sewer may have been installed since the septic tank was first installed, particularly if there have been any new developments in the area.

2. You will need to get building regulations consent to upgrade your system and may also require planning permission.


Arrange a consultation with an established off-mains drainage specialist of good repute, preferably one whose team includes a drainage engineer. Ask them to check your existing system and discuss options with you.

British Water runs an accreditation scheme for wastewater accredited technicians which you may want to check before contacting a specialist – Accreditation Certification/.

The two main options are –

1. Install a drainage field, for use with an existing septic tank, or

2. Install a small sewage treatment plant.


A drainage field consists of a specifically designed arrangement of perforated pipework, laid in trenches to allow the effluent from a septic tank to filter into the ground. They are designed to ensure aerobic contact between the effluent and the subsoil which allows treatment of the effluent through aerobic digestion.

Drainage fields cover a large area, so you will need to check whether your property has sufficient space to install one.


Drainage fields are a potential source of groundwater pollution and there are strict rules about their location, to avoid contaminating drinking water.

A drainage field must be a minimum of 15 metres from a building or 50 metres from a water abstraction point, and the field must be at least 10 metres from a watercourse or ditch, as the whole purpose of installing a drainage field is to convert a discharge to surface water into a discharge to the ground.

Check with your contractor whether any of the environmental designations listed in the Rules applies. Discharges are not permitted within a Groundwater Source Protection Zone (‘SPZ1’), and sometimes not within SPZ2 and SPZ3 zones either.

If you are unsure whether your property is affected by one of these designations, you can check whether your land has SPZ1 status by using an interactive Environment Agency map known as the Magic Map – MagicMap.

The map is quite tricky to navigate and you may be tempted to contact the Agency for assistance. If you do, take care not to alert the Agency to the fact that your current system breaches the Rules.

You should check that your choice of drainage field conforms to the relevant British Standard and refer to British Water’s List of approved equipment. A reputable specialist will only recommend products that do so.

Make sure that your contract for the works expressly confirms that the system your contractor will be installing complies with all the relevant General Binding Rules and Approved Document H2 of the Building Regulations 2010.


Drainage fields can only be installed where the soil conditions are suitable. Your contractor should carry out a series of tests to determine this. They are mandatory under the Building Regulations and involve –

• Digging a trial site assessment hole (‘TSAH’) to determine the position of the groundwater table which should never come within one metre of the bottom of the drainage field pipework.

• A percolation test to measure how quickly water drains away from the soil. This tests the porosity of the soil immediately below and surrounding the 300mm of drainage stone in the trench below the pipe. Different soil types will produce a variety of results: clay is usually unsuitable.


If a drainage field is unsuitable for your property, the Rules say that either the sewage will need to be treated by a small sewage treatment plant or you will need an environmental permit.

Small sewage treatment plants

Small sewage treatment plants (‘SSTPs’) include an additional process to remove most of the polluting materials in the sewage before it is discharged into the environment.

Because of this, SSTPs can discharge into the ground or directly to a watercourse without risk of contamination. However, they do require a power source and may require a permit.


The terms ‘soakaway’ and ‘drainage field’ are sometimes used synonymously so you must guard against contractors offering to install a crate soakaway to deal with your sewage. These are only suitable for collecting and storing rainwater.

The Environment Agency expressly forbids their use with septic tanks but that does not stop some contractors recommending them, as they are less expensive and easier to install.

If you think that your system does not apply with the Rules, or you want to upgrade and are unsure how to proceed, please contact Deborah Caldwell or Sarah Keens to discuss your options.

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