Summary

Attenuation tanks exist to solve a specific planning and drainage problem: when a site is developed — a new build, an extension, a driveway, or any works that increase the impermeable area of a plot — the rate at which rainwater runs off that site increases compared to the undeveloped (greenfield) condition. Sewers, watercourses, and downstream drainage systems are sized around the slower, more gradual runoff of undeveloped land. Left unmanaged, increased hard surfacing from development contributes to surface water flooding both on-site and downstream.

SuDS policy addresses this by requiring developments to control the rate and, increasingly, the volume of surface water discharge to no worse than the pre-development condition. An attenuation tank is one of several ways to achieve this — alongside soakaways (where ground conditions allow infiltration, see soakaway installation guide), permeable paving, and above-ground storage such as ponds or swales. Where infiltration isn't viable (heavy clay, high water table, contamination risk, or simply insufficient garden space) and there's no suitable watercourse for direct discharge, an attenuation tank connected to a flow-controlled outfall to the public sewer or watercourse is often the practical solution.

The most common failure mode on domestic and small-commercial jobs isn't the tank itself — it's getting the sizing calculation and the flow control device specification wrong, either through a shortcut sizing assumption or by fitting the wrong flow control component. An undersized tank floods on-site during a design storm; an incorrectly set flow control device discharges too fast and fails the planning condition it was installed to satisfy, which can hold up a final Building Control sign-off or a section 104/106 sewer adoption agreement.

Key Facts

Quick Reference Table

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Tank type Typical void ratio Best suited to Relative cost
Modular plastic crate 90–95% Domestic/small commercial, irregular footprints, non-trafficked or trafficked (load-rated crates) Lower
Precast concrete ~100% (chamber volume) Larger capacity, heavy trafficked loading, longer design life Higher
GRP tank ~100% (chamber volume) Defined smaller capacity, single-piece delivery, tight access sites Mid–higher
Design storm event Purpose
1-in-30-year Tank must contain full runoff volume without on-site flooding
1-in-100-year + climate change allowance Exceedance check — safe overland flood route required if tank capacity is exceeded
Component Function
Silt trap / catchpit Removes debris and silt before it reaches the tank and flow control device
Geotextile membrane Prevents fines ingress into modular crate voids
Impermeable liner (where used) Retains full stored volume, prevents infiltration loss where attenuation-only is required
Orifice plate / vortex flow control Restricts outflow to the calculated permitted discharge rate
Access/inspection chamber Maintenance access at inlet, outlet, and along larger systems

Detailed Guidance

Why SuDS drives the requirement for attenuation

Sustainable Drainage Systems policy sits behind almost every attenuation tank installed in the UK. The non-statutory technical standards for SuDS set out the principle that developments should not increase flood risk elsewhere by discharging surface water faster or in greater volume than the site did before development. Local planning authorities typically impose a SuDS condition requiring surface water discharge to be limited to the greenfield rate (or, on previously developed sites, an agreed brownfield rate).

For domestic extensions and smaller developments, this condition is most commonly discharged by demonstrating either that surface water will infiltrate to ground via a soakaway (see soakaway installation guide for sizing method and installation), or — where infiltration isn't viable — that it will be attenuated on-site and released at a controlled rate to a watercourse or sewer. Attenuation tanks are the practical mechanism for the latter route wherever there isn't space or suitable topography for above-ground storage such as a pond or swale.

Sizing methodology — the core calculation

Attenuation sizing works from a straightforward principle even though the underlying calculation can get detailed: the tank must store the difference between the volume of runoff generated during a design storm and the volume permitted to leave the site at the calculated discharge rate over that same storm duration. Longer, lower-intensity storms and shorter, higher-intensity storms both need checking, because the "critical storm duration" — the one requiring the largest tank — isn't always obvious without running the calculation across a range of durations.

In practice this is normally run using a recognised UK drainage design method or software package referencing the relevant national SuDS technical standards and rainfall data (commonly FEH/FSR-derived statistics for the site location). Which method or software a given local authority or water company will accept varies — some accept a simplified spreadsheet for small domestic sites, others require modelling software output as part of a formal drainage strategy submission.

Key inputs to the calculation:

Tank types and when to specify each

Modular plastic crate systems dominate the domestic and small-commercial market. Individual crate units (commonly 0.3–1.0m³ each) interlock to build a tank of almost any footprint and depth, wrapped in geotextile (and an impermeable liner, where the tank must not lose water to ground). Their high void ratio (90–95%) means less excavation for a given storage volume than a rubble-fill alternative, and their modularity suits irregular garden shapes and phased installation around existing services.

