Soakaway Regulations in the UK

What Is a Soakaway and What Are the UK Rules for Rainwater?

A soakaway is an underground structure that collects surface water runoff from roofs, driveways, and paved areas, then allows it to disperse gradually into the surrounding soil. For rainwater, it remains the legally preferred drainage option in England where ground conditions allow. But it must meet specific technical rules under Building Regulations before it can be installed.

The legal framework sits within Approved Document H: Drainage and Waste Disposal, which is part of the Building Regulations 2010. Section H3 sets out the rainwater drainage hierarchy:

1. An adequate soakaway or other filtration system (preferred)
2. A watercourse, where a soakaway is not reasonably practicable
3. An appropriate sewer, where neither of the above is feasible

A soakaway must always be the first choice for surface water disposal, but it must not be used within 5 metres of a building or road, 2.5 metres of a boundary, or in an area of unstable land. It also must not be installed in ground where the water table reaches the bottom of the soakaway at any point of the year.

These are the minimum legal distances under Approved Document H. Some planning authorities and drainage field rules require greater separation, particularly near watercourses and boreholes.

Do Soakaways Still Need to Meet Building Regulations in 2025?

Yes. In many cases planning permission is not required for a soakaway, but building regulations still apply. Installing a soakaway as part of a new build or extension will usually require Building Control approval, even if planning permission is not needed.

The distinction between planning permission and Building Regulations trips people up. They’re separate approval processes covering different things.

A soakaway does not normally require an application for planning permission provided it deals with surface water and does not discharge into a public sewer. Additional regulations may apply if you are in a conservation area or within the boundary of a listed building. Building Regulations, however, apply regardless.

When you will typically need Building Control sign-off:

• New build or extension where drainage is being designed
• Installing a new rainwater soakaway as part of a paving, driveway, or landscaping project above a certain scale
• Replacing or upgrading a foul drainage soakaway or drainage field
• Any work where a soakaway connects to a changed drainage layout

When replacement work may not require formal approval:

If you are installing a replacement soakaway then you will generally not require planning permission. For all types of soakaway, Building Control guidelines are required to be followed.

If you’re unsure which category your project falls into, the safest route is to check with your local Building Control office before starting work. Completing a soakaway without the correct sign-off creates problems at the point of sale.

What Is a Percolation Test and Is It Required?

A percolation test is a site assessment that measures how quickly water drains into the subsoil at the proposed soakaway location. It’s required before a soakaway can be properly sized and, in most cases, before Building Control will approve an installation.

For soakaways serving areas greater than 25m², reference should be made to BS EN 752-4 or BRE Digest 365 Soakaway Design, which is the most commonly used document. BRE Digest 365 describes the design and construction procedures for soakaways and explains how to calculate rainfall design values and soil infiltration rates.

The test itself follows a standard procedure. A trial pit is excavated to the proposed invert depth. It’s filled with water, allowed to drain, then refilled. The rate at which water drops over a measured period gives the soil infiltration rate, expressed as metres per second.

Soakaways should discharge from full to half-volume within 24 hours, according to BRE Digest 365 guidance. If the soil fails to meet this requirement, a conventional soakaway won’t work in that location.

A BRE365 soakaway test is necessary to assess the suitability of a new or substitute soakaway for surface water discharge, while a BS6297 percolation test is required when a septic tank or sewage treatment plant is being proposed for foul water treatment. These are different tests for different purposes. Mixing them up leads to costly design errors.

For small soakaways draining 25m² or less, a design rainfall of 10mm in five minutes may be assumed to give the worst-case scenario as detailed in the Approved Document. Larger systems need the full BRE Digest 365 calculation.

Can You Use a Soakaway for Foul Drainage or Septic Tank Effluent?

No. This is one of the most important distinctions in UK drainage regulation, and it’s one of the most commonly misunderstood.

You cannot use a soakaway (designed for draining rainwater), well, or borehole for discharging effluent to ground. Instead you must either upgrade to a drainage field or apply for a permit so that the Environment Agency can assess the risk of using this sort of system in your location.

This rule was formalised when the General Binding Rules for small sewage discharges came into force on 1 January 2015 under the Environmental Permitting (England and Wales)(Amendment)(England) Regulations 2014. The general binding rules set out the conditions that allow a septic tank or sewage treatment plant to be used without an environmental permit.

