How to Quote and Run a Commercial Drainage Installation

What you will learn

• How to read a drainage layout and price pipe runs, manholes, SuDS elements, and reinstatement as separate cost items.
• Which regulatory approvals - Building Regulations Part H, Section 106, and Section 104 - affect your programme and when to start them.
• How to structure stage payments around CCTV testing, Building Control sign-off, and adoption milestones.
• When unexpected ground conditions or uncharted services become a variation, and how to document and price them correctly.
• What the installation sequence looks like from pre-start survey through to CCTV testing and reinstatement sign-off.

Commercial drainage is one of the more unforgiving sub-trades to price. You are working from someone else's design, excavating ground that may not match the trial pit data, coordinating with Building Control and water authorities whose approval timelines are outside your control, and handing over a system that must pass a CCTV survey and air test before anyone will sign off the final payment. Get the quote wrong and you absorb the overage. Get the programme wrong and you hold up the whole site.

This guide covers how to price a commercial drainage contract accurately, run the installation to a clear sequence, manage variations caused by ground conditions or design changes, and hit the handover milestones that release your final invoice.

Reading the Design and Scoping the Quote

Commercial drainage projects are always quoted from a design - a drainage layout prepared by a civil engineer or drainage designer showing pipe runs, pipe sizes, invert levels, manhole positions, and connection points. Before pricing anything, confirm that the design is complete and issued for construction, not still in development. Quoting from a preliminary design means quoting against assumptions that may change, and the contractor absorbs the difference.

The key items to price from the drawing are:

• Foul drainage pipe runs - measured in linear metres, with pipe diameter and depth noted for each run
• Surface water pipe runs - same measurement approach, but check whether SuDS elements are required at discharge
• Manholes and inspection chambers - number, depth, and material (precast concrete rings or HDPE chambers)
• Connection to existing sewer or watercourse - type, depth, and whether a Section 106 application is within your scope
• Excavation - trench width and depth determine plant requirements and spoil volumes
• Backfill specification - granular fill, compacted in layers, to the engineer's requirements
• Reinstatement - tarmac, concrete, block paving, or topsoil; each has a different cost and timeline

SuDS elements need separate pricing. An attenuation tank, soakaway crate system, or channel drain network is not simply additional linear metres of pipe - each has its own excavation profile, installation sequence, and material cost. Mainline Groundworks publishes indicative rates for commercial drainage pipe runs: surface water from around £30-80 per linear metre, foul drainage from around £40-90 per linear metre. Those ranges are wide enough to make a detailed quantity take-off essential. GCS Contractors notes that commercial attenuation systems typically start from £5,000, rising with storage volume and ground conditions. Both sets of figures confirm that a single rate-per-metre applied across a whole scheme will either miss the budget or leave margin on the table.

Pricing the Work

The mistake drainage contractors make when pricing commercial work is treating it like a rate-card job. Linear metre rates give you a starting point, but the real cost drivers are depth, access, and reinstatement - none of which appear explicitly on a rate card.

Depth uplift. Standard pipe-in-trench pricing typically assumes trench depths of around 1.0-1.5 metres. Below 1.5 metres, you need different plant, longer cycle times for backfilling in layers, and possibly shoring or trench boxes. Price deeper runs separately and check the invert level schedule for every section before submitting your quote.

Ground conditions. The design is based on a ground investigation report, but actual ground conditions often differ. If the trial pit data shows firm sub-grade and you find made ground or soft clay, your excavation cost changes materially. Include a ground condition risk note in your quote and agree in the sub-contract how additional works will be instructed if conditions prove worse than the design assumed.

Access constraints. A clear greenfield site allows efficient machine excavation. A site where drainage runs adjacent to live services, beneath a road open to traffic, or under existing hardstanding has a completely different cost profile. Price access restrictions explicitly - that prevents disputes when the constraint turns out wider or deeper than either party expected.

Traffic management. If any drainage runs require working in or adjacent to the public highway, you will need a traffic management scheme and potentially a Temporary Traffic Regulation Order (TTRO) for road closures. Council fees for TTRO approval and the time to prepare and manage the scheme add meaningful cost to any contract that crosses public road. This should be a separate line item in your quote, not absorbed into the earthworks rate.

Reinstatement. Tarmac reinstatement to highway standard, inspected and signed off by the highways authority, costs substantially more than topsoiling and seeding. Check the reinstatement specification before pricing - it is not always clear on drainage drawings and frequently falls between scopes when a groundworker and a surfacing contractor are both on site.

Regulatory Approvals and Their Programme Impact

Commercial drainage installation is not a start-on-Monday trade. Several approvals need to run in parallel with - or ahead of - your excavation programme, and if you start without them you will either have to stop or face a costly retrospective application.

Building Regulations Part H covers drainage and waste disposal for new buildings and extensions. Your installation must comply with Part H requirements. Building Control will need to inspect certain stages - particularly the pipe installation before backfilling - and must sign off the system before handover. Find out at mobilisation who has appointed the Building Control body (local authority or an approved inspector) and agree an inspection notification procedure before you start excavating.

