Last Updated: January 2026
How to Achieve 10% Biodiversity Net Gain on a Housing Development
Housing developments in England must achieve a minimum 10% biodiversity net gain (BNG) under the Environment Act 2021. The most effective approach combines wildflower meadow creation via hydroseeding (achieving sign-off in 12-18 months), retention of existing high-value habitats, and strategic use of SUDS features. Developers should calculate baseline biodiversity units using Defra's Statutory Biodiversity Metric 4.0 , follow the biodiversity gain hierarchy, and secure habitats for a minimum 30-year management period through Section 106 agreements or conservation covenants.
Understanding the Legal Requirement
Since February 2024, mandatory BNG applies to all major housing developments (10+ dwellings) in England. Small sites (1-9 dwellings) have been covered since April 2024.
The requirement is straightforward: the post-development biodiversity value must exceed pre-development value by at least 10%, measured in standardised biodiversity units.
Key legislative framework:
- Environment Act 2021 (Schedule 14)
- Town and Country Planning Act 1990 (Schedule 7A)
- Biodiversity Gain Requirements (Exemptions) Regulations 2024
- Defra Statutory Biodiversity Metric 4.0 (mandatory calculation tool)
The Biodiversity Gain Hierarchy
Developers must follow this hierarchy, in order of priority:
| Priority | Action | Requirement |
|---|---|---|
| 1 | Avoid | Prevent adverse effects on medium, high and very high distinctiveness habitats |
| 2 | Mitigate | Reduce unavoidable impacts through design changes |
| 3 | Compensate on-site | Create or enhance habitats within the development boundary |
| 4 | Compensate off-site | Deliver gains on registered off-site land |
| 5 | Statutory credits | Purchase government credits as a last resort |
Local planning authorities will scrutinise applications to ensure developers have genuinely attempted each step before moving to the next.
Calculating Biodiversity Units
The Statutory Biodiversity Metric 4.0 calculates units using this formula:
Distinctiveness Scores
Examples relevant to housing sites:
| Habitat Type | Distinctiveness | Score |
|---|---|---|
| Developed land (sealed surface) | Very Low | 0 |
| Modified grassland | Low | 2 |
| Other neutral grassland | Medium | 4 |
| Species-rich grassland | High | 6 |
| Traditional orchard | Very High | 8 |
Condition Multipliers
| Condition | Multiplier |
|---|---|
| Good | 1.0 |
| Fairly good | 0.66 |
| Moderate | 0.44 |
| Fairly poor | 0.31 |
| Poor | 0.2 |
As an example, a housing site on agricultural land (modified grassland in moderate condition) might have a baseline of around 0.88 units per hectare. To achieve 10% BNG, the development would need to deliver approximately 0.97 units per hectare post-development. Actual figures will depend on the specific site assessment.
Habitat Creation Methods Compared
Different methods suit different site conditions, budgets and timescales. The following figures are indicative and will vary depending on site-specific conditions (costs estimated as of Q1 2026):
| Method | Cost per ha | Time to Target Condition | Typical Establishment Rate | Best For |
|---|---|---|---|---|
| Wildflower hydroseeding | Generally £3,500-£8,500 | 12-18 months | Generally 85-95% | Large areas, slopes, SUDS features |
| Conventional seeding | Generally £2,000-£5,000 | 18-24 months | Generally 70-85% | Flat amenity areas, lower budgets |
| Plug planting | Generally £15,000-£25,000 | 24-36 months | Generally 90-98% | Small areas, high-profile locations |
| Wildflower turf | Generally £45,000-£65,000 | 6-12 months | Generally 95-99% | Immediate impact, show homes |
| Natural regeneration | Generally £500-£1,000 | 36-60 months | Generally 50-70% | Low-intervention areas with seed bank |
Cost per BNG Unit
The following are broad estimates including 30-year management (costs estimated as of Q1 2026). Actual costs will depend on site conditions, specification, and management arrangements:
| Method | Typical Estimated Cost per Unit |
|---|---|
| Wildflower hydroseeding | £800-£1,500 |
| Conventional meadow seeding | £1,200-£2,000 |
| Plug planting | £2,500-£4,000 |
| Off-site unit purchase | £25,000-£50,000 |
| Statutory credits | £42,000+ |
Hydroseeding can offer strong value for housing developers needing rapid establishment across large public open space areas.
