Foul Drainage Assessment (FDA1): When You Need One

The Foul Drainage Assessment — commonly known by its form reference, FDA1 — is one of the less glamorous but critically important documents in the planning application process. It is required whenever a proposed development cannot connect to the public mains sewer for foul drainage, and its absence from a planning application is one of the most common reasons for applications being returned as invalid.

Despite this, the FDA1 is poorly understood by many applicants, and the requirements for non-mains foul drainage are frequently underestimated. This article explains when an FDA1 is required, what it should contain, how to navigate the regulatory framework, and the practical considerations for non-mains foul drainage systems.

What Is the FDA1?

The FDA1 is a standard form published by the government (available on the Planning Portal) that provides information about the proposed foul drainage arrangements for a development. “Foul drainage” means the disposal of sewage — wastewater from toilets, bathrooms, kitchens, and laundry.

The form requires the applicant to set out:

• Why a connection to the public mains sewer is not possible
• What type of non-mains drainage system is proposed
• The design and capacity of the proposed system
• The discharge arrangement (to ground via infiltration, or to a watercourse)
• The environmental and public health implications

The FDA1 is not itself a design document — it is a planning form that summarises the proposed arrangements. However, for all but the simplest installations, supporting technical information is needed to demonstrate that the proposed system is adequate and compliant with regulations.

When Is an FDA1 Required?

An FDA1 is required as part of a planning application whenever the development proposes to use non-mains foul drainage. Specifically:

New dwellings not connected to the public sewer. Any new residential property that will not be connected to the public foul sewer network requires an FDA1.

Extensions and conversions that increase foul drainage demand. If an extension or change of use increases the number of bedrooms or the foul drainage load, and the property relies on non-mains drainage, an FDA1 may be required.

Commercial and agricultural developments. Non-residential developments that generate foul drainage and are not connected to the mains sewer also require an FDA1.

Replacement of existing systems. Where an existing septic tank or treatment plant is being replaced as part of a planning application, an FDA1 should be submitted.

The key trigger is the absence of a mains sewer connection. If the development will connect to the public foul sewer, an FDA1 is not required — although the drainage strategy should still address foul drainage arrangements.

The Hierarchy of Foul Drainage Options

The regulatory framework for foul drainage in England follows a hierarchy, set out in the Building Regulations (Approved Document H) and the Environment Agency’s guidance:

1. Connection to the Public Foul Sewer

The preferred option is always a connection to the public foul sewer. Under Section 106 of the Water Industry Act 1991, property owners have a right to connect to the public sewer, subject to the water company’s approval of the connection details.

If a public foul sewer is available within a reasonable distance of the development site, the development should connect to it. “Reasonable distance” is not precisely defined, but the general expectation is that a connection should be made if a public sewer exists within the vicinity of the site, unless the cost of connection would be disproportionate.

The Environment Agency’s General Binding Rules state that a non-mains system should not be used if it is reasonable to connect to a public sewer. Planning officers and the EA will scrutinise the reasons for not connecting.

2. Package Treatment Plant with Discharge to Ground

If a mains sewer connection is not available, the next preferred option is a package treatment plant discharging treated effluent to the ground via a drainage field (also known as an infiltration system or soakaway).

A package treatment plant is a self-contained sewage treatment system that treats sewage to a standard suitable for discharge to the environment. Modern package treatment plants are compact, reliable, and produce an effluent quality that meets the standards set out in the Environment Agency’s General Binding Rules or the conditions of an environmental permit.

Discharge to ground via a drainage field is preferred over discharge to a watercourse because:

• It avoids the introduction of treated effluent to surface waters
• It recharges groundwater
• It provides additional natural treatment as the effluent percolates through the soil

3. Package Treatment Plant with Discharge to Watercourse

If discharge to ground is not feasible (because the soil does not have adequate permeability, or because groundwater levels are too high), the next option is a package treatment plant with discharge to a watercourse.

This requires either compliance with the Environment Agency’s General Binding Rules (for small domestic discharges) or an environmental permit from the EA.

4. Septic Tank with Drainage Field

A septic tank is a simpler system than a package treatment plant. It provides primary settlement of sewage but does not provide the secondary biological treatment that a package treatment plant offers. The effluent from a septic tank is therefore lower quality and must be discharged to a drainage field for further treatment in the soil.

A septic tank must not discharge directly to a watercourse. This is a legal requirement under the Environment Agency’s General Binding Rules (introduced in 2015, with a compliance deadline of 1 January 2020 for existing systems). Any septic tank that discharges to a watercourse is non-compliant and must be replaced with a package treatment plant or fitted with a secondary treatment stage.

5. Cesspool

A cesspool (or cesspit) is a sealed tank that collects sewage without treating or discharging it. The tank must be emptied regularly by a licensed waste carrier.

