Guidance for Returning Officers administering Local Government Elections in England

If a casual vacancy occurs in the six months before the date on which that elected member would have retired, an election is not to be held, unless more than one third of the authority’s seats are vacant.1
 
The relevant dates are the date the vacancy is deemed to have occurred and the date that the elected member would have ordinarily retired (not the date of the scheduled election).2
 
Also, the six month rule only applies if the elected member would have ordinarily retired within six months of the vacancy occurring. This means that in cases where the authority is elected by thirds or halves, the Proper Officer will need to check whether the relevant elected member would have in fact retired within six months.