Step-by-Step Process for Housing Developers
1. Baseline Assessment (Pre-Application)
Appoint a suitably qualified ecologist (such as FISC Level 4 — the Field Identification Skills Certificate for habitat survey — or equivalent accreditation) to:
- Map all habitats within the red-line boundary using UK Habitat Classification (UKHab)
- Assess condition against Defra criteria
- Complete the Statutory Biodiversity Metric 4.0 calculation tool
- Identify protected species requiring separate mitigation
Typical baseline habitats on housing sites:
- Arable land: Low distinctiveness, poor condition
- Modified grassland: Low distinctiveness, poor-moderate condition
- Hedgerows: Medium-high distinctiveness (linear units calculated separately)
- Scrub/woodland: Medium-high distinctiveness (retention prioritised)
2. Design for BNG (RIBA Stage 2-3)
Integrate biodiversity into the masterplan:
- Retain existing hedgerows (much higher unit value than replacement)
- Design SUDS basins with wildflower slopes (dual function: drainage + habitat)
- Allocate public open space for species-rich grassland, not amenity turf
- Create wildlife corridors connecting habitats
Indicative unit gains from design choices (actual values depend on condition achieved and strategic significance):
| Feature | Typical Unit Yield |
|---|---|
| SUDS basin slopes (wildflower seeded) | 4-6 units/ha |
| Species-rich meadow on POS | 4-5 units/ha |
| New native hedgerow | 2-4 units per 100m |
| Native woodland (>25 years to condition) | 8-10 units/ha |
| Biodiverse green roofs | 2-3 units/ha (limited application) |
3. Planning Application Submission
Submit with your application:
- Completed Statutory Biodiversity Metric 4.0 spreadsheet
- Biodiversity Net Gain Report demonstrating 10%+ gain
- Draft Habitat Management and Monitoring Plan (HMMP)
- GIS mapping files (baseline and post-development)
- Evidence of hierarchy compliance
4. Pre-Commencement: Biodiversity Gain Plan Approval
Before any development can commence, the local planning authority must approve a Biodiversity Gain Plan containing:
- Final metric calculations
- Spatial mapping of habitat creation/enhancement
- Confirmation of how 10% is achieved (on-site, off-site, or combination)
- Evidence that off-site gains are registered (if applicable)
5. Implementation
For on-site habitat creation, typical implementation involves:
Ground preparation:
- Strip topsoil to reduce fertility (wildflowers need low-nutrient soil)
- Remove perennial weeds
- Create appropriate substrate (often using subsoil)
Seeding:
- Hydroseeding achieves rapid, even coverage on slopes and large areas
- Application rates: typically 25-35g/m² for wildflower mixes
- Germination visible within 7-14 days under normal UK conditions
- Full establishment in 6-8 weeks
Nurse crops: Including a low percentage of fast-establishing grasses (e.g., 80:20 wildflower:grass ratio by weight) protects soil and aids wildflower establishment.
6. Long-Term Management (30 Years)
All significant on-site and off-site BNG must be secured for minimum 30 years via:
- Planning conditions (on-site only)
- Section 106 agreements
- Conservation covenants
Typical management regime for species-rich grassland:
| Activity | Timing | Frequency |
|---|---|---|
| Cut and remove hay | Late July-September | Annual |
| Remove arisings | Within 7 days of cut | Annual |
| Weed control (spot treatment) | Spring | As required |
| Monitoring surveys | Summer | Years 1, 2, 3, 5, 10, 15, 20, 25, 30 |
When to Use Each Habitat Creation Method
Use wildflower hydroseeding when:
- Treating slopes steeper than 1:4 (14°)
- Covering large areas (>0.5 hectares) efficiently
- Programme speed is critical
- Creating habitat on SUDS features
- Difficult access prevents machinery
Use conventional seeding when:
- Site is predominantly flat
- Budget is constrained
- Amenity areas where some grass cover is acceptable
- Overseeding into existing vegetation
Use plug planting when:
- Areas are too small for hydroseeding
- Maximum species control is required
- High-profile locations need guaranteed establishment
- Supplementing seeded areas with specific species
Common Challenges and Solutions
Challenge: High baseline value
Problem: Site contains existing species-rich grassland or hedgerows that score highly in the metric.