Cesspools are the least preferred option because:

• They require frequent and costly emptying (a typical domestic cesspool may need emptying every 4-8 weeks)
• They generate significant vehicle movements for tanker access
• They have a high risk of environmental pollution if they overflow or leak
• They are expensive to maintain over the lifetime of the development

Cesspools should only be proposed as a last resort where all other options have been demonstrated to be unfeasible.

The FDA1 Form: Section by Section

Section 1: Applicant Details

Standard applicant and site information.

Section 2: Existing Drainage

Describe the existing foul drainage arrangements on the site:

• Is there an existing non-mains drainage system?
• What type (septic tank, treatment plant, cesspool)?
• What is its condition and capacity?
• Does it comply with current regulations?

Section 3: Reason for Not Connecting to Mains Sewer

This is a critical section. You must explain why a connection to the public foul sewer is not proposed. Acceptable reasons include:

• No public sewer within reasonable distance. Provide evidence — a plan showing the site location and the nearest public sewer, ideally based on water company sewer records.
• Topographical constraints. If the site is significantly lower than the nearest sewer and pumping would be required, this may justify a non-mains system.
• Capacity constraints. If the water company has confirmed that the existing sewer does not have capacity for additional connections, this may justify a non-mains system (although the water company has a duty to provide capacity in most circumstances).

“Cost” alone is not usually accepted as a sufficient reason for not connecting. The EA expects a connection to be made if a public sewer is available, even if the connection is expensive.

Section 4: Proposed System

Describe the proposed non-mains drainage system:

• Type: Package treatment plant, septic tank with drainage field, or cesspool
• Make and model: Identify the specific product (e.g., “Klargester BioDisc BA”)
• Capacity: State the population equivalent (PE) or the number of bedrooms served
• Treatment standard: State the expected effluent quality (typically in terms of BOD, suspended solids, and ammonium)

Section 5: Discharge Arrangement

Describe how treated effluent will be disposed of:

• To ground via drainage field: Provide details of the drainage field design, including the size, location, and the results of percolation testing
• To watercourse: Identify the receiving watercourse and confirm compliance with the General Binding Rules or the terms of an environmental permit

Section 6: Percolation Test Results

If the system includes a drainage field, the FDA1 requires the results of percolation testing carried out in accordance with BS 6297:2007+A1:2008. Percolation testing determines the soil’s ability to accept treated effluent and is essential for sizing the drainage field.

The test involves:

1. Excavating a trial hole to the proposed drainage field depth (typically 300-900mm below ground level)
2. Pre-soaking the hole for at least 12 hours to saturate the surrounding soil
3. Filling the hole with water to 300mm depth and measuring the time taken for the water level to fall from 75% to 25% of the initial depth
4. Repeating the test at least three times and averaging the results

The result is expressed as a “Vp value” — the average time in seconds for the water level to fall 1mm. An acceptable Vp value is between 12 and 100 seconds per mm. Soils with a Vp below 12 are too permeable (risk of groundwater contamination); soils with a Vp above 100 are too impermeable (the drainage field will not function effectively).

Section 7: Environmental Considerations

Address the environmental implications of the proposed system:

• Groundwater protection: Is the site in a Source Protection Zone? Is the groundwater used for drinking water supply?
• Ecological sensitivity: Is the receiving watercourse or surrounding area ecologically sensitive (SSSI, SAC, SPA)?
• Flooding: Is the site in a flood zone? (A drainage field that is inundated during a flood event will not function correctly.)

The General Binding Rules

The Environment Agency’s General Binding Rules (GBRs) for small sewage discharges set out the conditions under which a non-mains sewage discharge can operate without an environmental permit. The GBRs apply to:

• Discharges of 2 cubic metres per day or less to ground (approximately 12 people equivalent)
• Discharges of 5 cubic metres per day or less to surface water (approximately 33 people equivalent)

The key conditions of the GBRs include:

• The system must be installed and operated correctly
• The discharge must not cause pollution of surface water or groundwater
• The treatment system must comply with British Standard BS EN 12566
• The drainage field must comply with BS 6297
• Septic tanks must not discharge to a watercourse (treatment plants may, subject to conditions)
• The system must not be in a Source Protection Zone 1 (for ground discharges)

If the proposed discharge does not meet the GBR conditions — for example, because it exceeds the volume threshold, or because the site is in a Source Protection Zone 1 — an environmental permit must be obtained from the Environment Agency.

Common Mistakes

Not Checking for a Mains Sewer

The most common mistake is assuming that no mains sewer is available without actually checking. Water company sewer records (available through a drainage search or direct enquiry) should be consulted to confirm the location of the nearest public foul sewer.

