SuDS Storage Volume, Tank Dimensions & Cost Estimator – Updated 2026
This calculator estimates the attenuation storage volume required for a development or impermeable surface using the simplified UK SuDS volume method. Enter your catchment area, site region (for rainfall data), design return period, pre- and post-development runoff rates, and the calculator produces the required storage volume, recommended tank dimensions, and supply cost estimate. This is a simplified preliminary estimate — final attenuation design must be carried out by a qualified drainage engineer using full hydrological analysis and the relevant local authority SuDS requirements.
An attenuation tank (also called a rainwater attenuation system, SuDS attenuation tank, or storm water detention tank) is an underground storage structure that temporarily holds excess rainwater runoff from impermeable surfaces — roofs, driveways, car parks, roads, and commercial hardstandings — during a rainfall event, and releases it slowly at a controlled rate to the downstream drainage system or watercourse. The fundamental principle is to prevent a peak flow surge from entering the combined or surface water sewer network or watercourse, which would otherwise contribute to flooding downstream.
Attenuation tanks have become mandatory for new developments in the UK following the introduction of SuDS (Sustainable Drainage Systems) requirements in the planning system. In England, SuDS are required for most new major developments under the National Planning Policy Framework (NPPF) and the technical standards set by the Lead Local Flood Authority (LLFA). From April 2024, Schedule 3 of the Flood and Water Management Act 2010 was enacted in Wales, making SuDS approval a statutory requirement for all new developments above a threshold. Scotland and Northern Ireland have their own equivalent SuDS requirements.
The standard requirement — in England, Wales, and Scotland — is that post-development runoff rates must not exceed pre-development (greenfield) rates for the design return period, typically 1-in-100 years plus a climate change allowance. Since development typically replaces permeable vegetated ground with impermeable hardstanding, runoff rates increase dramatically without attenuation. The attenuation tank provides the temporary storage buffer needed to limit the post-development peak discharge to the pre-development greenfield equivalent.
The simplified method for calculating attenuation storage volume — used for preliminary design and planning applications — is based on the difference between the incoming peak runoff rate and the permitted outgoing discharge rate:
For detailed design, the simplified method is replaced by full hydrological modelling (InfoWorks, MicroDrainage, or equivalent) which routes the design hyetograph through the catchment and optimises the tank volume and outflow rate simultaneously. The simplified method always produces a conservative (larger) volume than full modelling, so it is appropriate for preliminary design and budget estimation.
The table below gives approximate design rainfall intensities for UK regions at the 1-in-100-year (1% AEP) return period for common storm durations, derived from FEH and FSR data. These are simplified regional averages for preliminary design only — full FEH/FSR analysis using the NRFA data set and the specific site grid reference should be used for final design.
| UK Region | Annual Rainfall (mm/yr) | 1-in-100yr, 60 min (mm/hr) | 1-in-100yr, 120 min (mm/hr) | 1-in-100yr, 360 min (mm/hr) | 1-in-100yr+CC, 120 min (mm/hr) | Notes |
|---|---|---|---|---|---|---|
| South East England / East Anglia (dry) | 600–750 | 16–20 | 11–14 | 6–8 | 14–18 | Lowest UK rainfall; critical for sewer capacity not usually attenuation volume |
| South East England / Midlands (moderate) | 750–900 | 20–25 | 13–17 | 7–10 | 17–22 | Most common UK development region; standard LLFA design basis |
| South West England / South Wales | 900–1,200 | 22–28 | 15–19 | 8–12 | 20–25 | Higher rainfall; larger attenuation volumes typically required |
| North East England / Yorkshire | 750–950 | 20–26 | 13–18 | 7–10 | 17–23 | Similar to South East; upland areas significantly higher |
| North West England / North Wales | 1,000–1,500 | 24–32 | 16–22 | 9–14 | 21–29 | Higher than national average; Manchester / Lake District area significantly higher |
| East Scotland / Central Belt | 700–900 | 18–24 | 12–16 | 7–9 | 16–21 | East coast drier; intensive convective events possible in summer |
| West Scotland / Highlands | 1,500–3,000+ | 28–42 | 19–28 | 11–17 | 25–36 | Highest UK rainfall; attenuation volumes can be very large; specialist design essential |
| Northern Ireland | 900–1,200 | 22–30 | 15–20 | 8–12 | 20–26 | Higher than SE England; DfI NI Roads guidance applies for adoptable drainage |
CC = Climate Change allowance: +30% to rainfall depth for England (following UKCP18 projections and EA guidance); +20% for Scotland; confirm with LLFA / SEPA / NRW. Design return period should be 1-in-100yr (1% AEP) for all surface water attenuation; add CC allowance for final design. Intensities are approximate mid-range regional averages — use FEH Web Service or Wallingford HydraSoftware data for site-specific design.