Solution: Retain and enhance existing habitats where possible. Losses of high-distinctiveness habitats require like-for-like replacement, which is costly and time-consuming to establish.
Challenge: Nutrient-rich soils
Problem: Former agricultural land has high fertility that suppresses wildflowers.
Solution: Strip topsoil and use subsoil for wildflower areas. Alternatively, sow yellow rattle ( Rhinanthus minor ) as a hemi-parasite that weakens competitive grasses.
Challenge: Tight programme
Problem: Development timeline doesn't allow for traditional establishment periods.
Solution: Hydroseeding delivers visible vegetation in 7-14 days, with establishment in 6-8 weeks, which can be considerably faster than conventional methods. Consider starting habitat creation on early phases while construction continues on others.
Challenge: Long-term management funding
Problem: Who pays for 30 years of management on residential developments?
Solution: Management typically funded through service charges to residents, management company arrangements, or commuted sums to local authorities. Establish clear legal arrangements early.
Worked Example: BNG Calculation for a 150-Home Development
6-Hectare Residential Site
The following is a simplified worked example showing how the Statutory Biodiversity Metric 4.0 applies to a typical housing development. Actual calculations involve additional factors including temporal multipliers and spatial risk, but this illustrates the core process.
Baseline Assessment
| Habitat | Area (ha) | Distinctiveness | Condition | Units |
|---|---|---|---|---|
| Modified grassland | 4.5 | Low (2) | Poor (0.2) | 1.80 |
| Arable cropland | 1.0 | Low (2) | Poor (0.2) | 0.40 |
| Dense scrub | 0.3 | Medium (4) | Moderate (0.44) | 0.53 |
| Existing buildings/hardstanding | 0.2 | V. Low (0) | N/A | 0.00 |
| Total baseline | 6.0 | 2.73 |
10% BNG target: 2.73 × 1.1 = 3.00 area units required post-development.
(Plus separate hedgerow unit targets calculated on a linear basis.)
Post-Development Habitats
| Habitat | Area (ha) | Distinctiveness | Condition Target | Units |
|---|---|---|---|---|
| Buildings, roads, hardstanding | 4.0 | V. Low (0) | N/A | 0.00 |
| Species-rich meadow on POS (hydroseeded) | 0.8 | Medium (4) | Fairly good (0.66) | 2.11 |
| Wildflower SUDS slopes (hydroseeded) | 0.4 | Medium (4) | Fairly good (0.66) | 1.06 |
| Amenity grassland (gardens, verges) | 0.5 | Low (2) | Poor (0.2) | 0.20 |
| Retained & enhanced scrub | 0.3 | Medium (4) | Fairly good (0.66) | 0.79 |
| Total post-development | 6.0 | 4.16 |
Net gain: (4.16 − 2.73) ÷ 2.73 = +52.4% — comfortably exceeds the 10% requirement.
Hydroseeding cost for 1.2 ha of wildflower habitat: generally £4,200-£10,200
Equivalent off-site credit cost for the same units: £79,000-£158,000+
On this site, the 1.2 ha of hydroseeded wildflower habitat alone generated 3.17 units — enough to exceed the 3.00 unit target before any other habitats were counted.
Timeline
- Seeding completed in autumn (Phase 1 completion)
- Visible germination within 7-14 days
- First flowering display by the following spring
- Target condition of "fairly good" achieved within 18 months
Note: This example uses simplified unit calculations for illustration. The full Statutory Biodiversity Metric 4.0 applies additional multipliers for strategic significance, spatial risk, and temporal factors that affect final unit totals. Always engage a qualified ecologist to run the formal calculation.
