Landfill gas and the consultation distance around old tips

Development within 250 metres of land that is, or in the previous thirty years has been, used for the deposit of refuse or waste triggers consultation with the Environment Agency under the Development Management Procedure Order 2015. That is the origin of the 250 metre figure people quote, and it is a consultation trigger rather than a prohibition. What decides the application is ground gas: whether methane and carbon dioxide are migrating towards the site, how much, and what protection the buildings need.

Landfill gas is a different problem from chemical contamination of the soil, and a site can be clean on one and difficult on the other.

Why a buffer exists around a closed landfill

Decomposing waste generates methane and carbon dioxide for decades after a tip closes. Both migrate laterally through permeable strata, along service trenches and under hardstanding, and both accumulate in voids and below floor slabs.

Methane is flammable at concentrations between roughly 5 and 15 per cent by volume in air, and carbon dioxide is an asphyxiant that displaces oxygen in confined spaces. Neither is detectable by occupants. That combination is why the planning system treats proximity to a tip as a matter requiring positive evidence rather than reassurance.

Distance alone tells you very little. Gas can travel hundreds of metres through a gravel horizon and almost nowhere through clay, so a site 200 metres from a tip on London Clay may need less protection than one 400 metres away on river terrace gravels. The geology matters more than the radius.

Settlement is the other legacy issue. Where a site sits on made ground or on the edge of a tip, differential settlement and the consequences for foundations need a separate assessment, and the Geotechnical Assessment page sets out what that covers.

Finding historic tip boundaries and licensing records

Tips are poorly recorded and their mapped boundaries are frequently wrong. Assemble the evidence from several sources and expect them to disagree.

1. The Environment Agency's historic landfill dataset, available through its open data service, which gives recorded boundaries, dates and waste types where known.
2. Historic Ordnance Survey mapping across several editions, which shows quarries, brickworks, gravel pits and marked spoil that later became tips.
3. The council's own records, including waste licences, contaminated land register entries and planning history for the tip site itself.
4. Aerial photography, which often shows tipping in progress on a date the records do not mention.
5. Borehole records held by the British Geological Survey, which sometimes reveal made ground of unexpected depth.

Dates matter as much as boundaries. A tip closed in 1968 that took inert construction waste is a different proposition from one closed in 1994 that took municipal solid waste, and the gas generation curve for the second is still meaningful decades on.

Unlicensed tipping is the gap in the record. Small scale filling of pits and ponds by farmers and local builders was normal and was never licensed, and it shows up only on old maps, in local memory or in the first trial pit. Treat an absence of records as an absence of information rather than as evidence of a clean site, and put the check on the standing list in checking a site for constraints before you make an offer.

Monitoring rounds and how many are enough

Gas monitoring is the part clients try to compress and the part that cannot be compressed. A single visit tells you nothing useful, because ground gas emission varies with atmospheric pressure, rainfall, temperature and the water table.

The approach in CIRIA C665 is to monitor across conditions rather than to a fixed count, with the number of visits increasing with the risk. In practice that means several rounds spread over a period long enough to cover seasonal variation, including at least some visits during falling atmospheric pressure, when gas is drawn out of the ground most strongly.

Each round should record gas concentrations, borehole flow rate, atmospheric pressure and its trend, groundwater level and weather. Concentration on its own is not enough, because the gas screening value that drives the design combines concentration with flow rate. A high concentration at negligible flow is a much smaller problem than a moderate concentration at a measurable flow.

Two practical points. Install the wells so they respond to the right horizon rather than to the topsoil, with response zones targeted at the permeable layer. And programme the monitoring before the planning application rather than accepting a condition, because a condition requiring monitoring to be completed before development means a delay of months at the worst possible point in the programme.

Characteristic situations and gas protection design

Design follows the classification in BS 8485, which places a site in a characteristic situation from very low to high risk based on the gas screening values and the conceptual model. Each situation carries a required level of protection expressed as points achieved through a combination of measures.

The measures that fail are almost always detailing rather than the membrane itself. Service penetrations, lap joints, junctions at the perimeter and the transition to a suspended slab are where gas enters, and a correctly specified membrane installed badly performs worse than a simpler system installed well.

Passive ventilation is preferred to active systems because it needs no power, no maintenance and no management company. An active system on a residential scheme brings a long term obligation that somebody has to fund and operate for the life of the buildings, and councils are right to be cautious about it.

Design the protection for the building form you are actually proposing. A scheme that changes from houses with ventilated voids to flats over a podium during the design process needs the gas design revisiting, not carrying across.

Who is consulted and what they ask for

The Environment Agency is the statutory consultee within the 250 metre distance, and its interest is in controlled waters, gas migration and the tip itself as a permitted or formerly permitted facility.

The council's contaminated land or environmental health officer usually carries more weight on the detail, because they review the risk assessment and draft the conditions. Building control has a separate interest through the requirements on site preparation and resistance to contaminants. On a large scheme the water undertaker will also comment where services cross made ground.

Talk to the council's contaminated land officer before the investigation is scoped. They will tell you which characteristic situation classification and which guidance version they expect to see, whether they want a particular monitoring period, and what their standard verification requirement is. An investigation scoped to satisfy the officer who will read it is worth two that were not.

Conditions and verification reporting

Expect a phased condition, and read the wording before it is imposed. The usual structure requires a site investigation and risk assessment to be approved, then a remediation and protection strategy, then a verification report before occupation, with a separate limb covering unexpected contamination found during works.

The verification stage is where schemes get stuck. Verification of gas protection follows the approach in CIRIA C735, and it needs photographic evidence of installation as it happens, integrity testing, records of penetrations and sign off by a suitably qualified person. None of that can be reconstructed after the slab is poured and the walls are up.

Build the verification into the construction programme and brief the contractor at tender stage. A verification report assembled from memory six months after the event is refused, and a refusal at that point means an occupation delay on finished houses.

Land contamination reports are work for suitably qualified and experienced persons, prepared in accordance with BS 10175 and the current national guidance, and the professional input is a legal expectation rather than a recommendation. A Contaminated Land Assessment drafted in PlanScope is a starting point for that professional, not a substitute for them.

What to submit

The application needs a conceptual site model that a reviewer can follow: sources, pathways and receptors, with the tip position, the dates, the waste types where known and the intervening geology.

Add the monitoring data in full, as a table of every round with the atmospheric pressure and trend, the gas screening values calculated from concentration and flow, the characteristic situation classification with the reasoning, the proposed protection measures with the points calculation, and typical details for the membrane, the void and the service penetrations.

Then add the verification plan. Naming who will verify, what evidence will be collected and when, at application stage, removes the argument later and often produces a shorter and more workable condition.

You can draft the narrative sections of the contamination and ground gas reporting in PlanScope, with your investigation data inserted, and send it for professional review and sign off. Browse the full set at the PlanScope report index.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.