The two administratively independent unitary authorities, which were established on 1 April 1996 following the breakup of the County of Avon, are North Somerset and Bath and North East Somerset.
These unitary authorities include areas that were once part of Somerset before the creation of Avon in 1974.
The local authority must have a legal control over the site, by owning or leasing it or having an agreement with the owner.
The smallest is Wellington Basins, which covers 0.53 hectares (1.3 acres) of small ponds and surrounding grassland and woodland.
The woodland provides a habitat for mammals including deer, badgers, foxes and bats.