| System Type | Supply Cost per m³ Storage (excl. VAT, 2026) | Installation Cost per m³ (excl. VAT) | Total Installed Cost per m³ | Notes |
|---|---|---|---|---|
| Plastic modular crate (Polystorm / AquaCell type) | £80–£140/m³ | £60–£120/m³ | £140–£260/m³ | Most common; 95% void ratio; traffic-rated options available; flexible configuration |
| Geocellular plastic (heavy-duty commercial) | £120–£200/m³ | £70–£130/m³ | £190–£330/m³ | Higher load-bearing capacity; deeper installations; car park and road applications |
| Precast concrete ring tank | £180–£280/m³ | £100–£180/m³ | £280–£460/m³ | More durable; heavier to install; better for deep / high traffic; longer lead time |
| Large-diameter HDPE pipe storage | £100–£180/m³ | £80–£140/m³ | £180–£320/m³ | Good for linear sites; efficient for smaller volumes; limited to diameter available |
| Open attenuation basin (earthworks) | £10–£30/m³ | £20–£50/m³ | £30–£80/m³ | Lowest cost where land available; requires ongoing maintenance; flow control device essential |
| Flow control device (vortex / Hydro-Brake) | £800–£4,000 per unit (supply only) | £1,500–£8,000 installed | Size-dependent; larger devices for higher flows and larger sites cost more | |
Attenuation and SuDS resources UK 2026: CIRIA – SuDS Manual C753 | Environment Agency – Flood Risk | SuDrain – SuDS Guidance | SEPA – Scotland Drainage | NRW – Wales SuDS
| Item | Typical Cost (2026) | Notes |
|---|---|---|
| Drainage design and SuDS approval report | £1,500–£8,000 | Full hydrological design, SuDS strategy document, planning submission; fee depends on site size and complexity |
| LLFA / SuDS approving body fee (Wales) | £350–£2,000 statutory fee | SAB (SuDS Approving Body) application fee in Wales; varies by LPA; England: LLFA comment is part of planning consultation (no separate fee) |
| Geotextile membrane (per m²) | £0.80–£2.50/m² | Wraps modular crate system to prevent fines ingress; area = footprint perimeter × depth + top and base |
| Bedding and surround (granular) | £35–£55/tonne delivered | Clean 10mm or 20mm single-size aggregate for bedding concrete pipe attenuation systems; not required for modular crate |
| Excavation and disposal | £15–£40/m³ excavated | Depends on ground conditions; soft clay / sand lower cost; rock or contaminated ground significantly higher |
| Inspection / access chamber | £400–£1,500 each | Required for access to flow control device and tank inlet; precast concrete or HDPE; minimum 1 per system |
| Inlet pipe, outlet pipe and connections | £200–£800 | HDPE or PVC-u pipe connecting tank to site drainage network; size depends on design flows |
| Bypass sewer / overflow | £500–£3,000 | Emergency overflow for events exceeding design return period; required by most LLFAs; connects to watercourse or highway drain |
| Traffic-rated top slab / cover slab | £80–£180/m² | Required where tank is under trafficked area; reinforced concrete or purpose-made plastic lid system |
| CCTV survey (post-installation) | £200–£600 | Post-construction verification of pipe connections and flow control device installation; may be required by LLFA or adopting authority |
| Adoption and commuted sum | £500–£5,000+ | Where drainage is to be adopted by water company or LPA; commuted sum for ongoing maintenance; varies significantly |
| Annual maintenance (inspection and jetting) | £200–£800/year | Annual CCTV inspection and high-pressure jetting of tank and flow control device; essential to maintain performance |
Sustainable drainage systems — including attenuation tanks, permeable paving, rain gardens, and swales — are now required for most new major developments in England, Wales, Scotland, and Northern Ireland. In England, the NPPF requires Local Planning Authorities to ensure SuDS are incorporated in major development. In Wales, Schedule 3 of the Flood and Water Management Act 2010 (effective April 2024) requires SuDS approval for all new surface water drainage in developments above a threshold. The basic principle is that post-development runoff must not exceed pre-development (greenfield) runoff rates.
