Properties that cannot connect to a public foul sewer may need an off-mains drainage system, such as a septic tank, sewage treatment plant or cesspit. These systems are common in rural locations, but owners cannot simply install a tank and discharge wastewater wherever space is available.
The main off-mains drainage rules in England cover how sewage is collected, treated and discharged. They are designed to protect groundwater, rivers, streams, neighbouring land and public health.
The exact requirements depend on the type of system, where the effluent goes, how much sewage is discharged and whether the installation is new or existing. Septic tanks and sewage treatment plants may operate without an environmental permit where all applicable general binding rules are met. If the rules cannot be met, an Environment Agency permit may be required. Cesspits are different because they are sealed storage tanks and do not discharge sewage effluent into the environment.
This article explains the main requirements for properties in England. Different environmental regulators and rules apply elsewhere in the UK.
When can a property use off-mains drainage?
Off-mains drainage is generally considered where connecting to a public foul sewer is not reasonable or practical.
The first option for foul drainage should usually be a connection to the public sewer where one is available. For a new discharge, the Environment Agency requires operators to consider whether it is reasonable to connect to a public foul sewer before relying on a septic tank or treatment plant.
Whether a connection is reasonable may depend on:
- The distance to the nearest public sewer
- Ground levels and whether gravity drainage is possible
- The cost and complexity of the connection
- Whether third-party land must be crossed
- The sewerage undertaker’s requirements
- The capacity and location of the public network
A public sewer connection may involve significant work, but cost alone does not automatically justify choosing an off-mains system. The available options should be assessed properly before a new installation is designed.
Where mains drainage is not reasonably available, the main alternatives are a septic tank, sewage treatment plant or cesspit.
What is the difference between the main systems?
The rules vary because the systems manage sewage in different ways.
Septic tanks
A septic tank separates and partially treats domestic sewage. Solids settle inside the tank, while liquid effluent leaves through an outlet.
That effluent must normally discharge into the ground through a properly designed drainage field or infiltration system. A septic tank must not discharge directly into a river, stream, ditch, canal or other surface water.
The tank needs periodic emptying because solids accumulate over time.
Sewage treatment plants
A sewage treatment plant provides a higher level of treatment than a conventional septic tank.
Depending on the system, location and applicable rules, treated effluent may discharge either:
- Into the ground through an infiltration system
- To surface water, such as a river or stream
A treatment plant discharging to surface water must meet the relevant general binding rules or operate under an environmental permit.
Many treatment plants require electricity, regular servicing and mechanical components such as pumps, blowers or control equipment.
Cesspits
A cesspit, sometimes called a cesspool, is a sealed tank that stores sewage without treating or discharging it.
The contents are removed by a registered waste carrier and taken away for treatment. Because there is no environmental discharge, the general binding rules for septic tanks and sewage treatment plants do not apply in the same way. GOV.UK confirms that separate rules apply to cesspools.
A cesspit must remain watertight and requires regular emptying. It may be considered where no acceptable discharge route is available.
Who is responsible for following the rules?
The person responsible for the system is known as the operator.
You may be the operator if:
- You own the property using the system
- Several properties share the system and you are jointly responsible
- An agreement, such as a tenancy agreement, makes you responsible for it
The operator is responsible for ensuring that the system complies with the applicable rules. This includes operation, maintenance, servicing, repairs and preventing pollution.
Where several properties share one installation, responsibility should be clearly recorded. Shared ownership does not remove the need to maintain the system or respond when a fault occurs.
What are the general binding rules?
The general binding rules are legal conditions that allow qualifying small sewage discharges in England to operate without an individual environmental permit.
They apply to discharges from septic tanks and small sewage treatment plants. The precise conditions differ depending on whether the discharge goes into the ground or to surface water. They also distinguish between existing discharges and new discharges that began, or were significantly changed, on or after 2 October 2023.
The broad requirements include:
- Using a suitable treatment system
- Operating and maintaining it properly
- Staying within the permitted daily discharge volume
- Preventing pollution
- Using an appropriate discharge route
- Meeting location and installation requirements
- Keeping maintenance records
- Using competent installers and service providers where required
- Applying for a permit when the rules cannot be met
Meeting most of the conditions is not sufficient. All applicable rules must be followed for the discharge to remain exempt from permitting.