A drainage field is not the same thing as a soakaway. A drainage field, also called an infiltration system, consists of perforated pipes laid in trenches at a specific depth. It’s designed to allow treated effluent to disperse into the soil over a much larger surface area, with sufficient distance from groundwater, buildings, and watercourses.

Systems must conform to BS EN 12566 (sewage treatment plants) and BS 6297:2008 (drainage fields).

If you’re connected to a septic tank and you’ve been using a rainwater soakaway as the discharge point, you’re not compliant. Your local authority or the Environment Agency can require you to upgrade. Properties with septic tanks in Cornwall are common, particularly in rural areas where mains drainage hasn’t reached. If you’re unsure about the status of your system, a septic tank survey is the right starting point.

Why Do Soakaways Fail in Cornwall, and What Does That Mean for Compliance?

Ground conditions in Cornwall make soakaway performance harder to predict than in most parts of England. Passing the regulatory requirements on paper isn’t enough if the underlying geology works against you.

Permeable sub-soils such as sands and sandy loams are usually good news for soakaways, whereas hard igneous rocks such as granites and basalts, and heavy claggy soils may mean a soakaway is not going to be feasible.

Cornwall sits on precisely those difficult materials.

Heavy Clay Subsoils

Clay is the most common cause of soakaway failure in the county. Clay’s low permeability means water has nowhere to go, leaving you with a failed drainage system and potential flooding issues. A percolation test in clay will often return an infiltration rate far too slow for a compliant soakaway to be viable. In that scenario, the regulations require you to move to the next option in the hierarchy: discharge to a watercourse, or connection to a sewer where one is available.

Granite Bedrock

Much of west Cornwall sits on Cornish granite. If a soakaway is not placed deep enough to reach well-draining soil, it will not function as intended. It is not uncommon in areas with impermeable bedrock to encounter a layer of clay over the top, and if the permeable layer is not reached, the soakaway will fail.

Many properties across the Penwith and Kerrier areas hit granite within a metre or so of the surface, which severely limits both the depth and the viability of a conventional soakaway design.

High Seasonal Water Tables

The base of a soakaway should be 1.0m above the water table. Areas with a high water table can be liable to groundwater flooding, and even water below the surface can prevent infiltration if it’s too high.

Cornwall’s coastal location and high annual rainfall push water tables up through winter. A soakaway that drains adequately in August can sit in saturated ground by January. This is why percolation tests should be conducted in winter or early spring, not in dry summer conditions. A test carried out during a dry spell gives a misleading picture of how the system will perform when it’s under pressure.

These ground conditions also affect garden drainage solutions more broadly. Waterlogged gardens, lifting paving, and damp creeping towards buildings are often symptoms of exactly the same geological factors that make soakaways unreliable.

What Are the Exact Distances and Siting Rules for a Rainwater Soakaway?

The siting requirements under Approved Document H are not guidelines, they’re legal minimums. A soakaway installed in breach of these rules is non-compliant and can be required to be repositioned.

Approved Document H minimum distances for rainwater soakaways:

• At least 5 metres from any building or road
• At least 2.5 metres from a boundary
• Not in an area of unstable ground
• Not where the water table reaches the base of the soakaway at any point during the year

Additional distances that apply near septic drainage fields under the General Binding Rules:

• At least 10 metres from a watercourse, 15 metres from a building, and 50 metres from a borehole or spring. (These apply specifically to drainage fields discharging effluent, not to standard rainwater soakaways, but are worth noting if your property has both systems.)

Beyond distance rules, the soakaway must be sized to handle the drainage area it serves. Soakaways are probably the most common form of surface water disposal and are usually suitable for areas less than 100m². Larger areas require more detailed design calculations under BRE Digest 365, taking into account the drainage area, the 1-in-100-year rainfall event, and a 40% climate change allowance.

Undersized soakaways are among the most common compliance failures. A system that just about copes in a normal year can overflow in a heavy winter, sending water towards foundations, neighbours’ land, or a nearby watercourse, all of which carry their own legal consequences.

How Clear Stream Handles Soakaway Assessments and Garden Drainage

Clear Stream carries out soakaway assessments as part of its garden drainage service, working across Cornwall and Devon. The starting point is always a site assessment to understand what’s actually happening underground, not just on the surface.

Ground Assessment Before Any Design

Before recommending a soakaway, the team assesses the site for the ground conditions that determine whether one is viable. Across Cornwall, that means identifying clay depth, proximity to granite, seasonal water table behaviour, and available land within the required setback distances. Where a soakaway won’t work, alternatives are proposed honestly rather than pushing ahead with a system that’ll fail.

