Summary
Attenuation tanks have gone from a rare specialist item to a routine part of domestic extension and small-development drainage over the last decade, as local authorities have tightened SuDS (Sustainable Drainage Systems) conditions attached to planning permission. For groundworkers, drainage contractors, and builders picking up this work, pricing it correctly means understanding that the tank itself — the crates, the concrete chamber, the GRP shell — is often the smaller part of the total cost. Excavation volume, the flow control device, access chambers, and getting the SuDS calculation right before committing to a dig are what actually determine whether a job is profitable.
This guide sets out the cost components for pricing an attenuation tank installation, typical UK price ranges by tank type and capacity, and a worked example for a common domestic scenario: a rear extension increasing impermeable roof and patio area, where the planning permission carries a SuDS condition requiring controlled surface water discharge. For the underlying sizing methodology and installation technique this guide is priced around, see attenuation tank sizing and installation.
The most common pricing mistake on this type of job is quoting from tank capacity alone without pricing excavation volume, access, and the flow control device separately — a 5m³ crate tank in a rear garden with clear digger access is a very different job cost-wise from the same 5m³ tank under a driveway needing trafficked-rated crates and reinstated block paving.
Key Facts
- SuDS drainage strategy/calculation — required before sizing; price as a standalone line item (£300–£800 typically for a domestic scheme, more for larger/commercial sites requiring formal modelling software output) — see attenuation tank sizing and installation for the sizing method
- Modular plastic crate units — £60–£110 per crate unit (typically 0.3–1.0m³ per unit depending on brand/load rating) for standard non-trafficked rating; trafficked/load-rated crates for driveways cost 30–60% more per unit
- Precast concrete tank — £250–£450 per m³ of storage capacity supplied, before crane hire and installation labour; higher for smaller bespoke sizes
- GRP tank — £180–£350 per m³ of storage capacity supplied, for typical domestic/small-commercial sizes
- Geotextile membrane — £1.50–£3/m² non-woven wrap for modular crate systems
- Impermeable liner (where specified) — £4–£8/m² additional, for attenuation-only tanks that must not lose water to infiltration
- Excavation — dominant labour cost; mini/midi digger hire £90–£180/day self-drive, £250–£450/day with operator, depending on tank depth and excavation volume
- Flow control device (orifice plate) — £80–£200 supplied and fitted for a simple calculated-diameter orifice plate
- Flow control device (vortex/"hydrobrake"-type) — £350–£900 supplied and fitted, depending on calculated discharge rate and pipe diameter
- Silt trap/catchpit upstream of tank inlet — £150–£400 supplied and fitted, depending on size and depth
- Access/inspection chambers — £100–£300 each, depending on depth and cover rating (pedestrian vs trafficked)
- Labour rate — £180–£280/day per groundworker outside London/SE; £280–£400/day London/SE
- Crane hire (for precast concrete tank placement) — £400–£900/day depending on tank weight and site access
- Reinstatement — topsoil/turf £8–£15/m² for garden areas; block paving/tarmac reinstatement over a driveway installation £40–£90/m² depending on surface type
- Building Control/planning drainage discharge fee — typically pass-through, £150–£400 depending on local authority, separate from the trade cost
- Minimum job charge — most contractors should set a £2,000–£3,000 minimum for an attenuation tank installation given the fixed costs of mobilisation, excavation plant, and the flow control device regardless of tank size
- VAT — standard rate 20% applies; no reduced-rate exemption for attenuation tank installation alone
Quick Reference Table
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Try squote free →| Job type | Storage volume | Tank type | Typical UK price (ex VAT) |
|---|---|---|---|
| Small domestic (single-storey extension) | 3–5m³ | Modular plastic crate | £3,000–£5,500 |
| Standard domestic (larger extension/new outbuilding) | 5–10m³ | Modular plastic crate | £5,000–£8,500 |
| Domestic under trafficked driveway | 5–10m³ | Load-rated modular crate | £6,500–£11,000 |
| Larger domestic/small commercial | 10–25m³ | Precast concrete or GRP | £12,000–£28,000 |
| Commercial/multi-unit development | 25m³+ | Precast concrete | £25,000+ (site-specific, needs full quote) |
Figures assume reasonable digger access, straightforward ground conditions, and exclude Building Control/planning fees, which are typically £150–£400 separately.