What counts as a new discharge?
A discharge is treated as new where the system was installed or significantly changed on or after 2 October 2023.
A significant change could include alterations to the system or discharge arrangements that materially affect how it operates. New discharges are subject to additional general binding rules beyond those applying to older installations.
Property owners replacing an old tank should not assume that the replacement can automatically use the same arrangement. The new system and discharge point may need to satisfy current requirements.
How much sewage can be discharged without a permit?
The general binding rules include maximum daily discharge limits.
For a small sewage discharge operating under the rules, the commonly applicable limits are:
- Up to 2 cubic metres per day to the ground
- Up to 5 cubic metres per day to surface water
The correct daily volume must be calculated from the number of people or properties using the system and the nature of the premises. A permit is generally required where the discharge exceeds the relevant limit or cannot meet another applicable rule.
Two cubic metres is 2,000 litres, while five cubic metres is 5,000 litres. However, these figures should not be treated as a simple tank-size allowance. They relate to the maximum volume discharged each day.
Commercial, holiday accommodation and shared residential systems may serve fluctuating numbers of people. The design calculation should account for realistic peak use rather than only typical quiet periods.
Can a septic tank discharge into a ditch or stream?
No. A septic tank must not discharge directly into surface water.
Surface water includes:
- Rivers
- Streams
- Ditches carrying water
- Lakes
- Canals
- Estuaries
- Coastal waters
A septic tank only provides limited treatment. Its effluent should normally pass through a drainage field, where the soil provides further treatment before the liquid reaches groundwater.
Where an existing septic tank discharges directly to surface water, the installation may need to be upgraded or replaced with a suitable sewage treatment plant. Another potential solution is to redirect the septic tank effluent to a compliant drainage field, provided the ground and site are suitable.
The correct option depends on land availability, soil permeability, groundwater, access and the condition of the existing system.
What rules apply to discharges into the ground?
A discharge to the ground normally uses a drainage field or infiltration system.
The general binding rules require the discharge to avoid causing pollution and to comply with restrictions intended to protect groundwater and drinking water sources. Certain locations, including groundwater source protection zone 1, can require an environmental permit.
The site assessment may need to consider:
- Soil permeability
- Seasonal groundwater levels
- Nearby wells, springs and boreholes
- Watercourses
- Property boundaries
- Buildings
- Available land area
- Ground slope
- Flood risk
- Other nearby sewage discharges
A drainage field is not the same as a simple rubble-filled soakaway. Environment Agency guidance says sewage effluent should not normally discharge into a soakage pit because it concentrates the discharge and bypasses soil layers that would otherwise help treat contaminants.
Why are percolation tests important?
A percolation test helps determine whether the soil can absorb treated effluent at a suitable rate.
If the soil drains too slowly, wastewater may rise to the surface or back up towards the tank. If it drains too quickly, the ground may not provide enough treatment before the effluent reaches groundwater.
The results help calculate the size of the drainage field. The available land must be large enough to accommodate the required infiltration area while maintaining appropriate separation from buildings, boundaries and water sources.
A test performed during unusually dry or wet conditions may not represent year-round performance. The wider site and seasonal groundwater conditions should also be considered.
Can a treatment plant discharge into surface water?
A sewage treatment plant may be able to discharge treated effluent into surface water where the relevant conditions are met.
The plant must provide an appropriate standard of treatment and should conform to the British Standard that applied when it was installed. Current government guidance identifies BS EN 12566 as the relevant standard for small sewage treatment plants.
The discharge must not cause pollution, and the system must remain within the applicable daily volume limit.
A surface water discharge may need a permit where:
- The general binding rules cannot be met
- The volume exceeds the exempt limit
- The discharge is in a restricted location
- The treatment equipment does not meet the required standard
- The receiving water is environmentally sensitive
- Several nearby discharges create a cumulative concern
The presence of a stream at the edge of the property does not automatically mean it is an acceptable discharge point.