UK attenuation systems must now be designed for the 1-in-100 year (1% AEP) storm event plus a climate change allowance. In England, the Environment Agency's Climate Change Allowances (updated 2021) require a +30% increase in rainfall intensity for new development using the 'higher central' scenario for most catchments. This is a significant increase — a tank designed for 1-in-100 years alone may be 25–30% undersized compared to the correct climate change standard. Always confirm the required climate change allowance with the Local Lead Flood Authority (LLFA) for the specific site.
An attenuation tank is only effective if paired with a correctly sized and installed flow control device that limits the discharge rate to the permitted level. The most common UK device in 2026 is a vortex flow control (Hydro-Brake type) — a passive, self-cleaning device that uses vortex action to limit outflow regardless of head. These devices operate from approximately 0.5 l/s (domestic driveway) up to 100 l/s+ (large commercial site). The flow control device must be installed in an accessible inspection chamber, cleaned annually, and inspected by CCTV periodically.
The vast majority of UK domestic and commercial attenuation installations in 2026 use plastic geocellular modular crate systems — brands including Polystorm (Wavin), AquaCell (Hydro International), StormTech (Advanced Drainage Systems), Atlantis, and many own-brand equivalents. These systems are lightweight, fast to install, achieve 95% void ratio, are available in traffic-rated versions (for under car parks and roads), and can be configured in any shape. A typical domestic driveway attenuation system (5–20 m³) can be installed in one day by two groundworkers.
The pre-development (greenfield) runoff rate — the permitted discharge rate from the site — is typically calculated using the Institute of Hydrology Report 124 (ReFH2 or FEH) approach for the specific site. For most UK clay and loam soil sites, typical values are 2–5 l/s/ha at the 1-in-100 year standard. Wetter, more permeable sites may be higher. The lower this value, the larger the attenuation tank needed. For very low greenfield rates (clay, flat catchments), attenuation volumes can become very large — sometimes making alternative SuDS (permeable paving, infiltration) more economical.
If the site has suitable ground conditions (percolation rate, depth to water table, distance from contamination), permeable paving with infiltration can eliminate the need for an attenuation tank entirely by allowing rainwater to percolate into the ground rather than running off. Permeable resin-bound, block paving, or gravel surfaces allow rainfall to infiltrate at rates of 100–1,000 l/s/ha depending on the surface and subsoil. A percolation test (BS EN ISO 22282) must confirm adequate infiltration rates before permeable paving can be relied upon as the sole SuDS measure.
CIRIA C753 (the SuDS Manual) recommends that SuDS should be designed for two design criteria: the 1-in-30 year storm (for the minor drainage system — gutters, pipes, and gullies flowing full but not overflowing) and the 1-in-100 year + climate change storm (for the major drainage system — overland flow routes, attenuation storage, and flood risk management). Most attenuation tanks are therefore sized for the 1-in-100 year + CC event. The 1-in-30 year event is used to size the pipe network that feeds the tank.
In England, a front garden driveway exceeding 5 m² of impermeable surface requires either permeable paving (no attenuation needed) or planning permission. If planning permission is granted for an impermeable driveway, attenuation may be required as a planning condition. In practice, for single residential driveways, many local authorities accept a simple soakaway instead of a full attenuation tank — provided the ground conditions allow infiltration. Small modular attenuation systems (1–5 m³) are available for domestic driveways at approximately £500–£2,500 installed where attenuation is required.