Proposing a Septic Tank Discharging to Watercourse

This has been prohibited since 1 January 2020 under the General Binding Rules. Any FDA1 that proposes a septic tank discharging to a watercourse will be rejected.

Inadequate Percolation Testing

Percolation tests must be carried out in accordance with BS 6297. Common errors include:

• Not pre-soaking the test hole for 12 hours
• Testing in only one location (multiple tests at different locations are recommended)
• Testing at the wrong depth
• Testing in the wrong season (testing during very dry or very wet conditions may not be representative)

Undersizing the Drainage Field

The drainage field must be sized based on the percolation test results and the expected effluent volume. Undersized drainage fields will fail, causing sewage to surface and creating a public health and environmental nuisance.

Ignoring Maintenance

Non-mains drainage systems require regular maintenance — desludging, inspection, and servicing. The FDA1 should demonstrate that adequate access is available for maintenance vehicles and that the ongoing maintenance arrangements are viable.

Locating the System Too Close to Sensitive Receptors

The Building Regulations and the General Binding Rules set minimum distances between drainage systems and sensitive receptors:

• At least 7 metres from any habitable building
• At least 10 metres from any watercourse or ditch
• At least 50 metres from any spring, well, or borehole used for drinking water supply
• At least 15 metres from any building not forming part of the development

The Environmental Permit Route

Where the proposed discharge does not meet the conditions of the General Binding Rules, an environmental permit must be obtained from the Environment Agency. This is a separate process from the planning application and can take several months.

Common reasons for needing an environmental permit include:

• The discharge volume exceeds the GBR thresholds
• The site is in Source Protection Zone 1
• The receiving watercourse is environmentally sensitive
• The proposed system does not meet the GBR conditions for another reason

The permit application requires detailed technical information, including:

• A risk assessment for the proposed discharge
• Evidence that the system will meet the required effluent quality standards
• Details of the proposed monitoring arrangements
• Evidence that the system will be properly maintained

Cost and Timescale

An environmental permit application costs between GBP 3,500 and GBP 7,500 depending on the complexity, and the EA’s determination period is typically 4-6 months. This should be factored into the project programme.

The Relationship with Planning

The FDA1 is a planning document, and the LPA will consider the foul drainage arrangements as part of the planning application. The LPA will typically consult:

• The Environment Agency — on the environmental implications of the proposed discharge, particularly for discharges to watercourses and for sites in Source Protection Zones
• The water company — to confirm whether a mains sewer connection is available
• Environmental Health — on public health implications, particularly for cesspools and for systems near residential properties

If the LPA is not satisfied that the proposed foul drainage arrangements are adequate, it may:

• Refuse the application
• Impose a condition requiring the submission of detailed drainage plans before development commences
• Impose a condition requiring connection to the mains sewer when it becomes available (known as a “connection condition”)

Practical Considerations

Site Investigation

Before designing a non-mains drainage system, carry out:

• A drainage search to confirm the location of public sewers
• Percolation testing to determine whether a drainage field is feasible
• A ground investigation to determine groundwater levels
• A topographic survey to determine the site layout and fall

System Selection

The choice between a package treatment plant and a septic tank depends on:

• Discharge destination: If discharge to a watercourse is necessary, a treatment plant is required
• Effluent quality: Treatment plants produce a higher quality effluent than septic tanks
• Running costs: Treatment plants require electricity; septic tanks do not
• Maintenance: Both require regular desludging, but treatment plants also require annual servicing
• Site conditions: The drainage field requirements differ between septic tanks and treatment plants

Cost

The typical costs for non-mains drainage systems are:

System Type	Installation Cost	Annual Running Cost
Septic tank + drainage field	GBP 4,000 - GBP 8,000	GBP 200 - GBP 400 (desludging)
Package treatment plant + drainage field	GBP 6,000 - GBP 12,000	GBP 400 - GBP 800 (electricity + servicing)
Package treatment plant + watercourse	GBP 6,000 - GBP 12,000	GBP 400 - GBP 800 (electricity + servicing)
Cesspool	GBP 3,000 - GBP 6,000	GBP 3,000 - GBP 8,000 (emptying)

These costs are indicative and will vary depending on site conditions, system capacity, and local contractor pricing.

How Aegaea Can Help

Aegaea’s civils team prepares FDA1 assessments and designs non-mains foul drainage systems for developments across England, Scotland, and Wales. We carry out percolation testing, design drainage fields, specify treatment systems, and prepare the supporting documentation for planning applications.

We also assist with environmental permit applications where the General Binding Rules do not apply, and we can advise on the most appropriate foul drainage solution for your site.

If you need an FDA1 or advice on foul drainage for a planning application, contact us for a no-obligation quote.