Why the planning authority is asking
In 2022 Natural England advised planning authorities across 27 river and estuary catchments that new development adding nutrients to already-overloaded protected habitats could not be permitted unless it was nutrient neutral. The advice rests on the Habitats Regulations, so it is not something a council can waive: without an assessment showing a budget at or below zero, or mitigation secured to bring it there, the application cannot be determined. That is why a site in Poole Harbour’s catchment, the Solent’s or Stodmarsh’s can sit undetermined for months with everything else agreed.
The assessment is a calculation with a defined method. Natural England published the methodology and a calculator for each catchment; the planning authority’s ecologist checks the arithmetic and the assumptions behind it. Get the assumptions right first time and the assessment is a week’s delay, not a season’s.
What the assessment contains
The nutrient. Each catchment is designated for nitrogen, phosphorus or both — Poole Harbour and the Solent are nitrogen, the Somerset Levels and the River Avon phosphorus, Stodmarsh and the Broads both. The wrong nutrient is the commonest mistake in a DIY submission.
The waste-water load. The number of dwellings and an occupancy rate give the population; a daily water-use figure — Natural England’s standard allowance, or the tighter figure where a 110 litres-per-person planning condition is in place — gives the flow; and the treatment works’ permitted nutrient concentration, or the performance of a private package plant, converts flow into kilograms a year. Where the works’ permit tightens in April 2030 there are two figures, and the mitigation has to cover the higher.
The land-use change. The nutrient the site exports today — as arable land, pasture, woodland or hardstanding — is taken off, and what it will export after development is added, using the export coefficients for that catchment. A greenfield site on fertilised arable land can come out with a negative land-use term that offsets much of the waste-water load; a brownfield site cannot.
Sustainable drainage. SuDS features that treat run-off earn a credit against the land-use term where they are designed and evidenced for nutrient removal.
The buffer and the budget. The net figure is multiplied by 1.2 — Natural England’s 20% precautionary buffer — to give the budget in kg/year. Above zero, mitigation is required and the report says how much.
Two ways to get it done
Do it yourself, free. NutrientMonkey is our free browser-based nutrient neutrality calculator. It implements Natural England’s methodology for all 27 catchments — the right nutrient, the standard occupancy and water-use figures, editable export coefficients so you can paste the official catchment values, the SuDS credit, the 20% buffer and the pre- and post-2030 split — and shows the budget as you type. The planning-ready PDF, with every line of the arithmetic and a report ID the ecologist can verify online, is £79 +VAT. For a straightforward housing scheme on mains drainage it is all most applicants need.
Have us prepare it. Where the scheme is larger or the questions are harder — a private treatment plant, a phased scheme straddling the 2030 permit change, a land-use history the authority may dispute, a holiday-let or care-home occupancy that needs justifying, or simply an authority that wants a consultant’s signed assessment — we prepare the assessment, agree the assumptions with the authority’s ecologist where useful, size the mitigation and set out the credit options. As energy and sustainability consultants working from Dorset, with Poole Harbour on the doorstep and the Solent and Avon catchments alongside, it is a calculation we do regularly.
Mitigation and credits
A positive budget has to be secured before permission, not before occupation. The usual route is nutrient credits from a scheme in the same catchment — wetlands, land taken out of intensive agriculture, upgraded treatment works — bought in perpetuity at a price per kilogram that varies widely between catchments — the nutrient credits calculator turns a budget into the credits and, where a scheme publishes a price, the likely cost. On-site options exist for larger sites: land-use change within the red line, a treatment plant with better nutrient removal than the works, or SuDS designed for nutrient stripping. The assessment sizes whichever route you take, and the catchment pages on the calculator carry the credit position where one is published.
What happens next
The Planning and Infrastructure Act 2025 creates Environmental Delivery Plans funded through a Nature Restoration Fund: once an EDP covers a catchment, a developer will be able to pay a levy in place of securing bespoke mitigation. Until then the existing process applies in full, and an assessment done now is an assessment that can be re-run under whatever replaces it. We track the changes in our news section, and the catchment list itself is published as open data.
Common questions
What is a nutrient neutrality assessment?
A calculation, in kilograms of nitrogen or phosphorus per year, of the nutrient load a development adds to a protected water body — from its waste water and from the change in land use on the site — worked through Natural England's published methodology with a 20% precautionary buffer. If the result is above zero the development must secure mitigation, usually by buying nutrient credits, before the planning authority can grant permission under the Habitats Regulations.
Which developments need one?
Anything that creates overnight accommodation in one of the 27 catchments: new dwellings, conversions and barn conversions, houses in multiple occupation, holiday lets, care homes, student accommodation, hotels and campsites. Extensions to an existing home usually do not, unless they create a separate dwelling or an annexe with its own overnight accommodation. Non-residential development is caught only where it adds waste water, for example a new commercial kitchen or workforce accommodation.
Which catchments does nutrient neutrality apply in?
The 27 catchments Natural England has advised on, covering large parts of the south and east of England: Poole Harbour, the Solent and Chichester Harbour, the River Avon around Christchurch, the Somerset Levels and Moors, Stodmarsh in Kent, the Broads and the River Wensum, the Tees, Eden and Kent catchments in the north, and others. Some are nitrogen, some phosphorus, and a few both. Each has its own page on our calculator with the designated site and the nutrient that applies.
How is the nutrient budget calculated?
In stages. Waste water: occupancy per dwelling multiplied by daily water use gives the flow, and the treatment works' permitted nutrient concentration — or the performance of a private treatment plant — gives the load, less the load the site already discharges. Land use: the nutrient exported by the current land cover is taken off, and the export from the developed site is added. Sustainable drainage that removes nutrients earns a credit. The net figure is multiplied by 1.2 to give the budget in kg/year.
How many nutrient credits will I need?
One credit is normally one kilogram of the nutrient per year in perpetuity, so a budget of 4.2 kg/year of nitrogen needs 4.2 credits — or more precisely, the higher of the pre- and post-2030 budgets where the treatment works' permit tightens in April 2030. Credit prices vary by catchment and scheme; the catchment pages on our calculator carry the position where one is known.
Can I do the assessment myself?
Yes. NutrientMonkey is our free browser-based calculator implementing Natural England's methodology for all 27 catchments; the budget is free on screen and the planning-ready PDF, with the full arithmetic shown and a verifiable report ID, is £79 +VAT. It suits straightforward residential schemes. Where the drainage is unusual, the land-use history is contested, or the authority wants a signed professional assessment, we prepare it.
What do you need from me?
The site address or a location plan; the number and type of dwellings, or bedrooms for HMOs and holiday lets; the foul drainage route — mains sewer and which treatment works, or a private treatment plant; the current use of the site with its area; and any surface-water proposals. A red-line plan and the planning reference, if there is one, save a round of questions.
Is nutrient neutrality changing?
The Planning and Infrastructure Act 2025 introduces Environmental Delivery Plans funded through a Nature Restoration Fund, under which developers will be able to pay a levy instead of securing their own mitigation. Until an EDP covers your catchment the existing process applies: a nutrient budget, and mitigation secured before permission is granted.