Biodiversity net gain formula: plain English plus open-source TS

Baseline units (per habitat row)

baselineUnits = distinctiveness x condition x area_ha x strategicSignificance

Post-development units (per habitat row)

postDevUnits = distinctiveness x condition x area_ha x strategicSignificance
             x temporalRiskMultiplier
             x spatialRiskMultiplier
             x difficultyMultiplier

10% uplift requirement

requiredDelta = max(0, sum(baselineUnits) * 1.10 - sum(postDevUnits))

Open-source TypeScript reference

// JP039 v4.0 core arithmetic
export function unitsForRow(row: HabitatRow): number {
  const d = DISTINCTIVENESS_SCORE[row.band];   // 0,2,4,6,8
  const c = CONDITION_SCORE[row.condition];    // 1,2,3
  const s = STRATEGIC_SIGNIFICANCE[row.ss];    // 1.0,1.1,1.15
  return d * c * row.areaHa * s;
}

export function postDevUnits(row, years, difficulty) {
  return unitsForRow(row)
    * temporalRiskMultiplier(years)
    * difficultyMultiplier(difficulty);
}

Divergence from the bound JP039 spreadsheet

The bound spreadsheet adds: per-habitat trading-rule enforcement; module-level summation (area, hedgerow, watercourse); and condition-card guidance baked into validation cells. This implementation reproduces the core arithmetic and surfaces the trading-rule check as a boolean; for formal submission the bound spreadsheet remains the authoritative output.[S2]