Detailed Guidance
Building the price bottom-up
Price these components as separate line items, then present a summed fixed price to the customer:
- SuDS calculation and drainage strategy — either produced in-house (if qualified to do so) or by a drainage engineer/consultant; establishes the required storage volume, discharge rate, and flow control device specification before any dig work is costed
- Excavation — calculated from the tank footprint, depth, and void ratio (crate: storage ÷ 0.9–0.95; precast/GRP: effectively 1:1 with chamber volume), plus bedding, cover, and working space allowance
- Tank materials — crate units, precast chamber, or GRP tank, plus geotextile and (where specified) impermeable liner
- Inlet works — silt trap/catchpit and pipe run from the drainage system being controlled
- Flow control device — orifice plate or vortex unit, sized to the calculated discharge rate; this is a precision component and should be quoted from the actual calculation output, not estimated
- Access chambers — inlet, outlet, and any intermediate inspection points
- Reinstatement — garden, driveway, or paved surface make-good over the excavation
- Building Control/planning drainage discharge — pass-through fee or budgeted if handled on the customer's behalf
Worked example — rear extension, 6m³ modular crate tank, garden location, good access
Typical job: single-storey rear extension plus new patio adding roughly 55m² of impermeable area, planning permission carries a SuDS condition requiring surface water attenuation before controlled discharge to the existing surface water sewer connection. SuDS calculation returns a required storage volume of 6m³ and a permitted discharge rate requiring a vortex flow control device. Clear rear garden access for a midi digger.
| Item | Cost |
|---|---|
| SuDS calculation and drainage strategy (drainage engineer) | £450 |
| Midi digger hire, 1 day, self-drive | £140 |
| Excavation labour, 2-person team, 1 day | £440 |
| Modular crate units (12 × 0.5m³, non-trafficked rating) | £900 |
| Non-woven geotextile wrap, ~30m² | £75 |
| Impermeable liner, ~30m² | £180 |
| Bedding/surround material | £180 |
| Silt trap/catchpit at inlet | £280 |
| Vortex flow control device, supplied and fitted | £520 |
| Inlet and outlet access chambers (2 × pedestrian-rated) | £400 |
| Pipework, inlet and outlet runs (~12m total, 110/160mm) | £160 |
| Reinstatement (topsoil + turf, ~15m² disturbed) | £150 |
| Cost subtotal | £3,875 |
| Margin (30%) | £1,163 |
| Quoted price (ex VAT) | £5,038 |
| Quoted price (inc VAT) | £6,046 |
| Building Control/drainage discharge fee (pass-through, separate) | £150–£300 |
This sits comfortably within the standard domestic range for a 5–10m³ modular crate installation with good access.
Worked example — driveway installation, 8m³ load-rated crate, restricted access
Typical job: new driveway on a previously grassed front garden, exceeding the GPDO Class F 5m² impermeable threshold, requiring SuDS attenuation as the compliance route rather than permeable paving. Tank sits beneath the trafficked driveway surface, requiring load-rated crates, with restricted access requiring a smaller excavator and phased spoil removal.
| Item | Cost |
|---|---|
| SuDS calculation and drainage strategy | £480 |
| Mini digger hire, restricted access, 1.5 days | £240 |
| Excavation labour, 2-person team, 1.5 days | £660 |
| Spoil removal, skip hire | £320 |
| Load-rated modular crate units (16 × 0.5m³, trafficked rating) | £1,760 |
| Non-woven geotextile wrap, ~38m² | £95 |
| Impermeable liner, ~38m² | £230 |
| Bedding/surround material (trafficked-rated backfill) | £280 |
| Silt trap/catchpit at inlet | £320 |
| Vortex flow control device, supplied and fitted | £580 |
| Inlet and outlet access chambers (2 × trafficked-rated) | £520 |
| Pipework, inlet and outlet runs (~15m total) | £210 |
| Driveway reinstatement (block paving make-good, ~20m²) | £900 |
| Cost subtotal | £6,595 |
| Margin (30%) | £1,979 |
| Quoted price (ex VAT) | £8,574 |
| Quoted price (inc VAT) | £10,289 |
| Building Control/planning drainage discharge fee (pass-through, separate) | £200–£400 |
The trafficked loading requirement and restricted access push this toward the upper end of the standard domestic range.
Tank type and cost trade-offs
Modular plastic crate systems are the default specification for most domestic and small-commercial jobs because of their flexibility (any footprint, any depth within reason) and lower material cost relative to their void ratio. They're the right default recommendation unless site-specific factors point elsewhere.
Precast concrete tanks become cost-competitive, and sometimes cheaper overall despite higher unit material cost, where a site needs a large single-chamber tank with minimal on-site assembly time, or where structural loading (heavy vehicle access, limited cover depth) makes crate systems impractical. Crane access is the deciding practical factor — if a crane can get to the excavation, a precast tank installs fast; if it can't, crate systems remain the only realistic option regardless of cost comparison.
