Historic landfill records and sites next door to one

A historic landfill site next door changes the technical work on a development more than most mapped constraints, and it appears on no local plan proposals map. The mechanism is ground gas. Waste decomposes, produces methane and carbon dioxide, and those gases migrate laterally through permeable strata and made ground into buildings that sit outside the tip boundary.

The planning trigger is specific. Schedule 4 to the Development Management Procedure Order 2015 requires the authority to consult the Environment Agency on development within 250 metres of land used for depositing refuse or waste at any time in the previous 30 years. Many councils then ask for a gas risk assessment as a validation item within a similar distance.

The historic landfill dataset and what it records

The Environment Agency publishes a historic landfill dataset covering sites that operated and closed before the current permitting regime, alongside a layer of currently permitted waste sites. Both are visible through the agency's spatial data service and on the MAGIC map.

Two caveats matter more than the data itself.

The boundaries are approximate. They were digitised from paper licences, plans and correspondence of varying quality, and the mapped line can be tens of metres out. A site shown as 300 metres from a tip may be 200 metres from it. Treat the polygon as an indication and confirm the real extent from historic mapping, aerial photography and the council's records.

The dataset is incomplete for older tips. Deposits made before licensing began, informal village tips, filled brickpits, quarries backfilled with domestic waste and wartime demolition rubble often appear only on old Ordnance Survey editions as "old gravel pit" or nothing at all. The Phase 1 desk study has to read the maps rather than only the dataset.

Consultation distances used by councils

The 250 metre figure in Schedule 4 is the statutory consultation trigger, and it has become the working distance for gas risk assessment in most local validation lists. Some authorities apply a shorter distance for a small tip and a longer one for a large or gassing site, and some ask for a screening statement at any distance where made ground is suspected.

The Environment Agency's own interest is narrower than people expect. It comments on risks to controlled waters and on its own historic records, while the human health and gas protection questions usually sit with the council's environmental health or contaminated land officer. Write to both. The environmental health officer is the one who will draft the condition.

Ground gas as the main planning issue

For a site adjacent to a tip rather than on it, the conceptual site model is straightforward to describe and easy to get wrong.

The source is the waste mass next door. The pathway is permeable strata, made ground, service trenches, drainage runs and any granular layer that connects the tip to your site. The receptors are the occupants of the new building, and the building itself. A development on clay with no granular pathway is a different proposition from one on sand and gravel with a shared drainage corridor.

That is why service trenches deserve their own paragraph in the assessment. A new sewer or gas main running from the direction of the tip is a ready made conduit, and sealing the ends of ducts is one of the least costly protection measures available.

Gas monitoring rounds and how long they take

Monitoring is the part of the programme people underestimate. Wells have to be installed, left to stabilise, and then visited repeatedly.

A workable expectation:

1. Install monitoring wells in positions that sit between the source and the development, at depths that intercept the likely pathway.
2. Allow a stabilisation period after installation.
3. Carry out at least six rounds over a minimum of three months for a site assessed as low risk, with more rounds over six to twelve months where the source is large or the first results come back high.
4. Include visits during falling atmospheric pressure, because that is when gas emission is greatest.
5. Record concentrations and flow rates at each visit, with the atmospheric pressure and weather conditions.
6. Derive gas screening values from concentration and flow, and use them to place the site in a characteristic situation.

Work to a recognised method and say which one. The gas investigation guidance in BS 8576 and the wider approach in the Environment Agency land contamination risk management guidance are the usual references, and the Phase 1 and Phase 2 land contamination reports should cite them.

Because monitoring takes months, the sequence matters commercially. Commission the desk study during due diligence, before exchange where you can, and start the monitoring as soon as access allows. A scheme that waits for a planning condition to require monitoring loses a season.

Protection measures and verification

Where protection is needed, the design follows the approach in BS 8485 for gas protective measures in new buildings, which scores the contribution of the structural barrier, the gas resistant membrane and any ventilation layer against the characteristic situation.

The practical components are a reinforced concrete floor slab of defined quality, a gas resistant membrane with properly formed joints and service penetrations, and a passively ventilated void or granular layer below the slab with vents on opposing faces.

Two points that decide whether it works. Installation quality is everything, because a membrane with a torn corner around a soil pipe provides no protection at the one point it is needed. And verification has to be evidenced, which means an independent inspection during installation, photographic records, and a verification report following the recognised verification guidance. Authorities condition the verification report, and occupation is often tied to its approval.

Illustrative example. A ten flat conversion and extension on a site whose rear boundary is 120 metres from a landfill that closed in the 1970s, on sandy soils. The desk study identifies the tip from the dataset and confirms its extent from 1960s mapping. Six wells, eight rounds over four months, and results placing the site in a low characteristic situation. The design is a reinforced slab, a gas membrane and a passively vented sub-floor void, with sealed service entries and a verification report by condition. None of that is remarkable. All of it takes five months from instruction, which is the point.

Settlement, odour and the other issues

Gas dominates the discussion and three other issues deserve a check.

Settlement and stability where any part of the site includes fill from the same operation, or where the tip edge slopes into your site. That is a Geotechnical Assessment question, and it affects foundation choice and the design of external levels and drainage.

Leachate and controlled waters where groundwater flows from the tip towards a watercourse or an aquifer beneath your site. This is the part the Environment Agency will comment on.

Odour, vermin and dust where the neighbouring facility is still operating rather than closed. An active waste transfer or composting operation next to new housing raises an amenity objection in its own right, and an Odour Assessment may be needed. It also raises the agent of change question, since a new residential use should not undermine an existing lawful operation.

What to submit when the site adjoins a former tip

A proportionate pack for a small scheme:

• A Phase 1 desk study with a conceptual site model that names the landfill, its recorded operating dates and waste types where known, and the pathway analysis.
• Historic mapping extracts showing the tip and its relationship to the site.
• A gas monitoring report with well locations, installation records, all rounds, atmospheric pressure at each visit and derived gas screening values.
• A characteristic situation conclusion and a proposed protection scheme referenced to BS 8485.
• A Phase 2 investigation where soil or water contamination is also plausible, with analysis against appropriate assessment criteria.
• A remediation or protection strategy and a proposed verification plan.
• Any correspondence with the Environment Agency and the council's contaminated land officer.

Offer a condition structure the officer will recognise: investigation, remediation strategy, implementation and verification, with a clause dealing with unexpected contamination. Doing so tends to shorten the negotiation and avoids inheriting a condition that requires work you have already done.

PlanScope drafts the Phase 1 and Phase 2 land contamination narratives and the geotechnical documentation from your site data, inserting a clear flag wherever monitoring results or site-specific figures have to be supplied. 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, and land contamination reporting is expected to be prepared in accordance with BS 10175 by suitably qualified and experienced persons.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

Enter a postcode on PlanScope to see which of the 66 report types by category the ground conditions on a site are likely to need.