Habitat Suitability Indices (HSI) are widely used in conservation and in pre-development surveying. We tested a commonly-used HSI to assess its effectiveness at predicting the presence of a European protected species, the great crested newt Triturus cristatus, at the edge of its range. This HSI is used to understand species’ conservation needs, and in assessing the need for, and designing, mitigation. Given the cost of surveying to developers, it is essential that they can have confidence in the index used in targeting work and in Environmental Impact Assessments. We found that nine of the ten factors which make up the HSI are robust in the region, even in a disjunct population believed to have been isolated for around 3000 years. However, we propose modification of the geographic factor, based upon an improved knowledge of the species’ distribution since the HSI was originally devised.