Biodiversity Net Gain measures the net change in biodiversity units pre- and post-development. To calculate this, the baseline habitats (pre-development) must be classified and measured, and any planned changes to them. This means for each area of habitat, you must quantify how much of that will be retained (unchanged), enhanced, or lost from the development. New habitats can be created, but only when baseline habitats are lost. The baseline is your site boundary, so you cannot extend the size of the site post-development, nor can you leave empty space that was previously a habitat.

The tool will perform several checks to validate user inputs to ensure that retained, created, and enhanced habitats are the correct amounts, such that the site area remains the same pre- and post-development. An error will appear before calculation of units is complete if any validation fails.

Every habitat present on site pre-development must be added to the baseline habitats question. For each habitat, input the area in hectares, assign a condition (if applicable) and a strategic significance value.

It is recommended to complete the baseline before adding any habitats in to the post-development questions (created and enhanced). Once complete, it may be useful at this stage to create additional designs and copy the data to them, so that if you want to compare multiple design options on the same site the baseline data is already prepared and you can start adding different habitats to the post-development questions.

Below is an example error message when validation of area inputs has failed. In this example, the error is indicating that the total area of created habitats (1 ha) does not equal that of the baseline habitats that was lost (2 ha).

The image below shows the full range of inputs for this example. The inputs can be validated by either:

Adding enhanced habitats to the assessment is different to adding created habitats. When creating new habitats, the available space can be used for as many different parcels of habitat as long as the total area of new habitat equals that lost from the baseline. When enhancing habitats, the original habitat from the baseline remains in the same location and the area affected must remain the same.

There are two ways of enhancing habitats: enhancing by distinctiveness and enhancing by condition. Enhancing by distinctiveness means changing the baseline habitat into a different habitat with a higher distinctiveness value and, therefore, a higher relative value for biodiversity. Enhancing by condition means the condition of the post-development habitat will be higher than it was pre-development. It is possible to enhance both by distinctiveness and condition at the same time.

The distinctiveness value for each habitat is in the data card, which you can access by clicking the ? next to a habitat. The value is under the Environmental profile section:

For example, if you have one baseline grassland habitat that you will enhance a portion of, you should indicate how much using the Area enhanced (ha) field, as seen below:

Then, the same baseline habitat should be added to the Enhanced habitats question, with the same Baseline condition. You can then add a Target habitat and Target condition, the options for which will be dynamically filtered based on the Baseline habitat and Baseline condition.

The example below is what the enhancement for the above example would look like. The Baseline habitat and Baseline condition are the same, and the area of 1 ha equals the 1 ha in the Area enhanced (ha) field, and we can now select a Target habitat and Target condition

The tool will also validate areas of enhanced habitats. In this example below, the error is indicating that the total area of enhanced habitats (0.5 ha) does not equal that in the area enhanced input in the baseline question (1 ha).

The image below shows the full range of inputs for this example. The inputs can be validated by either:

If the first solution is chosen, this may produce the first error example due to an area of the baseline now being lost, and not accounted for in the Area retained (ha) field or by adding 0.5 ha to the Created habitats question.