How to Price a Soakaway Installation

Summary

Soakaways sit in an awkward pricing spot: the materials cost is modest (a plastic crate and some gravel), but the labour, access, and — critically — the sizing decision drive the real cost, and get this wrong and either the customer has a job that floods within a year, or you've built something twice the size it needed to be and eaten your own margin. This guide is for groundworkers, landscapers, drainage contractors, and general builders pricing soakaway installation for domestic clients: new extensions needing a surface water disposal point, replacement of a failed or undersized existing soakaway, and paving/driveway projects triggering a SuDS compliance requirement.

Building Regulations Approved Document H3 puts a soakaway (infiltration to ground) at the top of the surface water disposal hierarchy — before a watercourse, before a surface water sewer, and well before a combined sewer connection, which now requires Section 106 approval and is actively discouraged where infiltration is viable. This means almost every new roof, driveway, or patio catchment in the UK defaults to "soakaway required" unless the ground genuinely won't take it (heavy clay, high water table, contaminated land). Pricing this work accurately, including the percolation test that determines feasibility, is now a routine part of extension, driveway, and patio quoting — not a specialist add-on.

The single biggest pricing mistake on soakaway jobs is quoting a fixed price for "a soakaway" before knowing the catchment area, soil type, or access. A 1m³ crate on sandy loam with clear machine access and a 30-minute excavation is a fundamentally different job from a 3m³ rubble-filled pit in heavy clay needing hand-dig access through a side gate. This guide sets out the cost components, a worked example at each end of that range, and the regional price variation to apply.

Key Facts

• BRE Digest 365 — the standard UK percolation test and sizing method; required (or at minimum expected by Building Control) before any soakaway is designed — see soakaway sizing for the full calculation method
• Percolation test cost — £150–£350 as a standalone visit (3 trial pits, water-fill timing, calculation and report); often waived/absorbed into the job price on straightforward domestic jobs where ground conditions are already known to be favourable
• Minimum distance from building — 5m (Approved Document H3); closer on clay subsoils increases subsidence risk and some contractors quote 7–10m as best practice
• Minimum distance from boundary — 2.5m typical, some local authorities/NHBC specify 3m
• Modular plastic crate (Wavin Aquacell, Polypipe Polystorm, Hydro Civil StormCrate etc.) — £45–£90 per 0.4–0.5m³ crate unit depending on load rating; 90–95% void ratio
• Rubble/granular pit fill — clean 40–75mm crushed stone £35–£55 per bulk bag (~0.5m³); only 30–35% void ratio, so needs roughly 3x the excavated volume of a crate soakaway for the same storage capacity
• Non-woven geotextile membrane — £1.50–£3/m², minimum 130g/m² per BRE Digest 365 specification
• Excavation — mini digger (1.5–3 tonne) hire £90–£150/day self-drive, £250–£400/day with operator; hand-dig only where access is blocked
• Labour rate — £180–£280/day per groundworker outside London/SE; £280–£400/day London/SE
• Typical crate soakaway excavation time — half a day to one day for a straightforward 1–2m³ soakaway with clear access; a full day plus for larger or hand-dig jobs
• Inspection/rodding access — proprietary inspection turret £40–£90; required by Building Control for crate soakaways
• Building Control notification — required for any soakaway forming part of new build, extension, or a change to the surface water disposal point; building control/approved inspector fees typically £150–£350 depending on local authority, separate from the trade cost
• CAT/Genny scan — budget £0 in-house or £60–£120 subcontracted before any excavation near a boundary or driveway
• Minimum job charge — most contractors should set a £700–£950 minimum for a soakaway installation, covering mobilisation, plant delivery, and setup regardless of size
• VAT — standard rate 20% applies; no reduced-rate exemption for soakaway work alone

Quick Reference Table

Figures assume normal vehicle/mini-digger access, straightforward sandy loam to sandy clay ground, and exclude Building Control fees, which are typically £150–£350 separately.