Precast concrete tanks suit larger capacity requirements or sites needing higher structural loading — under a shared access road, for example, or where ground conditions make an open excavation with crate assembly impractical. They arrive as factory-made chambers, craned into a prepared excavation, reducing on-site build time but requiring suitable crane access.

GRP tanks are a middle ground — single-piece, factory-sealed units for defined smaller capacities where delivery access allows a large single unit to be brought to site, common on tighter urban plots where phased crate assembly is harder than dropping in one sealed tank.

Installation sequence

  1. Excavation — to the calculated footprint and depth, with allowance for bedding material below the tank base and cover depth above (checked against any surface loading requirement and frost depth for shallow installations)
  2. Base preparation — compacted granular bedding layer, levelled and checked, providing a stable and even base for the tank or crate assembly
  3. Geotextile membrane — laid to wrap the excavation base and sides before crate placement (for modular systems), preventing fines migration from surrounding ground into the storage voids
  4. Impermeable liner (where specified) — fitted inside the geotextile wrap for attenuation-only tanks that must not lose stored volume to infiltration
  5. Tank/crate assembly — modular crates built up in interlocking layers to the design volume and footprint, or precast/GRP tank craned and positioned
  6. Inlet and outlet connections — pipework connecting the tank to the upstream drainage system (via a silt trap/catchpit) and to the downstream flow control device and outfall
  7. Flow control device fitting — orifice plate or vortex unit fitted at the outlet, set to the calculated permitted discharge rate; this is a precision component and must be installed and commissioned per the manufacturer's specification, not approximated on site
  8. Backfill — surrounding the tank with appropriate backfill material per the manufacturer's specification (often a clean granular surround for crate systems), compacted in layers
  9. Access chambers — installed at inlet, outlet, and intermediate inspection points, brought up to finished ground level
  10. Commissioning and record-keeping — flow control device checked against design discharge rate, as-built drawings and calculations retained for Building Control/planning discharge and future maintenance reference

The flow control device — the component that makes the whole system work

The flow control device is what actually delivers SuDS compliance — everything upstream is about storing enough volume to make this restriction work without flooding the site. Two common types:

Whichever device is specified, it must be commissioned to the exact calculated discharge rate from the drainage design — a device fitted "close enough" or substituted for a different size without recalculation risks failing the planning/Building Control SuDS condition, even if the tank itself is correctly sized.

Maintenance access and long-term function

Attenuation tanks are inspection-and-maintenance items, not fit-and-forget infrastructure. Silt and debris accumulate over time, particularly where the catchment includes driveway or unfiltered roof runoff, and a blocked flow control device or silted tank can lose effective storage capacity or fail the discharge rate it was designed to. Specify and clearly document:

Frequently Asked Questions

Do I always need Building Control or planning approval for an attenuation tank?

In most cases yes, in some form. New build and extension work that increases impermeable area typically triggers a SuDS condition either through the planning permission itself or through Building Regulations Part H drainage requirements at Building Control stage. It's normally part of the drainage strategy for the wider development rather than a standalone approval — always check the specific conditions attached to the permission and confirm the accepted calculation method with the local authority before finalising the tank design.

Can I use a smaller tank if I fit a bigger flow control orifice?

No — this inverts the relationship. The flow control device size is set by the calculated permitted discharge rate, not by the tank size. Making the orifice bigger increases the discharge rate beyond what's permitted, failing the SuDS requirement regardless of tank capacity. The tank must be sized to store the volume that accumulates while water is released at the correct, smaller, calculated rate — both are derived together from the same storm modelling and can't be substituted for each other.

What's the difference between attenuation and a soakaway?

A soakaway disposes of surface water permanently by infiltrating it into the surrounding ground — see soakaway installation guide. An attenuation tank does not dispose of water at all; it temporarily stores it and releases it at a controlled rate to a sewer or watercourse. Soakaways suit sites where ground conditions permit infiltration; attenuation tanks suit sites where they don't. Some schemes combine both — attenuation storage discharging to a soakaway at a controlled rate.

How deep does an attenuation tank need to be buried?

There's no fixed depth requirement — it depends on the site's available fall to the outfall connection, cover requirements (structural loading, frost protection if shallow), and the tank's own height once assembled. Modular crate systems offer flexibility here since the footprint can be adjusted (wider and shallower, or narrower and deeper) to suit available depth — an advantage over a fixed-height precast tank on constrained sites.

Regulations & Standards