UK design rainfall intensities are derived from two datasets: the older Flood Studies Report (FSR, 1975) and the updated Flood Estimation Handbook (FEH, 1999 updated to FEH3). FEH data from the NRFA Web Service should be used for all new designs, as it provides site-specific design rainfalls based on the grid reference rather than regional averages. FEH data subscriptions cost approximately £1,500–£4,000/year for professional drainage designers, or a single-site FEH extraction can be purchased for approximately £30–£60. This calculator uses simplified regional average intensities for preliminary design only.
Attenuation tanks and their flow control devices require regular maintenance to function correctly. Silt, debris, and vegetation can block outlets and reduce tank capacity over time. In England, the lead local flood authority (LLFA) can require the owner of a SuDS system to maintain it under the Flood and Water Management Act 2010. In Wales, SAB-adopted drainage must be maintained by the SAB (typically the council). For private developments, a drainage maintenance agreement should be included in the planning conditions, specifying minimum inspection frequency (typically annual) and maintenance standards (CCTV survey, jetting of outlet, vegetation management).
Where ground conditions allow, a soakaway (which disposes of runoff by infiltration into the ground) is preferred over an attenuation tank (which stores water and discharges slowly to a watercourse or sewer). Soakaways require adequate percolation rates (minimum 1×10⁻⁶ m/s, confirmed by BS EN ISO 22282 percolation test), sufficient depth above the water table (typically 1.0–1.5m), and appropriate setbacks from buildings and boundaries. Where infiltration is feasible, a soakaway eliminates the need for a controlled outlet, a flow control device, and a connection to a watercourse or sewer — significantly reducing cost and maintenance burden.
When specifying an attenuation tank from a manufacturer, two volumes matter: the storage volume (the actual usable volume for water storage) and the gross installed volume (the total volume of the excavation and tank structure). For modular plastic crate systems with approximately 95% void ratio, the gross installed volume is essentially the same as the storage volume. For concrete ring tanks, the usable storage is the internal diameter × depth. Always specify the required storage volume in m³ (not the gross tank volume) in procurement documents, and request confirmation of the void ratio from the manufacturer.
An attenuation tank is an underground storage structure that temporarily holds excess rainwater runoff during a rainfall event and releases it slowly at a controlled rate. During a heavy storm, runoff from roofs, driveways, and impermeable surfaces enters the tank and is stored. A flow control device (typically a vortex or orifice device) at the outlet limits the discharge rate to a pre-set maximum — the permitted greenfield runoff rate. Once the storm passes, the tank gradually empties through the flow control at the permitted rate, returning to empty capacity before the next storm. This prevents the sudden surge of runoff that would otherwise cause downstream flooding.
The required attenuation volume depends on: the size and type of the impermeable catchment; the local design rainfall intensity (derived from FEH data for the 1-in-100yr + climate change event); the permitted discharge rate (typically the pre-development greenfield runoff rate, approximately 2–5 l/s/ha for most UK sites); and the critical storm duration. As a rough guide: a domestic driveway (50–100 m²) typically requires 1–5 m³; a residential estate (0.5–1 ha) typically requires 50–200 m³; a medium commercial site (2–5 ha) typically requires 300–1,000 m³. The calculation must be carried out by a qualified drainage engineer — this calculator provides a preliminary estimate only.
For a new or extended driveway in England: if the total impermeable front garden area exceeds 5 m², you either need to use permeable paving (which doesn't require attenuation) or obtain planning permission. If planning permission is required for an impermeable surface, the local planning authority or LLFA may require attenuation as a condition. For house extensions, the drainage strategy must be agreed with building control — for new hard standing or roof areas, you may need to demonstrate that the post-development runoff does not increase flooding risk, which may involve a soakaway test or attenuation. For major residential or commercial development, attenuation is almost always required.
Attenuation tank costs in the UK in 2026 range from approximately £140–£260 per m³ of storage volume installed for standard plastic modular crate systems — the most common type. For a domestic driveway system (3–5 m³), total installed cost including the flow control device is typically £1,500–£4,000. For a medium-sized commercial site (100–200 m³), expect £25,000–£60,000 for the attenuation system alone, plus drainage design fees (£1,500–£8,000) and associated civil works. Precast concrete tank systems cost approximately £280–£460/m³ installed — significantly more but better suited to deep or heavily trafficked installations. Always obtain at least three specialist quotes.
