Planning documents for a scheme on a former landfill

Development on a former landfill needs a two-stage contamination evidence base, a gas risk assessment with enough monitoring rounds behind it to be credible, and a gas protection design that someone will verify on site. The planning application rarely resolves any of it. What the authority usually wants at submission is a Phase 1 preliminary risk assessment that identifies the fill, a Phase 2 intrusive investigation with gas and groundwater results, and a remediation strategy outline, with detailed design left to a pre-commencement condition.

Closed landfill sites also bring a consultee you cannot work around. The Environment Agency maintains an interest in landfill gas and leachate, and local authorities commonly consult it where a site sits within 250 m of a landfill boundary. That consultation shapes the programme more than the planning fee ever will.

Establishing the fill history and extent

Start with the desk study, because the extent of fill decides everything downstream. Historic Ordnance Survey mapping shows extraction pits and infilled ground. The authority's own contaminated land register, the Environment Agency's historic landfill dataset and British Geological Survey records give the rest.

Three questions matter more than the general history:

1. What was deposited, and in particular was it inert, industrial or putrescible household waste?
2. When did deposition stop, because gas generation declines over decades and a 1950s inert pit behaves nothing like a 1990s household waste cell?
3. Where exactly are the edges, including any unrecorded extension beyond the mapped boundary?

Trial pits along the presumed edge are often the only way to answer the third question. Boreholes in the middle of a cell tell you about depth and composition. They do not tell you whether the fill crosses under your proposed terrace.

A Land Contamination Phase 1 assessment and the Phase 2 investigation that follows are reports where a suitably qualified and experienced person is legally expected to take responsibility. The NPPF puts the burden on the developer, and authorities expect work in accordance with BS 10175 from a competent author. An AI-assisted draft is a starting point for that professional, never a substitute.

Gas monitoring rounds and the characteristic situation

Ground gas risk is assessed through gas screening values, which combine measured concentration with borehole flow rate. Those feed the characteristic situation classification in BS 8485, running from CS1 where no gas protection is needed to CS6 where the risk may make development unsuitable without major works.

The number of monitoring visits is where applications fail. One or two rounds will not be accepted on a putrescible landfill. Practitioners generally plan for at least six visits spread over three months for a low-risk site, and considerably more where screening values sit near a boundary between characteristic situations, with visits timed to capture low and falling atmospheric pressure because that is when gas emission peaks.

Record barometric pressure at every visit. A monitoring set with no readings below about 1,000 mbar invites the consultee to ask for another three months, and that request lands after the eight or thirteen week determination period has already slipped.

Gas protection design and verification

Protection measures are specified as a combination of points under BS 8485, drawn from ventilation beneath the slab, a gas-resistant membrane, structural floor type and site-wide measures. The design has to be internally consistent. A passive sub-floor ventilation void only works if the air path is continuous and the vents are not later blocked by landscaping.

Verification is the step that gets forgotten. BS 8485 expects installation to be verified and recorded by a suitably qualified person, with membrane integrity testing and photographic evidence, and the authority will normally condition a verification report before occupation. Budget for it at design stage rather than discovering it when the building control sign-off stalls.

Service entries are the common failure point. Every pipe and cable through the membrane is a potential gas path, and sealing details need to be drawn rather than left to the groundworker.

Settlement, foundations and service entries

Filled ground settles, unevenly and for a long time. Differential settlement across a building footprint that straddles the fill edge is the classic problem, and the usual answers are piled foundations through the fill to competent strata, a ground-bearing raft designed for the predicted movement, or avoiding the fill altogether in the layout.

But piling brings its own complication on a gassing site. A pile bore can create a preferential pathway for gas, and continuous flight auger techniques with careful sealing are often preferred over driven options that may also carry vibration issues for neighbours.

Flexible service connections and a drainage layout that tolerates movement both belong in the design from the start. Rigid drainage runs across settling fill crack, and a cracked foul run on a gassing site gives gas a route into the building.

A Geotechnical Assessment covering foundation options and settlement, informed by the same site investigation, is a report councils expect from a suitably qualified author, and professional review is strongly advised. The Geotechnical Assessment page sets out what the document covers.

Leachate, groundwater and the regulator's position

Leachate is the second reason the Environment Agency engages. Where fill sits above or within groundwater, the application needs to show that development will not mobilise contaminants towards a receptor, whether that is a watercourse, an abstraction or a drinking water source.

The surface water strategy has to work with that. Infiltration drainage into or through landfill fill is normally unacceptable because it drives leachate generation, so soakaways and permeable paving over the cell are usually ruled out and the drainage strategy has to find discharge capacity elsewhere. That single constraint often reshapes a layout, and it is worth testing before the site plan is fixed. Much the same logic governs schemes in a groundwater protection area, as set out in the guidance on documents for a scheme in a source protection zone.

If the landfill permit has not been surrendered, the regulatory position is more complicated again, because the permit holder retains obligations and the Environment Agency will want to understand how development interacts with them.

The pack and the condition discharge sequence

For a residential scheme on a closed landfill, a realistic submission pack runs to:

• a Phase 1 preliminary risk assessment
• a Phase 2 intrusive investigation with gas, soil and groundwater results
• a gas risk assessment giving the characteristic situation and an outline protection scheme
• a geotechnical appraisal covering foundations and settlement
• a drainage strategy that avoids infiltration over the fill
• a construction management plan covering dust, odour and excavated waste
• the standard Planning Statement and design documents

And the order matters as much as the list.

Expect conditions requiring the detailed remediation strategy, the gas protection design, a verification report, and reporting of unexpected contamination. Sequence the monitoring so the Phase 2 data exists before you submit, because the alternative is a condition you cannot discharge for a further three months.

Enter your postcode on PlanScope to see which of the 66 report types the site is likely to need, including the contamination and ground reports that a former landfill triggers, and review the contaminated land assessment page for what Phase 1 and Phase 2 cover. 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.