Soil and Inert Waste: WM3 Classification Explained

Soil is the heaviest, cheapest-looking waste most groundworkers, landscapers and skip firms deal with, and one of the easiest to get wrong. A few tonnes of clean subsoil can go to an inert site for very little. The same tonnage from a former garage forecourt can be a hazardous waste with a completely different route, price and paperwork. Nothing about the colour of the soil tells you which one you have. This guide explains how soil is classified, what WM3 is for, and when testing is needed before the first load leaves site.

What counts as soil waste

Soil becomes waste when the holder discards it or intends to discard it. Excavated topsoil, subsoil, clay, sand, gravel and stones from a dig are all in play, and so is made ground, the mix of natural soil, rubble, ash and fill that sits under much of urban Britain. Chapter 17 of the List of Wastes (England) Regulations 2005 (with a Welsh equivalent) covers soil from construction and excavation, and the common entries are:

• 17 05 04, soil and stones other than those mentioned in 17 05 03
• 17 05 03*, soil and stones containing hazardous substances
• 17 05 06, dredging spoil other than that mentioned in 17 05 05
• 20 02 02, soil and stones from garden and park waste, which is a chapter 20 municipal entry

17 05 04 and 17 05 03* are a mirror pair: the same material can fall under either, depending on what is in it. That is the reason soil needs assessing rather than assuming. The asterisk means hazardous, and a hazardous classification brings a consignment note and the rules in our guide to hazardous waste regulations.

Hazardous, non-hazardous and inert

Waste is classed as hazardous or non-hazardous under the waste classification rules. Inert is a third label, but it is not a separate legal class. It comes from the landfill rules, which define inert waste as waste that does not undergo any significant physical, chemical or biological change: it does not dissolve, burn, react, biodegrade or release meaningful pollution into water. Inert waste is a sub-set of non-hazardous waste, and it matters because inert landfills and some recovery sites accept it at a lower cost.

So two questions apply to any load of soil. The first is whether it is hazardous. If it is not, the second is whether it also meets the acceptance criteria of the site you want to send it to. A load can be properly classed as non-hazardous and still be refused by an inert site because it fails a leachate or contamination limit.

What WM3 is and how it is used

WM3 is the technical guidance on waste classification and assessment, published jointly by the environment agencies of the UK (the Environment Agency, Natural Resources Wales, SEPA and NIEA). It explains how to decide whether a waste is hazardous, how to treat mirror entries, and how to assess the fifteen hazardous properties (HP1 to HP15, covering things such as toxicity, flammability, corrosiveness and ecotoxicity). Always use the current edition from gov.uk, because the guidance has been revised since it was first issued.

For soil the working method is a stepwise one:

1. Identify the candidate codes. For excavated soil these are normally the 17 05 03* and 17 05 04 pair.
2. Establish what might be in it. Use the site history, any ground investigation report and what you can see and smell on site.
3. Get it tested for the relevant substances. A competent person chooses the analysis suite to fit the site history.
4. Compare the results with the thresholds. WM3 sets out how to assess concentrations against the hazardous property thresholds.
5. Record the outcome. Keep the assessment, the results and the reasoning with your waste records.

The responsibility sits with the producer or holder of the waste. Your carrier or skip firm may ask for the assessment, and the receiving site will do its own pre-acceptance checks, but neither takes over the duty to classify the waste correctly.

When testing is needed

There is no rule that says every load of soil needs a laboratory report. The question is whether you can reasonably be sure what is in it. Testing is usually needed when:

• The site has a history that points to contamination: petrol stations, garages and workshops, gasworks, foundries, scrapyards, chemical or tanning works, railway land, farm yards and former landfill.
• You find evidence while digging: staining, a sheen on water, odours, ash, clinker, slag, buried tanks or pipework, or fragments that look like asbestos cement.
• The ground is made ground or imported fill of unknown origin, not natural material.
• The receiving site, treatment operator or landfill asks for classification evidence before it will accept the load, which many do.
• The soil has been stockpiled alongside other materials, or mixed with other waste on site.

At the other end, natural soil from a site with no industrial history, taken from a clear and documented source, may be accepted by some sites on a producer's declaration, and inert sites can typically take soil and stones from uncontaminated sites without testing. The operator decides that under its permit, so ask what it will accept in writing before you dig.

Sampling and laboratories

A test result is only as good as the sample behind it. Samples need to represent the material that will actually be moved, taken at suitable depths and spacing across the area or stockpile, and handled so they are not cross-contaminated. The number of samples depends on the volume, how variable the ground is and what the receiving site requires, so there is no single figure that fits every job. A competent person, such as an environmental consultant or an experienced site engineer, should plan it.

Use a UKAS-accredited laboratory, and for soil analysis look for MCERTS accreditation where it applies. Ask the lab to report in a form you can use for WM3 assessment and for landfill acceptance, because the two tests look at different things.

One common trap is relying on a planning-stage ground investigation report. Those reports are written to assess risk to people and groundwater on the finished site, not to classify waste leaving it. They are a good starting point, but the sampling may not cover the material you are actually moving, and the analysis may not match what WM3 requires. Ask the consultant whether a waste classification can be added.

Where soil can go

The destination follows the classification and the site's permit:

• Inert landfill or inert recovery: for non-hazardous soil that meets the acceptance criteria, sometimes with a lower landfill tax rate.
• Non-hazardous landfill or a treatment site: for soil that is non-hazardous but fails inert criteria, usually at a higher cost.
• Hazardous landfill or a permitted treatment facility: for soil assessed as hazardous, with a consignment note.
• Reuse on another site: possible within a waste exemption or under an environmental permit, provided the material is suitable and the conditions are met. See waste exemptions explained.

Landfill tax is set by HMRC, and the rate depends on what the material is. Soil can qualify for the lower rate as a qualifying material, while contaminated or mixed loads may not. The landfill operator makes that call. Our guide to landfill tax rates in 2026 shows how the rates are structured; check HMRC's current guidance for the qualifying-material rules.

Avoiding waste in the first place

Excavated soil is not always waste. If it is reused in the same development, or moved to another site under a recognised approach, it may never need to be treated as waste at all. In England the industry route is the CL:AIRE Definition of Waste Code of Practice, which uses a materials management plan and a qualified person to show that the soil is suitable, certain to be used and used without further treatment. Where it applies, it saves transport, landfill and paperwork. It does not apply to everything, and the Environment Agency can still step in if the conditions are not met, so get agreement before you rely on it.

Segregation also saves money. Keep soil away from rubble, timber, plastics and general waste. Soil mixed into a general skip is charged as mixed waste, and a soil load with a few bags of rubbish in it can be refused by an inert site.

Paperwork for soil loads

Non-hazardous soil goes on a waste transfer note with a clear description. Something like: “Soil and stones from excavation, natural ground, no made ground, classified non-hazardous”, then the EWC code (17 05 04), the quantity in tonnes or loads, the carrier's registration number, and the permit or exemption details of the receiving site. Add the reference and date of your classification report so the link between the note and the assessment can be shown later. If the soil is hazardous, a consignment note is needed instead.

For a regular run from one project to one receiving site, a season ticket can cover repeat loads of the same material, provided the soil does not change. If the ground conditions change as the dig goes deeper, the description and classification need to be revisited. Keep records for at least two years for transfer notes (three for consignment notes), and keep the assessment alongside them.