Regulatory Standards and Guidance
Developers and their consultants should reference:
| Standard | Application |
|---|---|
| Environment Act 2021 | Primary legislation mandating BNG |
| Defra Statutory Biodiversity Metric 4.0 | Mandatory calculation tool |
| Natural England HMMP Template (JP058) | Habitat Management and Monitoring Plans |
| CIEEM BNG Technical Guide 2024 | Professional practice for ecologists |
| BS 8683:2021 | Process for designing and implementing BNG |
| BS 8616:2019 | Seeding and turf work in landscape operations |
Frequently Asked Questions
How long does BNG approval take in the planning process?
The Biodiversity Gain Plan must be approved pre-commencement. Most local planning authorities aim to determine these within 4-8 weeks of submission, provided the metric is correctly completed and supporting information is adequate. Incomplete submissions cause delays.
What's the cheapest way to achieve 10% BNG on a housing development?
Retention of existing habitats is almost always cheaper than creation. Where creation is required, wildflower hydroseeding can offer strong value compared to alternatives. Off-site credits and statutory credits are considerably more expensive and should be last resorts.
Can I use existing grassland for BNG units?
Existing grassland contributes to your baseline value, not your gain. To achieve BNG, you must demonstrate improvement beyond the baseline. This typically means enhancing existing habitat to a better condition or creating new habitat of higher distinctiveness.
What happens if BNG conditions aren't met?
Failure to deliver committed BNG can result in enforcement action, including potential criminal proceedings under Section 97 of the Environment Act 2021. Local planning authorities have powers to require remedial action. Developers remain liable for the 30-year management period.
How does BNG interact with protected species?
BNG does not replace existing protected species legislation. If your site has great crested newts, bats, badgers, or other protected species, you need separate mitigation and licensing. BNG is an additional requirement, not a replacement.
Can wildflower hydroseeding achieve 'good condition' for the metric?
Yes. Properly specified and managed wildflower hydroseeding can achieve 'moderate' to 'good' condition within 12-24 months, depending on species mix, soil preparation, and management regime. This compares favourably with plug planting (24-36 months) and natural regeneration (36-60 months).
What seed mix should I use for BNG wildflower areas?
Use native, local-provenance seed mixes appropriate to your soil type and region. For neutral grassland on housing developments, a mix containing 20+ native wildflower species plus fine grasses is typical. Mixes should target the condition criteria in Defra's metric guidance to ensure units are achieved.
Do hedgerows count separately from area-based habitats?
Yes. The Statutory Biodiversity Metric 4.0 calculates hedgerow units separately from area units. Hedgerows are measured in linear metres and have their own distinctiveness and condition criteria. Most native hedgerows score as medium or high distinctiveness, making retention highly valuable.
What monitoring is required over the 30-year period?
Monitoring frequency varies but typically includes surveys in years 1, 2, 3, 5, 10, 15, 20, 25, and 30. Each survey must assess whether habitats are progressing toward or maintaining target condition. Reports must be submitted to the local planning authority or responsible body.
Can I start habitat creation before planning permission is granted?
This is risky. If you enhance habitat before establishing your baseline, you may create a higher baseline that requires more net gain to exceed by 10%. Always complete baseline surveys before any site clearance or enhancement works.
Conclusion
Achieving 10% biodiversity net gain on housing developments requires early integration of ecology into site design, correct application of the Statutory Biodiversity Metric 4.0 , and commitment to 30-year habitat management. The most cost-effective approach for most developers combines retention of existing high-value habitats with creation of new species-rich grassland through hydroseeding on public open space and SUDS features.
Developers who engage ecologists early and design for BNG from the outset typically achieve compliance more efficiently than those who treat it as a planning condition to be satisfied at the last minute.
This guide provides general information based on regulations as of January 2026. BNG requirements may be subject to change. Always consult with a qualified ecologist and your local planning authority for project-specific advice.