When is an environmental permit required?
An Environment Agency permit is normally needed where a sewage discharge does not qualify under the general binding rules.
This can include situations where:
- The daily discharge volume is too high
- A location restriction applies
- The discharge is within groundwater source protection zone 1
- The system cannot meet the required equipment standard
- The discharge method is non-standard
- Pollution cannot otherwise be prevented
- Other general binding rules cannot be satisfied
Government guidance states that operators should first try to meet the general binding rules. A permit is intended for circumstances where compliance cannot reasonably be achieved, rather than as an alternative to installing a compliant system.
Permit applications can require technical information about the treatment process, discharge volume, proposed outlet, groundwater and nearby environmental features.
A permit may contain conditions covering maintenance, monitoring and effluent quality. The operator must comply with all conditions once it is issued.
Are planning permission and Building Regulations approval required?
Environmental compliance is only one part of installing an off-mains drainage system.
A project may also require:
- Planning permission
- Building Regulations approval
- Environment Agency permission
- Consent from a landowner
- Approval for work near a watercourse
- Electrical certification
- Compliance with manufacturer requirements
Approved Document H provides guidance for compliance with the Building Regulations in England. It covers foul drainage, underground pipes, inspection chambers, septic tanks, treatment systems, cesspools and drainage fields.
Planning permission and Building Regulations are separate processes. Receiving planning permission does not remove the need to meet environmental rules, and meeting the general binding rules does not automatically provide planning approval.
Local planning and building control authorities should be consulted before installation begins.
What maintenance is legally required?
An off-mains system must be operated and maintained so that it does not cause pollution.
Maintenance requirements vary by system, but may include:
- Regular emptying
- Mechanical servicing
- Checking alarms and control panels
- Inspecting the drainage field
- Keeping access covers secure
- Repairing damaged pipes
- Monitoring unusual smells or wet ground
- Retaining service and emptying records
- Using registered waste carriers
Where a permit applies, its maintenance and record-keeping conditions must be followed. Government permit guidance states that septic tanks and treatment plants operating under permits must be emptied at least annually, with sludge removed by a registered waste carrier. Actual emptying frequency may need to be higher depending on the system and use.
A sewage treatment plant should also be serviced in line with the manufacturer’s instructions.
What records should be kept?
Useful records include:
- Installation documents
- Planning and Building Regulations approvals
- Environmental permits
- Manufacturer information
- Service reports
- Emptying receipts
- Waste transfer documentation
- Repair invoices
- Drainage plans
- Inspection reports
These records help demonstrate that the system has been managed responsibly. They can also be valuable when selling the property or investigating a fault.
What does preventing pollution involve?
The operator must ensure that the discharge does not pollute surface water or groundwater.
Possible warning signs include:
- Sewage smells
- Effluent appearing on the ground
- Discoloured water in a ditch or stream
- Persistent wet areas around the drainage field
- Wastewater backing up into the property
- Unusually lush vegetation over part of the system
- A treatment plant alarm
- Sludge escaping from the tank
- Dead vegetation near the discharge point
A system can fail even if wastewater is still leaving the house. For example, a saturated drainage field may allow poorly treated effluent to reach the surface.
If pollution is occurring, simply arranging another emptying may not solve the underlying problem. The tank, plant, outlet pipe and discharge area may all need investigation.
Can surface water enter an off-mains sewage system?
Rainwater should not normally be directed into a septic tank, cesspit or sewage treatment plant designed for domestic foul sewage.
Roof water, driveway runoff and land drainage can overload the system, reduce treatment performance and increase emptying requirements.
Foul and surface water drainage should therefore be separated.
Surface water may require a separate solution such as:
- A soakaway
- An infiltration system
- A surface water sewer
- A rainwater harvesting system
- Another approved discharge arrangement
Approved Document H covers both foul and surface water drainage and provides guidance on keeping these systems appropriately managed.
What happens when an existing system does not comply?
Where an existing septic tank or sewage treatment plant does not meet the general binding rules, the operator must normally either connect to the public foul sewer where reasonable or upgrade the system so it becomes compliant.