Both are underground drainage structures for managing surface water, but they work differently. A soakaway disposes of water by allowing it to infiltrate into the surrounding ground — it has no outlet pipe, and water simply percolates away. Soakaways are suitable where the ground has adequate permeability (confirmed by a percolation test) and the water table is sufficiently deep. An attenuation tank stores water temporarily and releases it slowly through a controlled outlet (flow control device) to a watercourse or sewer — it does not rely on ground permeability. Soakaways are generally preferred by drainage engineers and LLFAs because they mimic the natural water cycle more closely, but they are only feasible where ground conditions permit.
The simplified method used for preliminary design calculates attenuation volume as: V = (Qin − Qout) × Tstorm, where V is storage in m³, Qin is the peak inflow rate (l/s), Qout is the permitted outflow rate (l/s), and Tstorm is the critical storm duration in seconds. Qin = Effective impermeable area (m²) × Rainfall intensity (m/s) = A × i/1000/3600. Qout = Pre-development greenfield rate (l/s/ha) × Area (ha). For detailed design, this simplified method is replaced by a full hydrological routing calculation using a design storm hyetograph (time-distributed rainfall) and the outflow rating curve of the flow control device. The simplified method typically gives a volume 10–30% larger than the detailed method.
An attenuation tank requires regular maintenance to remain effective: annual inspection and jetting of the flow control device and outlet pipe to clear silt and debris; periodic CCTV survey of the tank interior and connections (typically every 3–5 years); geotextile inspection where accessible — checking for blockage by root intrusion or silt; and vegetation management around inlet structures. The flow control device is the critical component — a blocked vortex insert can prevent the tank from draining between storms, eliminating its capacity before the next event. Maintenance should be carried out by a specialist drainage contractor and documented for planning conditions and insurance purposes.
Yes — where ground conditions allow, permeable paving with infiltration can eliminate the need for a separate attenuation tank. Permeable paving systems (resin-bound, block paving with permeable joints, or gravel) allow rainfall to infiltrate through the surface and into a granular sub-base, from where it percolates into the ground. The key requirements are: a percolation test (BS EN ISO 22282) confirming adequate infiltration rate (minimum 1×10⁻⁶ m/s for soakage); depth of at least 1.0–1.5m above the seasonal high water table; appropriate setbacks from buildings (typically 5m for residential); and no significant contamination risk to groundwater. Where these conditions are met, permeable paving is often the most cost-effective and lowest-maintenance SuDS option for domestic driveways and car parks.
A vortex flow control device (common brands include Hydro-Brake, Eurocell Vortex, and Econocell) is a passive, maintenance-friendly device installed at the outlet of an attenuation tank to limit the discharge rate. Water enters the device tangentially, creating a vortex that restricts flow — the vortex intensifies as head increases, providing an almost constant outflow rate regardless of the water depth in the tank. This self-regulating behaviour makes vortex devices ideal for attenuation applications where the outlet rate must remain constant across a wide range of storage heads. They are available in standard sizes from approximately 0.5 l/s (small domestic) to 50+ l/s (large commercial), and cost approximately £800–£4,000 supply-only depending on size and material.
An attenuation tank itself does not usually require a separate planning application — it is typically installed as part of a development with full planning permission that includes a drainage strategy. However, in some circumstances, particularly for retrofitting an attenuation tank to an existing developed site, permitted development rights may apply (in England) for below-ground drainage works within the curtilage of a house. In Wales, all surface water drainage for new developments above a threshold must be approved by the SAB (SuDS Approving Body). Where the tank is part of a highway or adopted drainage scheme, adoption by the relevant authority (water company, LPA, or highway authority) requires pre-approval of the design and post-construction inspection and certification.
Additional attenuation and SuDS resources UK 2026: CIRIA – C753 SuDS Manual | SuDrain | Environment Agency – Flood Risk | SEPA – Scotland | NRW – Wales | GOV.UK – SuDS Guidance