Stodmarsh Nutrient Neutrality Calculator

Stuck behind a Stodmarsh nutrient condition?

Stodmarsh near Canterbury halted thousands of homes across east Kent. The catchment covers a broad area of the Stour valley — Canterbury, Ashford, Dover, Folkestone & Hythe, Maidstone and more — and it’s assessed for both nitrogen and phosphorus.

If your site drains to Stodmarsh SAC / SPA / Ramsar, the planning authority can't grant permission until you've shown the scheme is nutrient neutral — that it won't add both nutrients to a catchment that's already overloaded. NutrientMonkey gets you the number: build the assessment on screen, watch your budget in kg/year update as you go, and export the planning-ready report when it's right.

Which nutrient applies here

Stodmarsh is a dual-nutrient catchment: both nitrogen and phosphorus are degrading its protected wetlands, so a compliant scheme has to be neutral for each. That matters, because the methodology and the credits differ by nutrient — and neighbouring catchments often differ. NutrientMonkey flags the Stodmarsh catchment for both nutrients and locks the tool to match, so the assessment is right from the start.

Credits in the Stodmarsh catchment

Stour Environmental Credits — the not-for-profit venture owned by Ashford Borough Council and Canterbury City Council — sells permanent credits at £235 per 0.1 kg of nitrogen and £3,500 per 0.1 kg of phosphorus, equivalent to about £2,350 per kg N/year and £35,000 per kg P/year. Temporary credits are not yet offered. Each nutrient budget is mitigated separately, so a dual-nutrient scheme buys both. Always confirm the current price and availability with the scheme.

There is now a second source of supply. In April 2026 Natural England approved the Stodmarsh Stream Enhancement Scheme — run by Greenshank Environmental with Dace Environmental across five sites and more than 2.5 km of small watercourses in the Stour catchment — under the Habitats Regulations. Its first phase is expected to unlock in the order of 3,000–5,000 homes across Ashford, Canterbury and the wider Stour catchment, with a second phase taking that toward 8,000, and units are reserved through the scheme’s own online platform. It does not publish a per-kg price, so credits here are by quotation — worth pricing against Stour Environmental Credits before you commit.

Two Canterbury City Council points change the arithmetic here. Occupancy: the council accepts 1.6 people per dwelling for flats, against Natural England’s 2.4 for houses. And Herne Bay is a special case — properties connected to the May Street works need only 0.13% of the standard credits, a 99.87% reduction, because only that share of May Street’s treated effluent reaches the Stour. The reduction applies to nitrogen and phosphorus alike. NutrientMonkey has Herne Bay as a sub-area option and applies it for you. Parts of Herne Bay drain to Swalecliffe instead and fall outside the catchment altogether, so it is worth checking the connection with Southern Water.

One credit is 1 kg of nutrient/year. The number you need is your governing budget — the greater of the pre- and post-2030 figures, floored at zero. NutrientMonkey shows this live so you can gauge how much mitigation you'll need before you commit. Scheme availability and prices change — always confirm the current position with the provider or your planning authority.

How the Stodmarsh calculator works

1. Pick Stodmarsh from the catchment list — it's pre-flagged for both nutrients.
2. Enter the dwellings & waste water — occupancy, water use (125 or 110 l/p/d plus NE's allowance) and the drainage route (septic, package plant or mains permit).
3. Add land use & SuDS — current vs future land cover and any sustainable drainage, with editable export coefficients for the Kent catchment values.
4. Export when ready — a branded, planning-ready PDF with the budget in kg/year and every input documented.

Honest about what it is

This is a nutrient-methodology assessment built to support your Stodmarsh planning application — fast, clear and ready to discuss with your case officer. The official Natural England catchment calculator remains the statutory tool, and your figures should reproduce in it; the report states this plainly. Want it handled end to end? We also prepare nutrient neutrality assessments for Stodmarsh as a service — we run the budget, size the mitigation and produce the report your ecologist and case officer sign off. Tell us about the site and we will quote, usually the same day.

Frequently asked questions

In a different catchment?

NutrientMonkey covers every Natural England catchment. If you are not certain which one a site falls in, the catchment map shows all 27 on one map of England, coloured by the nutrient that applies.

Or jump to another part of the country:

• Poole Harbour N
• Solent N
• Tees (Teesmouth) N
• Lindisfarne N
• The Broads N & P
• River Wensum N & P
• Chesil & the Fleet N & P
• Hornsea Mere N & P
• Oak Mere N & P
• Rostherne Mere N & P
• West Midlands Mosses N & P
• River Clun N & P
• River Avon (Christchurch) P
• Somerset Levels & Moors P
• River Itchen P
• River Axe P
• River Camel P
• Derwent & Bassenthwaite P
• River Eden P
• River Kent P
• River Lambourn P
• River Lugg P
• River Mease P
• Esthwaite Water P
• Peak District Dales P
• Roman Walls Loughs P

Or go back to the main NutrientMonkey calculator.

Wider planning support

Nutrient neutrality is rarely the only condition on a scheme. Our energy statements service and the wider SAP & compliance team cover the rest of the planning and Building Control picture — get a quote and we'll talk it through.