Detailed Guidance

Building the price bottom-up

For any soakaway job, price these six components as separate line items, then present a summed fixed price to the customer:

1. Percolation test and sizing — either a standalone survey visit, or (on smaller domestic jobs where ground is known/assumed) folded into the first visit at reduced cost. Always document the f-value used, even on an informal assessment — see soakaway sizing for the BRE Digest 365 method.
2. Excavation — the dominant cost on most jobs. Calculate pit volume from the required storage (crate: storage ÷ 0.9–0.95 void ratio; rubble: storage ÷ 0.3–0.35 void ratio) plus bedding and cover allowance, then divide by daily output for the ground/access conditions.
3. Materials — crate units or granular fill, geotextile, bedding stone, inspection access turret.
4. Inlet connection — pipe run from the downpipe/gully to the soakaway, plus any inspection chamber upstream.
5. Reinstatement — topsoil, turf, or paving make-good over the excavation.
6. Building Control notification — pass through the local authority/Approved Inspector fee, or budget it into the job if you handle notification on the customer's behalf.

Worked example 1 — standard domestic soakaway, 60m² roof, sandy clay, good access

Typical job: single-storey rear extension adding 60m² of new roof catchment, existing surface water previously ran to a soakaway that's now inadequate for the extra area. Sandy clay ground (f ≈ 5×10⁻⁶ m/s, from a quick percolation check), clear rear garden access for a mini digger.

Comfortably within the standard UK range for this job type.

Worked example 2 — larger driveway catchment, clay ground, restricted access, London

Typical job: 120m² block-paved driveway on heavy clay in a London suburb, side access only (no rear vehicle access, hand-dig required through a 900mm side gate), percolation test formally required by Building Control as part of a driveway drainage condition.

This sits above the standard range, driven by hand-dig access, HGV-rated crates for driveway loading, and the mandatory oil interceptor for paved-surface runoff.

Regional variation

London and the South East run 40–60% above the rest of the UK on soakaway installation, driven by labour day rates (£280–£400/day vs £180–£280/day), plant hire and delivery costs, and — significantly — access constraints, since London gardens and driveways are more likely to require hand-dig-only working with no rear vehicle access. Scotland, Wales, and Northern England typically sit at or slightly below the "rest of UK" figures in the table above, though rural sites with long plant-delivery distances can push costs up regardless of region.

Percolation testing: price it, don't absorb it

The percolation test is the step most likely to get skipped or under-priced, and it's the step that determines whether the job is even viable. On a straightforward domestic job with known favourable soil (sandy loam, no history of waterlogging, similar soakaways working nearby), many contractors reasonably fold an informal test into the first visit at no separate charge. But where ground conditions are uncertain, where Building Control formally requires a documented BRE Digest 365 report (common on new-build extensions and any driveway drainage condition), or where the site has clay subsoil, price the test as its own £150–£350 line item and get it agreed before committing to a fixed installation price. Discovering post-quote that the ground won't take a standard-sized soakaway — and needs 3x the storage volume — is the single most common cause of a soakaway job going over budget.

Crate vs rubble pit: pricing the choice

Modular plastic crates have a 90–95% void ratio versus roughly 30–35% for a clean rubble/granular pit, meaning a rubble pit needs roughly three times the excavated volume to achieve the same storage capacity. For small jobs this often makes crates cheaper overall despite the higher unit cost, because the excavation saving outweighs the material cost difference. Rubble pits remain competitive on very small jobs (porch/garage roof, under 0.5m³ storage) where the excavation is trivial either way, or where site access genuinely can't accommodate crate delivery and a skip/grab-lorry approach to rubble fill is more practical.

Access and ground conditions — the two biggest cost swings

• Access: a rear garden with clear mini-digger access can be excavated in half a day for a standard-sized soakaway. The same job through a sub-900mm side gate, hand-dig only, can take 2–3x as long. Price this as an explicit access surcharge (25–50%) rather than a vague on-site variation.
• Ground conditions: clay soil doesn't just slow the dig, it directly increases the required storage volume per BRE Digest 365 (lower infiltration rate f means more storage needed for the same catchment) — so poor ground drives both the excavation cost and the materials cost upward simultaneously. This compounding effect is why clay-ground jobs can run 2–3x the price of an equivalent sandy-loam job, not just marginally more.