If neither option is reasonably possible, an environmental permit may be required.
Possible upgrades include:
- Replacing a septic tank with a treatment plant
- Installing a compliant drainage field
- Redirecting the discharge
- Replacing damaged tanks or pipework
- Increasing treatment capacity
- Repairing a failing outlet
- Separating surface water
- Improving access for maintenance
The solution should address the reason for non-compliance rather than only replacing the most visible component.
For example, installing a new septic tank will not solve the problem if the site has no suitable land for a drainage field.
What should be checked when buying a property?
Anyone buying a property with off-mains drainage should establish:
- What type of system is installed
- Where the tank and discharge point are located
- Which properties use it
- Who owns and maintains it
- Whether it has an environmental permit
- Whether it meets the general binding rules
- When it was last emptied and serviced
- Whether the system has caused previous problems
- Whether the drainage field lies within the property boundary
- Whether maintenance vehicles can reach it
The seller should provide relevant records where available.
A visual inspection alone may not reveal the condition of underground pipes or the drainage field. A specialist inspection can help identify defects, poor access and signs of non-compliance before the purchase is completed.
How should a new system be planned?
A new off-mains drainage installation should begin with a full assessment of the property and discharge options.
A typical process includes:
- Establishing whether a public sewer connection is reasonable
- Calculating the expected sewage flow
- Assessing the site and ground conditions
- Identifying watercourses, wells and groundwater restrictions
- Choosing the correct system type
- Designing the drainage field or discharge arrangement
- Checking planning and Building Regulations requirements
- Confirming whether an environmental permit is needed
- Planning access for installation and future emptying
- Installing and commissioning the system properly
CJ Groundworks installs septic tanks, cesspits and sewage treatment plants for domestic, commercial and rural properties.
The correct option should be chosen from the site evidence. A treatment plant may be appropriate for one property, while limited land, groundwater constraints or an unsuitable discharge route may make another solution necessary.
Frequently Asked Questions
Do septic tanks need to be registered?
Many qualifying small sewage discharges no longer require separate registration if all applicable general binding rules are met. However, an environmental permit is required in certain locations or where the rules cannot be satisfied.
Can treated sewage be discharged into a ditch?
A sewage treatment plant may discharge to suitable surface water where the general binding rules or permit conditions are met. Whether a particular ditch qualifies depends on where the water flows and the environmental circumstances. A septic tank cannot discharge directly to a ditch or other surface water.
How often should an off-mains system be emptied?
The frequency depends on the system, its capacity and how many people use it. It should be emptied often enough to prevent excess sludge affecting performance. Permit guidance requires at least annual emptying for systems covered by that guidance, although some installations need more frequent visits.
Can I install a septic tank myself?
Installation must satisfy environmental, planning and Building Regulations requirements. Excavation, levels, ventilation, pipework and the drainage field all affect performance. Professional design and installation reduce the risk of pollution and non-compliance.
Do the rules apply to shared systems?
Yes. Where multiple properties share a septic tank or treatment plant, the owners or other responsible parties may be jointly regarded as operators. They must ensure that the system complies and is maintained properly.
Meeting the rules starts with the right system
The rules for off-mains drainage systems are intended to ensure that domestic sewage is contained, treated and discharged without causing pollution.
A septic tank must normally discharge to a suitable drainage field rather than directly to surface water. A sewage treatment plant may discharge to the ground or surface water where the relevant conditions are met. A cesspit does not discharge effluent, but it must remain sealed and be emptied regularly.
Many small discharges can operate without an environmental permit under the general binding rules. However, the system must meet every applicable condition, including requirements covering capacity, location, equipment, maintenance and pollution prevention. Where compliance is not possible, an Environment Agency permit may be needed.
CJ Groundworks provides professional off-mains drainage installation across Sussex, East Sussex, Surrey and Kent. With more than 35 years of experience, the family-run team can assist with excavation, system installation, drainage fields and associated groundwork.
Call 0140 373 3214 or email chris@cjgroundworks.com to discuss your off-mains drainage requirements.
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