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Phosphate calculator

The first step in understanding the implications of development on phosphate levels is the publication of a Phosphate Calculator.

Read the Council’s press release.

The phosphate budget calculator can be used on proposed developments across Somerset. It will provide a transparent and rapid calculation of net phosphate loading from developments, including phosphate offsetting calculations for on or off site locations. The calculator has been approved by Natural England and it can therefore be used to provide a standardised and transparent decision making tool for the Local Planning Authority and Developers.

It is requested that developers use the calculator to calculate the level of phosphates a proposed development will generate and for this information to be submitted as part of a package of information to support the planning application as it will need to form part of the HRA. A planning application will only be able to proceed to a positive recommendation if the proposed development is phosphate neutral or there is identified mitigation that can be secured. The calculator will remain under constant review and updated to ensure that it reflects any data changes.

The calculator will also be updated in due course to take into account planned upgrades to Waste Water Treatment Works (WwTw) within Somerset by Wessex Water.

The calculator was updated on 8 March 2021 with the following amendments:

  • Correction of a glitch relating to the runoff coefficients of meadow / natural grassland
  • Removed wetland land use from the proposed land use options in stage 3 and replaced with an option to input bespoke values for wetland or SuDS following guidance by natural England that the previous value is no longer endorsed by them due to uncertainties.
  • Definitions have been added for all of the land uses
  • Land uses with the same coefficients have been collated where possible
  • The runoff coefficient for a bog changed from 0.00 to 0.02 kg/ha/year
  • Permit limits for some Wastewater Treatments Works were updated following advice from the Environment Agency
  • Various dwelling types and the appropriate occupancy rates have been added

Note: Calculator was further updated on the 19 March to correct an error in stage 7

The phosphate calculator can be accessed by following this link:

Note: Calculator updated on the 19 March to correct an error in stage 7.The calculator is subject to ongoing review and revision. When the document is updated a new link is provided to the calculator. As such any saved versions of the previous link will no longer take you to the calculator. You should always use the embedded link in our website to access the most up to date version.

We aim to implement further changes to the Phosphate Calculator before Easter 2023, in a coordinated way with updated advice from Natural England.

Following the publication of this calculator, some applicants for development will be able to clarify whether they have the capacity to deliver on site mitigation and therefore be able to present suitable land management proposals to allow their proposals to make progress.

For small scale schemes that can use Septic Tanks or Package Treatment Plants Somerset Ecology Services together with Natural England have prepared Interim Guidelines about the scope to achieve drainage solutions that do not trigger the significant effect test. View the Guidelines here.

Please note the original guidance issued in February 2021 has been replaced by updated guidance dated May 2021. The key impact of this change is to reduce the discharge threshold to 2 cubic metres per day.

For those applications where applicants do not have the capacity to provide an on-site solution the next step for the Somerset Council’s to develop an off-site mitigation strategy to include costed projects that can deliver the necessary mitigation and to apply a cost per dwelling (or alternative, impacted development type) that can allow new development to progress without resulting in a significant impact upon the protected site.
A video has been prepared which provides a step by step guide to help you use the calculator.

Next Steps 
Once you have used the calculator and have an understanding of how much mitigation a development will require, you will then be able to consider what your options are for mitigation. In order to help you move forward, some guidance has been produced which includes:

Last reviewed: August 2, 2023 by Jenny

Next review due: February 2, 2024

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