Nutrient Neutrality Assessment for Hampshire Scheme
Delivered a nutrient neutrality calculation and mitigation strategy that satisfied Natural England and unlocked planning for 120 homes in the Solent catchment.
Project challenges
The site fell within the Solent catchment nutrient neutrality zone, where Natural England had issued a standing objection to all new residential development that could not demonstrate nutrient neutrality. The 120-home scheme on a 4.5-hectare greenfield site would generate additional wastewater that, when treated at the receiving wastewater treatment works, would discharge nitrogen and phosphorus into the Solent's internationally designated habitats.
The nutrient neutrality requirement had effectively frozen planning decisions across the affected local authorities for over two years. Several other developments in the area had stalled because they could not identify viable mitigation to offset the additional nutrient load. The client had already invested significantly in the project and needed a credible pathway to planning approval to maintain their land option and funding arrangements.
The challenge was compounded by the site's greenfield status. Converting agricultural land to residential development changes the baseline nutrient loading from the site — while agricultural use generates diffuse nutrient runoff from fertiliser application, the residential development generates point-source nutrients through wastewater. The nutrient budget calculation needed to account for both the loss of the existing agricultural load and the addition of the new wastewater load to determine the net nutrient impact.
How we solved it
We prepared a detailed nutrient budget calculation following Natural England's published methodology (updated June 2025). The calculation quantified the existing nutrient loading from the site's current agricultural use (based on the actual farming regime, confirmed through consultation with the landowner) and the projected nutrient loading from the proposed residential development (based on occupancy rates, water usage, and the treatment performance of the receiving wastewater treatment works).
The calculation showed a net increase of 47 kg of total nitrogen per year — the amount that needed to be offset through mitigation to achieve nutrient neutrality.
We evaluated three mitigation options: on-site constructed wetland, off-site nutrient credit purchase, and a hybrid approach combining on-site land use change with a smaller credit purchase. The hybrid approach proved most cost-effective. We identified 1.8 hectares of the site that could be converted from arable agriculture to species-rich grassland with no fertiliser application, generating a baseline reduction of 32 kg of nitrogen per year. The remaining 15 kg per year was offset through the purchase of nutrient credits from a Natural England-approved wetland mitigation scheme operating within the Solent catchment.
We also designed the site's surface water drainage strategy to maximise nutrient removal. The SuDS management train included a treatment wetland as the final stage before discharge, providing additional nutrient stripping that, while not formally counted in the nutrient budget (to maintain a conservative calculation), provided an additional buffer of confidence.
Results delivered
Natural England confirmed that the mitigation strategy was acceptable and withdrew their objection to the planning application. The nutrient neutrality calculation and mitigation strategy were submitted as a standalone technical document alongside the flood risk assessment and drainage strategy.
Planning was granted by the local authority within 14 weeks of submission — a remarkable outcome in a catchment where many applications had been stalled for years. The hybrid mitigation approach saved the client approximately £180,000 compared to a fully credit-based solution, while the on-site land use change contributed positively to the scheme's biodiversity net gain assessment.
The project established a replicable model for nutrient neutrality mitigation on greenfield sites within the Solent catchment. Two subsequent schemes by the same developer adopted the same hybrid approach, with Aegaea providing the nutrient budget calculations and mitigation strategies for all three sites.
Project Overview
Nutrient neutrality has been one of the most significant barriers to housing delivery in parts of southern England since Natural England first issued their advisory guidance in 2019. The Solent catchment is one of the most heavily affected areas, with thousands of homes stalled by the requirement to demonstrate that new development will not increase nutrient loading to the Solent’s designated habitats.
This project demonstrates that the nutrient neutrality barrier can be overcome with a rigorous, evidence-based approach to the nutrient budget calculation and a creative mitigation strategy that combines on-site land use change with off-site credit purchase.
The Nutrient Budget Calculation
The nutrient budget calculation is the foundation of any nutrient neutrality assessment. It compares the existing nutrient loading from the site (the baseline) with the projected loading from the proposed development (the future scenario) to determine the net change.
Existing Baseline
The site was in active arable use, growing winter wheat in a typical rotation with spring barley and oilseed rape. We worked with the landowner to establish the actual fertiliser application rates and crop management practices, rather than relying on default assumptions. This site-specific approach produced a more accurate baseline than the generic agricultural loading rates published in Natural England’s methodology.
The existing nutrient loading from the agricultural use was calculated at 68 kg of total nitrogen per year across the 4.5-hectare site. This figure included both the nitrogen applied as fertiliser (accounting for crop uptake and soil retention) and the background atmospheric deposition.
Future Scenario
The proposed 120-home development would generate wastewater from approximately 290 residents (based on the national average occupancy rate of 2.4 persons per dwelling). This wastewater would be treated at the receiving wastewater treatment works, which had a permitted total nitrogen discharge concentration of 15 mg/l.
The future nutrient loading from wastewater was calculated at 97 kg of total nitrogen per year. The loss of the agricultural baseline (68 kg/year) reduced the net impact to 29 kg/year from the land use change. However, the nutrient budget must also account for the additional nutrient loading from the non-agricultural areas of the development (gardens, roads, car parks), which added a further 18 kg/year through urban runoff.
The total net increase in nutrient loading was therefore 47 kg of total nitrogen per year.
Mitigation Strategy
On-Site Land Use Change
The most cost-effective mitigation available on this site was converting a portion of the existing agricultural land to low-nutrient grassland. By ceasing fertiliser application and converting 1.8 hectares from arable to species-rich grassland, we could reduce the baseline nutrient loading from that area by 32 kg of nitrogen per year.
The land use change was secured through a Section 106 agreement requiring the grassland to be maintained in perpetuity, with a management company responsible for annual monitoring and reporting. The grassland specification included a diverse native seed mix that also contributed to the scheme’s biodiversity net gain assessment, creating a dual benefit from a single piece of land.
Off-Site Nutrient Credits
The remaining 15 kg of nitrogen per year was offset through the purchase of nutrient credits from a Natural England-approved mitigation provider. The credits were generated by a constructed wetland scheme within the Solent catchment that had been established specifically to provide nutrient mitigation for development.
The credit purchase was secured through a legal agreement between the developer and the mitigation provider, with evidence of the credit reservation submitted to Natural England and the local planning authority.
SuDS Treatment Wetland
As an additional (uncredited) measure, we designed the site’s surface water drainage system to include a treatment wetland as the final stage in the SuDS management train. The wetland provided nutrient removal from surface water runoff before discharge, reducing the actual nutrient loading below the calculated figure. While this treatment was not formally counted in the nutrient budget calculation (to maintain conservatism), it provided Natural England with additional confidence in the mitigation strategy.
Outcome
The combination of rigorous calculation, creative mitigation, and integrated drainage design delivered a viable pathway to planning approval in a catchment where many developers had been unable to progress. The project demonstrated that nutrient neutrality does not have to be a development stopper — it requires careful assessment, willingness to adapt the site design, and a pragmatic approach to mitigation.
The hybrid mitigation model developed for this project has since been adopted for two further schemes by the same developer, establishing a consistent and replicable approach to nutrient neutrality within the Solent catchment.
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