The ground gas section of a contaminated land assessment

Ground gas is assessed separately from soil contamination, by a different method, over a longer period, and it is the part of a contaminated land assessment most likely to hold up a self-build. Soil samples come back from the laboratory in a fortnight. Gas monitoring runs for months, in rounds, and the rounds have to catch the right weather.

If your site sits on made ground, near a former landfill, over an infilled pond or quarry, on alluvium or peat, or in a coal mining area, assume gas monitoring is coming and programme for it before you buy.

What triggers a ground gas assessment

The Phase 1 desk study identifies plausible source, pathway and receptor links. Ground gas enters that assessment where there is a credible source of methane or carbon dioxide beneath or near the site.

Typical sources are degradable material in made ground, a licensed or historic landfill within a few hundred metres, infilled ponds, pits and quarries, alluvial deposits and peat producing carbon dioxide and methane naturally, and mine gas in coalfield areas. Made ground of unknown origin counts, because unknown means unassessed rather than clean.

Radon is a separate question with its own mapping and its own protection requirements under the building regulations, although the protective measures often overlap with gas protection in practice. Do not let one be assumed to cover the other.

Where the desk study finds no credible source, say so with the evidence and stop. A contaminated land assessment that recommends gas monitoring on every site regardless of findings is not doing its job.

Monitoring rounds, duration and conditions

This is where programmes fail. Gas monitoring means installing standpipes or monitoring wells, then returning repeatedly to take readings, and the readings are only useful if the visits are spread properly.

The usual expectations are a minimum number of rounds over a period of months rather than weeks, with the number increasing with the risk. A low risk site with shallow made ground might be assessed on six rounds over three months. A site adjacent to a landfill needs considerably more, often over a year, and the contaminated land officer will say so.

The critical point is atmospheric pressure. Gas emissions rise as barometric pressure falls, so monitoring visits must include periods of low and falling pressure. Six rounds taken in settled high pressure weather tell you very little, and a competent reviewer will reject them. Each visit should record methane, carbon dioxide, oxygen, carbon monoxide where relevant, flow rate, atmospheric pressure and the water level in the well.

Water level matters more than people expect. A well flooded to the response zone cannot produce a meaningful reading, and a monitoring set full of flooded wells is a wasted season.

Deriving a characteristic situation

The assessment converts monitoring data into a gas screening value, broadly the product of the borehole flow rate and the gas concentration, and uses that to place the site in a characteristic situation ranging from very low risk upwards.

That characteristic situation then drives the protection required. The scoring approach in the British Standard for gas protection design allocates points to the structural barrier, the membrane and the ventilation layer, and the design has to reach the score required for the situation identified.

Two things go wrong. The first is a characteristic situation derived from too few rounds, which a reviewer will treat as unsupported. The second is a worst case assumption adopted to save monitoring time, which sounds prudent and costs real money, because moving up one situation can add a ventilated void and a verified membrane to a house that did not need either.

On a single plot self-build, the monitoring usually costs less than the extra protection it avoids. That is the calculation worth doing at the outset.

Protection measures and how they work

Gas protection is a system rather than a product, and the three elements work together.

Passive ventilation, either a clear subfloor void or a proprietary geocomposite vent layer, is the usual solution for housing. Active systems with fans belong to higher risk situations and bring a maintenance obligation that a private householder should not be signed up to without knowing it.

Service penetrations are where systems fail in practice. Every pipe, duct and cable through the slab needs a proper top hat detail, sealed to the membrane, and the sequencing has to allow that to be done before the trades that would damage it arrive.

Verification, and why it is a separate report

The planning condition is not discharged by installing the system. It is discharged by evidencing that the system installed matches the system designed.

Verification means an independent specialist inspecting the works as they proceed, with a photographic record of the membrane before it is covered, integrity testing where specified, records of penetration details, and a report confirming that the installed system achieves the required protection. Good practice guidance on testing and verification of gas protection systems sets out what that record should contain.

Programme it. Verification requires visits at points in the build that cannot be repeated, and there is no retrospective way to photograph a membrane under a slab. On a self-build where the ground worker is gone and the slab is poured, the honest options are a new investigation or an argument.

The report then goes to the council as part of discharging the contamination condition, usually before occupation rather than before works start.

Where monitoring gets cut short

It happens on nearly every tight programme. Two rounds are missed, the last visit falls in a high pressure spell, and the report goes in with a caveat.

Councils respond in one of three ways. They refuse to discharge the condition and require further monitoring, which costs a season. They accept the data with a conservative characteristic situation, which costs money in protection. Or they accept it as submitted, which happens where the risk is low and the data is consistent.

You can influence which one you get. Agree the monitoring scope with the contaminated land officer in writing before the wells go in, record the weather conditions for every visit, and explain any gaps rather than leaving them to be noticed. An officer who agreed the scope is far more likely to accept the result.

Land contamination assessment work is a field where the National Planning Policy Framework places responsibility on the developer, and councils expect reports prepared in accordance with BS 10175 by suitably qualified and experienced persons. That is a legal and professional requirement rather than a preference, and a drafted document is a starting point for that professional rather than a replacement for them.

Where the same investigation is being used for foundation design, coordinate with the Geotechnical Assessment so that one set of boreholes serves both purposes. Drilling twice on a small plot is a waste that a single conversation at the scoping stage avoids.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Browse the PlanScope report types to see how the contamination documents fit with the rest of a householder or small site submission.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.