The baseline chapter of an environmental statement

The baseline chapter is the only part of an environmental statement that every other chapter depends on, and it is the part most often assembled last. Schedule 4 of the Town and Country Planning (Environmental Impact Assessment) Regulations 2017 asks for a description of the relevant aspects of the current state of the environment and an outline of how they would evolve without the development. Get the environmental statement baseline chapter wrong and the significance conclusions in air quality, ecology, noise and transport all inherit the error.

For a promoter budgeting an evidence base, the baseline is also where money leaks. Surveys are commissioned early, the scheme changes, the application slips a season, and by submission the data no longer describes the site.

What Schedule 4 actually asks for

Two things, and they are not the same thing. First, the current state of the environment for the aspects likely to be significantly affected. Second, the likely evolution of that state without the development, so far as natural changes can be assessed with reasonable effort. The second limb is the future baseline, and it is the one most frequently missing.

Regulation 18 requires the environmental statement to be prepared by competent experts and to be accompanied by a statement of the expertise used. That is a legal requirement, not a preference. An Environmental Impact Assessment draft can structure a chapter and set out the questions each topic has to answer, but the assessment itself has to be authored and signed by suitably qualified specialists, and PlanScope's draft is a starting point for that team rather than a substitute for it.

The scoping opinion sets the boundaries. Where an authority has scoped a topic out, the baseline for it still needs enough description to show why the scoping decision holds.

Choosing a baseline year and defending it

Pick one year and state it plainly in the first page of every technical chapter. Mixed baseline years across topics are the single most common reason a statement reads as incoherent: traffic counted in one year, air quality modelled from a different year's monitoring, ecology surveyed across two seasons that straddle both.

Where a topic cannot share the common year for a real reason, say so and explain the reason. Traffic counts avoid August and December because flows are unrepresentative, so a scheme submitted in February may reasonably carry autumn counts from the previous year. Ecology follows survey seasons that no programme can compress. Those are defensible departures when they are declared.

What is not defensible is silence. An officer comparing a 2024 monitoring year in the air quality chapter against 2026 counts in the transport chapter, with no explanation, will ask whether the two chapters were written by people who spoke to each other.

Survey data currency and how long it lasts

There is no statutory shelf life for survey data. What exists is professional guidance per discipline and the practical question of whether anything has changed. Ecological survey data is usually treated as current for a limited number of seasons, with a preliminary ecological appraisal update expected where the application has sat for a long time, and the ecology institute's own guidelines are the reference professionals work to.

Refresh triggers are worth writing into the evidence programme at the start:

1. A material change to the red line boundary or the development parameters.
2. A change in the designation status of a receptor, such as a new conservation area or a revised air quality management area.
3. New development built or consented within the study area since the survey.
4. A change in the assessment guidance or the modelling tool, for example a new emissions factor release.
5. Elapsed time beyond the currency period the relevant professional body applies.

Write the trigger list into the scoping report. It converts an argument at determination into a decision already recorded.

The future baseline and committed development

The future baseline describes the site and its surroundings at the assessment year, assuming the development does not happen. It has to account for committed development, which normally means schemes with extant permission that are reasonably likely to be built, and sometimes allocations where delivery is programmed.

Building the committed development list is a research task with a date on it. Search the authority's register for major permissions in the study area, record the reference, the description, the decision date and the expected build-out, and state the cut-off date for the search. Consultees will test the list against their own knowledge, and a missing neighbouring scheme is an easy point for an objector to make.

Some topics have a future baseline that changes on its own. Background pollutant concentrations are projected to fall under national emissions factors, which affects the air quality assessment year. Ecological baselines shift with succession on a cleared site. Flood risk baselines move with climate change allowances, which is the same logic that runs through the sequential test section inside a flood risk assessment.

Recording data gaps without weakening the statement

Every environmental statement has gaps. The difference between a strong one and a weak one is whether the gaps are declared, bounded and justified.

Record each gap in the same place, usually a table in the methodology section of the chapter, with three columns: what is missing, why it is missing, and what assumption has been made instead. Then state whether the gap could change the significance conclusion. A gap that cannot change the conclusion is a footnote. A gap that could change it needs either more survey work or a precautionary assumption that is explicitly conservative.

Ground conditions are the classic case. A phase 2 investigation may be impossible on an operational site, so the baseline rests on a desk study and a limited intrusive programme, with a condition securing further investigation before development. The way that reasoning is set out in a ground gas section of a contaminated land assessment transfers directly to the environmental statement chapter.

Never fill a gap with a plausible number. An assumed figure that later proves wrong undermines the whole chapter, and an inspector will say so.

Presenting baseline consistently across topics

Readers move between chapters. If the study area is described as a 500 m radius in one chapter and a 1 km radius in another without explanation, the statement looks unplanned even where both are correct for their discipline.

A short baseline coordination note at the start of the process fixes this. Agree the common baseline year, the site description wording, the receptor naming convention, the figure numbering and the study areas per topic, then circulate it to every consultant. It costs a morning. It saves a round of consultee questions.

Use the same plan base for every figure. Different consultants working on different base drawings produce site boundaries that do not quite match, and that is exactly the kind of detail a determined objector will photograph and put in front of a committee.

Where consultees challenge the baseline

Statutory consultees rarely argue with the mitigation first. They start with whether the baseline describes the real site.

The natural environment regulator will ask whether survey effort matched the habitats present and whether the survey seasons were right. The environment regulator will ask about the hydrological and flood baseline and the data source behind it. The highway authority will ask when the counts were taken, on which days, and whether any nearby scheme has opened since. Environmental health will ask where the monitors sat and for how long.

None of those questions is answerable retrospectively. They are answerable by a methodology section that already says, for each topic, what was measured, where, when, by whom, against which standard, and what was assumed where measurement was not possible.

A scoping opinion request that sets out the proposed baseline year, study areas and survey programme in advance gives the authority the chance to object while there is still time to respond. It is the least costly insurance in the whole evidence base.

Browse the full set of supporting documents a major application tends to need at all 66 report types, or enter your postcode on PlanScope to see which of them a specific site is likely to trigger. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.