Does my loft conversion need BNG?

No — householder applications exempt. Same for rear extensions, side returns, garage conversions on a single dwelling. BNG kicks in when application creates new dwelling(s) or is commercial/mixed-use.

How much does on-site BNG cost?

Survey + metric £1,850–£4,850. On-site enhancement (green roof, wildflower planting, swift bricks, native hedge): £4,500–£18,500 for small infill site. Off-site biodiversity unit purchase: £20,000–£42,000/unit (typically 0.3–1.5 units needed). Statutory credits: £42,000+ — avoid via good design.

Who monitors the 30-year management plan?

LPA (planning authority) holds the legal agreement; designated 'Responsible Body' (often LPA itself or accredited conservation body) monitors. Reports submitted annually for 30 years. Failure triggers planning enforcement + financial penalty under conservation covenant terms.

Biodiversity Net Gain Domestic London 2026 | Builderr

Does Biodiversity Net Gain Apply to My London Domestic Renovation?

Biodiversity Net Gain (BNG) mandates 10% biodiversity uplift on most planning permissions under Environment Act 2021 from Feb 2024. Householder applications (extensions, loft conversions, single-dwelling work) are exempt. Minor development (1–9 new dwellings + site <1ha + GIA <1000m²) caught from April 2024. Most London single-home renovation outside scope. Garden-flat conversion creating 2+ dwellings triggers 10% BNG via metric calculation + 30-year management plan.

Scope + exemptions

Environment Act 2021 + Town + Country Planning Act 1990 Schedule 7A: mandatory 10% BNG on planning permissions from 12 Feb 2024 (major) + 2 Apr 2024 (minor). Exemptions: householder applications (Class GPDO Part 1 extensions, loft conversions, outbuildings — single-dwelling improvements); de minimis (<25m² habitat affected + no priority habitat); urgent Crown development; self-build/custom-build single dwellings on registered plot. Caught: new dwellings (1+), commercial, mixed-use, flat conversions creating new units. London-specific: most house extensions/loft conversions outside scope; HMO conversions creating self-contained units may be caught; rear-garden flat developments definitely caught.

Metric + units

DEFRA Biodiversity Metric 4.0 calculates baseline + post-development habitat units. Surveyor (CIEEM-registered ecologist) maps habitats by type + condition (good/moderate/poor) + distinctiveness (high/medium/low). Habitat units = area × distinctiveness × condition × strategic significance. 10% net gain required vs baseline. Achieved by: on-site enhancement (priority), off-site biodiversity units purchase (£20k–£42k/unit), statutory biodiversity credits (£42k–£650k/unit — last resort, government-set). Survey + metric calc cost £1,850–£4,850 for small site. 30-year monitoring + maintenance plan + legal agreement (Section 106 or conservation covenant) mandatory.

Practical impact on London garden-flat schemes

Typical scenario: subdivide garden of single dwelling to build new house at rear (1 unit). 10% BNG applies — baseline garden often medium-distinctiveness modified grassland + some shrub + tree. Post-dev: hardstanding + new dwelling + small landscaped gardens. Common solutions: green/brown roof on new dwelling, wildflower meadow strip, native hedge boundary, swift bricks/bee bricks integrated, retain mature trees, replace lawn with biodiverse meadow + bug hotels. Skilled ecologist designs to meet 10% on-site — avoids off-site credit purchase £20k+. Plan from RIBA Stage 2 — late retrofit often fails the metric.