CCTV Survey Integration

Where an existing soakaway has failed or is suspected to be non-compliant, a CCTV drain survey can identify whether the problem is structural, a blockage, or a fundamental ground capacity issue. This matters because replacing a failed soakaway like-for-like, in the same location, won’t fix a performance problem caused by poor soil.

Septic Tank Compliance

For rural properties with septic tanks, Clear Stream checks whether the discharge point is a rainwater soakaway (no longer compliant for effluent) or a properly designed drainage field. Where the distinction has been blurred over decades of incremental patching, a septic tank survey establishes the full picture before any work begins.

What you get with Clear Stream:

• No call-out fee to assess the site
• Fixed-price quote before work begins
• 1-2 hour response across Cornwall and Devon
• Engineers with direct experience of Cornwall’s clay, granite, and coastal ground conditions
• 5-Year Guarantee on all repair work

Clear Stream covers Falmouth, Truro, Penzance, Redruth, Camborne, St Austell, Newquay, Bodmin, and the full extent of Cornwall, as well as Devon.

Conclusion

Soakaway regulations in the UK sit across two pieces of legislation: Building Regulations Approved Document H, which governs the technical siting and sizing of rainwater soakaways, and the Environment Agency’s General Binding Rules, which prohibit soakaways from being used for foul drainage or septic tank discharge.

Getting it right means understanding which type of soakaway you need, conducting a proper percolation test, meeting the minimum distance requirements, and securing Building Control sign-off where required. In Cornwall, it also means understanding the ground. Clay, granite, and high water tables regularly rule out a standard soakaway altogether, requiring an alternative drainage strategy.

If you have questions about an existing soakaway, are planning a new installation, or suspect a system on your property may not be compliant, call Clear Stream on 01872 222555. We’ll assess the site, explain your options clearly, and give you a fixed-price quote before any work begins. No call-out fee, no pressure.

Visit clearstreamdrainage.co.uk for the full range of drainage services across Cornwall and Devon.

Frequently Asked Questions

Do I need planning permission to install a soakaway?

In most cases, no. A rainwater soakaway that handles surface water only and does not discharge to a public sewer generally falls outside the requirement for planning permission. However, if you’re in a conservation area, near a listed building, or installing a foul drainage field for the first time, planning permission may apply. Building Regulations still apply regardless of whether planning permission is needed, and Building Control sign-off is usually required.

Can I use a soakaway for my septic tank?

No. Since January 2015, the Environment Agency’s General Binding Rules have prohibited the use of a rainwater soakaway as a discharge point for septic tank effluent or sewage treatment plant output. Foul drainage must discharge to a properly designed drainage field built to BS 6297:2008. If your system currently discharges to a rainwater soakaway, it is non-compliant and should be assessed and upgraded.

How do I know if the ground is suitable for a soakaway?

A percolation test, carried out following BRE Digest 365 guidance, is the standard method for assessing ground suitability. A trial pit is excavated, filled with water, and the rate of infiltration is measured. Clay-heavy ground and sites underlain by granite or other impermeable rock types will often fail this test. In those cases, the regulations require a move to the next option in the drainage hierarchy: a watercourse connection or sewer, where available.

How far does a soakaway need to be from my house?

Under Approved Document H of the Building Regulations, a rainwater soakaway must be at least 5 metres from any building or road, and at least 2.5 metres from a boundary. It must also not be installed where the seasonal water table reaches the base of the soakaway. Drainage fields serving septic tanks operate under different and more restrictive setback distances.

What happens if my soakaway doesn’t meet regulations?

A non-compliant soakaway can be required to be repositioned, resized, or removed by Building Control or the local planning authority. If it’s causing pollution or discharging the wrong type of water to ground, the Environment Agency also has enforcement powers. Beyond regulatory risk, an incorrectly installed soakaway will eventually fail, sending water back towards the building or into neighbouring land. If you suspect a problem, a drainage survey is the right starting point before the issue escalates.

Is a soakaway the same as a drainage field?

No. A soakaway is designed to disperse rainwater and surface water runoff into the soil. A drainage field is a separate system, built from perforated pipes in trenches, designed specifically to receive treated effluent from a septic tank or small sewage treatment plant. They are governed by different regulations and built to different standards. Using a rainwater soakaway for foul drainage is a common compliance failure on older rural properties.