How to Price a French Drain Installation

Summary

French drains are one of the most commonly under-priced jobs in groundworks and landscaping. The trade description sounds simple — "dig a trench, put a pipe in it, fill with gravel" — and many tradespeople quote it like a day's digging. In reality the cost driver isn't the pipe, it's the excavation: depth, spoil disposal, access for machinery, and what's discovered in the ground (roots, services, rock, existing drains).

This guide is for landscapers, groundworkers, and general builders pricing French drain work for domestic clients — wet lawns, damp external walls, retaining wall hydrostatic relief, and driveway/patio interception drains. It covers labour and materials cost buildup, day-rate vs linear-metre pricing, machine hire decisions, and worked examples across the three common job types. For the installation technique itself (pipe sizing, fall, trench depth by application), see french drain installation — this guide stays on the money.

The single biggest pricing mistake is quoting a garden drain and a foundation-relief drain the same way. A 450mm garden run and a 900mm hydrostatic-relief run against a retaining wall can have the same linear length but the second costs 2–3x more in labour and gravel volume. Depth cubes the material cost, not doubles it.

Key Facts

• Trench excavation — 450mm deep runs are typically hand-diggable in soft ground; anything over 600mm or longer than ~15m usually needs a mini digger (1.5–3 tonne)
• Mini digger hire — £90–£150/day self-drive; £250–£400/day with operator (often cheaper to sub the whole excavation to a local groundworker with their own kit)
• Spoil disposal — Excavated soil from a 10m × 400mm × 450mm trench is roughly 1.8m³ — one van load, or a grab lorry at £150–£280 for larger jobs
• 20mm angular gravel — £45–£70 per bulk bag (~850kg, ~0.5m³ once compacted); a 10m run at 450mm deep needs roughly 1.5–2 bulk bags
• Perforated land drain pipe — 100mm PVC slotted coil £1.50–£3/m; 150mm £3–£5.50/m
• Geotextile filter membrane — £1–£2/m² for non-woven polypropylene BS EN ISO 10319 rated fabric
• Labour rate (groundworker/landscaper) — £180–£280/day per person outside London; £280–£400/day London/SE
• Two-person team productivity — 8–12 linear metres/day at 450mm depth in easy ground; 4–6m/day at 750mm+ or in clay/rooty ground
• Soakaway connection — Adds £300–£1,200 depending on whether an existing soakaway is used or a new one built (see soakaway sizing)
• CAT/Genny scan — Always budget £0 in-house or £60–£120 sub'd if you don't have your own detector; never skip this line
• Minimum job charge — Most tradespeople should set a £450–£650 minimum for French drain call-outs regardless of length — mobilisation, plant, and setup don't scale down
• VAT — Standard rate 20% applies to French drain work; no reduced-rate exemption unless part of a wider approved energy-efficiency or disabled-adaptation scheme
• Access surcharge — Add 20–40% where machine access is blocked (side gate under 900mm, steps, no vehicle access) and work becomes hand-dig only
• Discharge rules — Part H3 discharge hierarchy governs where the water can go; getting this wrong means a return visit that eats the margin — see part h drainage

Quick Reference Table

Figures exclude VAT and assume normal vehicle/machine access. Add 20–40% for hand-dig-only sites.

Detailed Guidance

Building the price bottom-up

For any French drain job, price these five components separately, then sum:

1. Survey and design time — 30–60 minutes on site to confirm the water source, trench route, fall, and outfall. Fold into the quote, don't bill separately for domestic jobs under £2,000.
2. Excavation — the dominant cost. Calculate trench volume (length × width × depth) and divide by your team's daily output in that ground condition.
3. Materials — pipe, gravel, geotextile, plus any pea shingle capping or turf/topsoil reinstatement.
4. Outfall/soakaway — either connecting to an existing soakaway (cheap) or constructing a new one (expensive — see the soakaway sizing calculator).
5. Waste disposal — spoil removal if it can't be reused on site (raising beds, filling low spots).

Worked example 1 — garden wet-patch drain, 12m run, 450mm deep

Typical job: lawn stays waterlogged after rain, client wants a French drain to a soakaway at the boundary.

Works out at roughly £74/linear metre inc VAT — mid-range for a straightforward garden run.

Worked example 2 — retaining wall hydrostatic relief drain, 8m run, 900mm deep

Sloped garden, block retaining wall showing damp staining and slight bulging, client's structural engineer has specified a drainage layer and French drain behind the wall.

Works out at roughly £331/linear metre inc VAT — reflects depth, machine hire, and structural drainage board.

