A very rough summary is that any structure which isn't there only temporarily needs planning permission. There are certain 'permitted development rights' that mean certain forms of development don't need permission. The detail is set out in the rather byzantine General Permitted Development Regulations. Class E covers things like sheds in your back garden but not structures between the front of your home and the street: http://www.legislation.gov.uk/uksi/2015/596/schedule/2/made Southwark Council could in the forthcoming New Southwark Plan could approve a 'Local Development Order' (LDO) in order to provide a borough wide exception for specified types of development. While Southwark Cyclists asked them to approve an LDO for cycle stores in front gardens, its suggestions appear to have been ignored in the most recent draft. Hope that helps!