Combining with other drainage work

Soakaways are frequently priced alongside related drainage work — flag these as upsells or bundle opportunities rather than quoting the soakaway in isolation:

• French drain connection where the soakaway is the outfall for a new land drain — see french drain installation pricing guide
• Driveway/patio SuDS compliance where the soakaway satisfies the GPDO Class F impermeable-surface threshold
• Gutter and downpipe alteration to route additional roof catchment to the new soakaway
• Building Control notification handling — some contractors offer this as a managed service for an extra £50–£100 admin fee rather than leaving the customer to deal with the local authority directly

Common pricing mistakes

• Quoting a flat "soakaway installation" price before establishing catchment area or soil type — the same nominal job can vary 3x in real cost
• Absorbing the percolation test into the price on jobs where ground conditions are genuinely uncertain, then discovering clay ground mid-dig with no budget headroom
• Forgetting the oil interceptor/trapped gully requirement on driveway and paved-area catchments — this is a compliance requirement, not an optional extra, and adds £150–£250
• Under-pricing hand-dig access jobs at the same rate as machine-accessible ones
• Not itemising Building Control notification separately — customers are often unaware this is a required step distinct from the trade cost

Frequently Asked Questions

Should I quote soakaway installation per cubic metre of storage or as a fixed job price?

Use cost-per-cubic-metre-of-storage as your internal costing benchmark (roughly £550–£900/m³ of storage for a standard crate soakaway with reasonable access, more for clay ground 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 or ground conditions, so a pure per-m³ rate invites disputes.

What margin should I target on soakaway work?

25–35% is standard, consistent with other groundworks and drainage categories. Push toward the top of that range on hand-dig, clay-ground, or Building-Control-notified jobs where risk and admin overhead are higher; the lower end is appropriate for straightforward machine-accessible jobs in known-good ground.

Is it worth pricing the percolation test as a free "no obligation" survey to win the job?

Generally no, for anything beyond the simplest domestic job. A properly conducted percolation test (three trial pits, timed water-fill readings, BRE 365 calculation) takes real time and determines whether the whole job is even viable — giving it away devalues that expertise and encourages customers to shop the free survey around multiple contractors. A modest £150–£250 test fee, creditable against the job if they proceed, is standard practice and filters out non-serious enquiries.

How much extra should I charge if the customer wants a new soakaway AND the existing failed one removed?

Price removal as a separate line item — typically £150–£400 depending on whether the old soakaway is a rubble pit (easier to simply abandon/backfill if not causing issues) or a crate system that's collapsed or silted (needs full excavation and disposal). Don't assume removal is a quick add-on; a collapsed or silted crate soakaway can be harder to excavate than installing a new one from scratch.

Does a soakaway need planning permission?

Usually no — soakaway installation itself is not normally a planning matter. However, if it's part of a wider project requiring planning permission (a driveway exceeding the GPDO Class F 5m² impermeable threshold without SuDS compliance, or work at a listed building), the wider project's planning status applies. Building Control notification is a separate, more commonly triggered requirement — see the Key Facts section above.

Regulations & Standards

• Building Regulations Approved Document H3 — surface water drainage discharge hierarchy; soakaway is the first-choice route ahead of watercourse or sewer connection

• BRE Digest 365 — the standard UK percolation test and soakaway sizing methodology

• BS EN 752:2017 — drain and sewer systems outside buildings, general principles referenced in soakaway design

• CIRIA C753 (2015) — The SuDS Manual; sustainable drainage systems design guidance for larger or development-scale schemes

• Water Industry Act 1991, Section 106 — governs connection to a public surface water sewer, the fallback route when soakaway infiltration isn't viable

• NHBC Standards Chapter 5.3 — drainage below ground, relevant to new-build warranty compliance

• CDM Regulations 2015 — excavation safety; trenches/pits over 1.2m deep require shoring or battered sides

• GOV.UK — Approved Document H, Drainage and Waste Disposal

• BRE — Digest 365 Soakaway Design

• CIRIA — SuDS Manual C753

• HSE — HSG47 Avoiding Danger from Underground Services

• NHBC Standards

• soakaway sizing — full BRE Digest 365 percolation test method and worked sizing calculation

• soakaways — overview of soakaway design and Part H3 discharge hierarchy

• soakaway installation guide — the installation technique this guide is priced around: excavation, crate/rubble construction, siting rules

• french drain installation pricing guide — pricing for French drain work that often connects to a soakaway outfall

• part h drainage — Building Regs Part H overview covering foul and surface water drainage requirements