I think its a combination of both. Off site can provide larger spaces that can support a wider range of diverse wildlife and also be enjoyed by the community. On site needs to be included in all new developments and be more comprehensive than just a few 'bat boxes'.
Minimum sized gardens, encourage (insist) owners maintain a wild section, stop cutting verges and turn to wild flower verges, have small community patches within the development that are wildflower and/or pond based. Include a minimum number of trees in each development, don't allow tall solid fences but have traditional hedges instead to enable the movement of wildlife through developments to create wildlife corridors linking to the bigger open spaces.
This will require education of the general public/new homeowners and some enforcement to ensure they don't make wholesale changes.