Worked example 3 — new soakaway required (add-on pricing)

If there's no existing soakaway to tie into, price the soakaway separately using BRE Digest 365 percolation-based sizing (see soakaway sizing). A typical crate soakaway for a small garden drain (1m × 1m × 1m pit, plastic crate system) runs £450–£800 in materials and labour; a rubble-filled pit is cheaper in materials (£150–£250) but needs more excavation volume for the same storage capacity. Always price the soakaway as its own line item — bundling it into a per-metre drain rate hides the real cost and undersells the job when a customer later asks "why does next door's cost less per metre."

Pricing for access and ground conditions

Two variables move the price more than anything else in the spec sheet:

• Access — if a mini digger can't reach the trench line (narrow side passage, steps, no rear vehicle access), the job becomes hand-dig only. Productivity drops from 8–12m/day to 3–5m/day. Price this as a 25–40% uplift, not a vague "on-site variation" — clients accept it far better when it's itemised upfront.
• Ground conditions — heavy clay, rooty ground near hedges/trees, or hitting rubble/rock all slow excavation. If you haven't dug a trial pit before quoting, price with a contingency line ("ground conditions allowance: £150–£400, only charged if encountered") rather than absorbing the risk or over-quoting every job to cover it.

Combining with other drainage work

French drains are frequently sold alongside other water-management work — it's worth flagging these as upsells rather than quoting the drain in isolation:

• Soakaway sizing/construction if the existing one is undersized or absent
• Gutter and downpipe diversion if roof water is contributing to the wet area
• Damp-proofing/tanking if the drain is solving a symptom of a deeper wall damp issue
• Patio or driveway drainage channels if surface water (not just subsurface) is the real problem — see patio installation pricing guide

Common pricing mistakes

• Quoting a flat "£X per metre" without stating the depth and ground assumption — leads to disputes when depth turns out to be double what was assumed
• Forgetting spoil disposal — 10m of trench at 450mm deep is nearly 2 tonnes of soil that has to go somewhere
• Under-pricing the soakaway or outfall connection as "included" when it's actually the most variable-cost item on the job
• Not budgeting a CAT scan line — hitting a service on a domestic garden drain is rare but the liability isn't worth skipping the £0 (own kit) or £60–£120 (subbed) check

Frequently Asked Questions

Should I price French drains per linear metre or as a fixed job price?

Use linear-metre pricing as your internal costing tool, but present the client with a fixed total, broken into the line items above (excavation, materials, outfall). Pure per-metre quoting invites disputes if depth or ground conditions change mid-job; a fixed price with a documented ground-conditions contingency protects both sides.

How much should I charge for a same-day emergency French drain (e.g. after a flood)?

Add a 25–50% call-out premium for emergency/urgent work — you're compressing planning, material sourcing, and possibly working outside normal hours. Standard job cost buildup still applies; the premium reflects disruption to your schedule, not extra technical difficulty.

Is it worth owning a mini digger for regular French drain work?

If you're doing more than roughly one drainage job a fortnight, a used 1.5–2 tonne mini digger (£8,000–£15,000) pays for itself against daily hire rates (£90–£150/day plus delivery £80–£150 each way) within 12–18 months. Below that frequency, hire remains cheaper once you account for maintenance, transport, and storage.

What margin should I target on French drain work?

25–35% is standard for groundworks/landscaping in this category. Push toward the top of that range on hand-dig, access-restricted jobs where your risk (ground conditions, time overrun) is higher, and the bottom on straightforward machine-accessible garden runs where competition on price is stronger.

How do I price a job where the customer wants the drain AND a new lawn/patio afterward?

Price the drain and the reinstatement as separate line items even if delivered as one job. Reinstatement (turf, topsoil, patio relay) is a different skill and cost structure from drainage excavation — bundling it obscures which part of the job is actually profitable.

Regulations & Standards

• Building Regulations Approved Document H — surface water drainage discharge hierarchy (H3); governs where French drain outfall can legally go

• Land Drainage Act 1991 — prohibits discharging water onto neighbouring land or the highway without consent

• BS EN ISO 10319 — geotextile tensile strength testing standard, relevant to filter membrane specification

• BRE Digest 365 — soakaway design and percolation testing methodology, needed when a new soakaway is part of the job

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

• HSE HSG47 — avoiding danger from underground services; applies to all excavation pricing risk assessment

• Planning Portal — Approved Document H, Drainage and Waste Disposal

• HSE — HSG47 Avoiding Danger from Underground Services

• BRE — Digest 365, Soakaway Design

• Land Drainage Act 1991 — legislation.gov.uk

• french drain installation — the installation technique this guide is priced around: pipe sizing, fall, and trench spec by application

• soakaway sizing — worked BRE Digest 365 sizing calculation for pricing new soakaway add-ons

• part h drainage — Building Regs discharge hierarchy that determines where French drain outfall can legally go

• clay soil drainage — broader clay-soil drainage context including mole draining as an alternative/complement