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Otters are large mustelids in the same family as badger, weasel and mink. They are primarily nocturnal and are rarely seen. Otter occur in rivers, streams, lakes, marshland and ditches, occupying territories of up to 40km and resting in holts beneath tree roots and within cavities near to watercourses. The UK population of otter declined dramatically during the 60s and 70s largely due to the use of agricultural pesticides. 

Otters and their habitats are protected under European and UK legislation including the Habitats Directive and are a species of Principal Importance in England under Section 41 of the NERC Act 2006. Local authorities are therefore required to protect the species when determining planning applications. Planning permission can be declined if insufficient information regarding otter on the site is held, and the discovery of otter or their signs during works may cause delays to the development.