GRP tanks sit between the two — useful where a defined smaller capacity is needed and delivery access allows a single large unit, common on tighter urban plots.
Flow control device — don't under-price this component
The flow control device (orifice plate or vortex/"hydrobrake"-type unit) is a small physical component but a disproportionately important cost and compliance item. It must be sized precisely to the calculated permitted discharge rate from the SuDS calculation — this is not a component to substitute for a cheaper generic part or estimate without the calculation output. Vortex devices cost considerably more than a simple orifice plate but are frequently specified (and sometimes mandated by the local authority or water company) where the calculated discharge rate would otherwise require an orifice small enough to be blockage-prone. Always quote the flow control device from the actual drainage calculation, and flag to the customer that this component's cost depends directly on that calculation's outcome.
Regional variation
London and the South East run 30–50% above the rest of the UK on attenuation tank installation, driven by labour day rates, plant and crane hire costs, and a higher likelihood of restricted access requiring hand-dig or smaller-plant working. Rural sites elsewhere in the UK can see costs pushed up by long plant-delivery distances even where labour rates are lower.
Common pricing mistakes
- Quoting tank capacity cost alone without separately pricing excavation volume, access chambers, and the flow control device — these routinely account for more than half the total job cost
- Estimating the flow control device cost before the SuDS calculation is complete — the device size and cost depend directly on the calculated discharge rate
- Under-pricing driveway/trafficked installations at the same crate specification as non-trafficked garden installations — load-rated crates and trafficked-rated access chambers cost significantly more
- Forgetting reinstatement cost for hard-surfaced areas (block paving, tarmac) — this can be a larger line item than the tank installation itself on a driveway job
- Not itemising the Building Control/planning drainage discharge fee separately, leaving customers unaware it's a required step distinct from the trade cost
- Treating the SuDS calculation as a free "no obligation" quote step rather than a priced line item — it determines job viability and required tank size before any other cost can be accurately estimated
Frequently Asked Questions
Should I quote attenuation tank installation per cubic metre of storage or as a fixed job price?
Use cost-per-cubic-metre-of-storage as an internal costing benchmark (roughly £550–£950/m³ for a standard modular crate installation with good access, considerably more for precast concrete, trafficked loading, or restricted access), but always present the customer with a fixed total broken into the line items above. Storage volume alone doesn't capture access difficulty, tank type, or the flow control device specification, so a pure per-m³ rate invites disputes.
What margin should I target on attenuation tank work?
25–35% is standard, consistent with other groundworks and drainage categories. Push toward the top of that range on restricted-access, trafficked-loading, or precast concrete jobs where risk, plant complexity, and admin overhead (Building Control liaison, drainage engineer coordination) are higher.
Who should carry out the SuDS calculation — can I do it myself as the installing contractor?
For straightforward domestic jobs, an experienced groundworker or drainage contractor familiar with the local authority's accepted calculation method can often produce an adequate scheme, but check what the specific local authority or water company will accept before relying on this — many require a formal drainage strategy from a qualified drainage engineer, particularly where the scheme forms part of a wider planning application. Price a drainage engineer's calculation as a separate line item where required rather than assuming it can be absorbed into the installation quote.
Is it worth offering a maintenance contract alongside the installation?
Yes, and it's an underused upsell on this type of work. Attenuation tanks need periodic silt trap emptying and flow control device inspection to keep working correctly, and most domestic customers have no mechanism for remembering this once the installation is complete and buried. A modest annual maintenance visit (£80–£150) covering silt trap emptying and a visual check of the flow control device is a straightforward add-on that also protects your own installation's reputation if the system is later found not discharging at the correct rate.
Regulations & Standards
Building Regulations Approved Document H (Drainage and waste disposal) — surface water disposal hierarchy and drainage requirements relevant to attenuation tank installation
Non-statutory technical standards for sustainable drainage systems — national SuDS design standards referenced by many local authorities
CIRIA C753 — The SuDS Manual — design guidance for attenuation and storage sizing referenced across the industry
Flood and Water Management Act 2010 — statutory framework underpinning SuDS policy
Water Industry Act 1991, Section 106 — governs connection to public sewers, relevant where attenuated discharge outfalls to a public surface water sewer
CDM Regulations 2015 — excavation safety; trenches/pits over 1.2m deep require shoring or battered sides
attenuation tank sizing and installation — the installation technique this guide is priced around: SuDS sizing methodology, tank types, and installation sequence
soakaway installation pricing guide — pricing for soakaway installation, the infiltration-based alternative to attenuation
soakaway sizing — BRE Digest 365 percolation test method for soakaway design
part h drainage — Building Regs Part H overview covering foul and surface water drainage requirements