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Nutrient mitigation

Nutrient mitigation

Excessive nutrients (principally nitrogen) in the Solent’s European sites are causing eutrophication, leading to an increase in algae growth. This has an adverse impact on the habitats and species within the European nature conservation designations sites in and around the Solent to which the Habitat Regulations apply.

Following recent case law, Natural England has advised that there is a likely significant effect on the Solent’s designated sites due to the increase in wastewater from new housing and any other development providing overnight accommodation which would discharge into the Solent.

The internationally-designated habitats of the River Avon to the west of the New Forest are also affected by elevated levels of nutrients – in this case phosphates. The scale of planned new development in the catchment of the River Avon that falls within the National Park is low, but any proposals for net new dwellings and other forms of overnight accommodation in this catchment must demonstrate it is phosphate neutral.

Natural England recommends that the impact of development on water quality in the catchments of the Solent and the River Avon is examined within the Appropriate Assessment process. For this reason, we are requesting the submission of a Nitrate Mitigation Checklist for any planning application for a net increase in dwellings or providing overnight accommodation.

In February 2024, the Government issued updated national guidance and tools for addressing nutrient neutrality. This includes revised nutrient budget calculators for both the River Avon and Solent catchments, which supersede the previous version of the calculators. Importantly, the revised budget calculators factor in the upgrades to certain wastewater treatment works required by 2030 under the provisions of the Levelling Up & Regeneration Act 2023.

This updated information and relevant budget calculators are available to download from the Government website here: Using the nutrient neutrality calculators – GOV.UK (


Water catchment areas in the New Forest National Park

Applicants are advised to carefully review the latest Natural England guidance (March 2022) in preparing their applications. We have also prepared a high-level map illustrating the respective water catchment areas for the National Park to indicate the areas affected by the respective nitrates and phosphates issues. This is available to download on the right hand side.

The New Forest National Park is covered by three main water catchment areas:

  • The Test
  • The New Forest
  • The Hampshire Avon

Detailed maps of these catchment areas can be viewed at Environment Agency – Catchment Data Explorer by searching using the three catchments named above.

Identifying the relevant catchment for mitigation measures – Natural England advice (June 2021)

In June 2021 Natural England provided the National Park Authority with updated advice on the geographic scope for nutrient mitigation for new development draining to the New Forest catchment. This latest advice is available on this website and confirms the following:

Area Fluvial Catchments Suitable mitigation locations to deliver nitrate neutrality
North East of the National ParkBartley Water (including Ashurst, Netley Marsh and Minstead)The Bartley Water catchment is included within the Test and Itchen catchments for mitigation purposes due to the tidal flows between the estuaries. The National Park Authority has an overarching legal agreement with Eastleigh Borough Council to enable development in the north east of the Park to be off-set in the Test & Itchen catchment.
New Forest RiversBeaulieu and Lymington rivers (including Brockenhurst & Lyndhurst)Mitigation should be located within the Beaulieu and Lymington river catchments as a first and preferred option. If it can be reasonably shown that mitigation cannot be delivered within the Beaulieu or Lymington catchments, it may be considered appropriate to locate mitigation in adjacent catchments – i.e. the wider New Forest southern coastal catchment, and/or the Western Yar and Newtown catchments on the Isle of Wight. The National Park Authority has an overarching legal agreement with the Isle of Wight Council to enable development in the New Forest Rivers catchment to be off-set on the Isle of Wight.
New Forest RiversAvon Water, Danes Stream, Dark Water, Sowley Stream (including Sway)Mitigation would be appropriate in the wider New Forest southern coastal catchment (including Beaulieu and Lymington rivers), and/or the Western Yar and Newtown catchments on the Isle of Wight. The National Park Authority has an overarching legal agreement with the Isle of Wight Council to enable development in the New Forest Rivers catchment to be off-set on the Isle of Wight.

It is the responsibility of the applicant/developer to find suitable and sufficient nutrient mitigation solutions. A list of nutrient mitigation suppliers is available on the Partnership for South Hampshire’s website at Mitigation schemes available to developers – Partnership for South Hampshire ( This information includes details of the catchments that the mitigation schemes serve. We have also prepared flowcharts of the process for securing nutrient mitigation for both the planning application and prior approval routes and these can be found under the ‘Solent nitrate mitigation’ and ‘Avon phosphate mitigation’ tabs on the right of this page.

Please note that the drop down boxes in the Nitrate Mitigation Checklist will not work if you open the document in Microsoft Edge or Internet Explorer. For these to work you will instead need to save the document to your computer and then open the document using Adobe Reader or other appropriate software.

National Park Member


'Read our guidance notes before applying to save yourself time in the long run.'

More New Forest Planning policy

Planning policy is a key statutory function and plays a central role in the achievement of the two National Park purposes and the related socio-economic duty.

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