Section 106 agreements cover new connections to an adopted public sewer. The water authority - Anglian Water, Thames Water, Severn Trent, or whichever regional authority holds the sewer - must approve the connection point, method, and reinstatement before you work on their asset. Applications typically take four to six weeks to process and must be submitted before connection work begins. If the Section 106 application is within your scope, the submission date belongs on your programme from day one.

Section 104 adoption agreements apply when new drainage on a development will be adopted by the water authority as public infrastructure. Adoption requires the system to be built to the water authority's specification, inspected, CCTV surveyed, snagged, and formally accepted before they take it on. This affects material choices - many water authorities specify particular pipe materials and joint types for adopted sewers - and adds a snagging and adoption period to the tail end of your programme that must be accounted for in your final payment terms.

Running the Installation

Once approvals are in progress and the programme is agreed, the installation follows a consistent sequence on most commercial drainage contracts.

1. Carry out a pre-start survey. Walk the site with the drainage layout and mark all known services. Use a CAT scan to locate buried services and compare findings against utility records. Note any discrepancies between the drawing and site conditions - this is your baseline if ground conditions or uncharted services become a variation claim later.
2. Set out and excavate trenches. Excavate to the depths and widths specified, checking invert levels as you proceed. Lay granular bedding material to the correct depth and compaction before placing any pipe.
3. Install pipework and manholes. Lay pipe from the lowest invert upward, maintaining falls and checking alignment. Install manholes and inspection chambers as you progress, keeping benching clean and channel profiles correct - these are the details CCTV surveys expose if they are wrong.
4. Make connections. Foul connections to the public sewer must be made under the approved Section 106. Notify the water authority when you are ready and follow their site supervision requirements for the connection work.
5. Backfill and compact. Backfill in layers - typically 150mm compacted layers in the pipe zone, then selected fill as specified. Maintain compaction records if the specification or adoption agreement requires them.
6. Test the system. Air testing or water testing of all drain runs, CCTV surveys of installed sections, and a pre-handover check of manholes and inspection chambers for cleanliness and damage. Complete your own internal pre-inspection before presenting the system to Building Control or the water authority.
7. Reinstate and complete. Reinstate to the specified standard. For highway reinstatement, follow the relevant code of practice and arrange the highways authority inspection before final sign-off.

Variations, Ground Conditions, and Cost Recovery

Ground conditions are the biggest source of variation on drainage contracts and also the hardest to recover cost on without the right paperwork in place from the start.

When excavation reveals conditions that differ materially from the ground investigation - softer material, higher groundwater, buried obstructions, uncharted services - stop and notify the client or main contractor in writing the same day. Record what you found, where you found it, and how it differs from the design assumption. Photograph everything before proceeding.

Agree how the additional works will be instructed before any additional work is carried out. The options are typically a formal variation order at agreed rates, daywork at a pre-agreed schedule of rates, or a lump sum for a defined additional scope. Proceeding on verbal instruction without a written record leaves you pricing the variation retrospectively, at which point the client has much less incentive to accept your costs.

Uncharted services are a distinct category. If excavation hits a gas, electricity, water, or telecoms service that was not on the utility records and diverts your route or delays your programme, that is almost always a client risk event. Notify immediately, document the obstruction, and agree a revised route or programme instruction before continuing.

Stage Payments and Handover Documentation

Commercial drainage contracts pay most cleanly against measurable milestones rather than a monthly assessment. The most defensible payment stages on a typical contract are:

• Mobilisation complete, plant and materials on site
• Excavation and pipe installation complete, pre-test CCTV passed
• Connections made and approved under Section 106, Building Control inspection signed off
• Reinstatement complete and inspected
• Final handover: CCTV records, air test certificates, Section 104 or 106 documentation issued, and all snagging complete

Each stage payment should be supported by a written record of what has been completed: photographs, test results, inspection certificates, and water authority correspondence. Disputes about stage payments are almost always disputes about what "complete" means. The cleaner your evidence, the shorter the argument.

Drainage is a trade where handover documentation matters as much as the quality of the installation itself. A system that functions correctly but lacks the CCTV record, the air test results, the Section 106 consent reference, and the Building Control sign-off certificate cannot be handed over cleanly - and in most cases cannot be invoiced in full until it can. Build documentation discipline into your process from day one. Zigaflow's project tracking features let you attach test records and certificates directly to the job, so the handover pack is assembled during the project rather than chased at the end.

For a broader view of how civil groundworks projects are managed from start to final account, see the guide on how to run a groundworks and civils project. For more on managing the construction trades, visit Zigaflow for construction contractors.

Sources

• Drainage Installation Cost UK | Foul & Surface RatesMainline Groundworks · accessed 2026-08-05
• Drainage Contractors Cambridge | GCS Contractors LtdGCS Contractors Ltd · accessed 2026-08-05

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