This could be due to the resignation of a councillor, the passing away of a councillor, the disqualification (or ceasing to be qualified) of a councillor, or a failure to take up office.
Town and Community Councils
When a councillor's seat becomes vacant the Town/Community Clerk will display a 'Notice of Vacancy' in the community area as soon as practicable.
The notice explains that an election can be called by a request made in writing to the Democratic Services Manager by 10 electors from the community area (if the community is warded, they must only come from the community ward), within 14 working days of the date of the notice.
If an election is called it must be held within 60 days of the notice. If no request is received to call an election then the Town/Community Council may then co-opt someone into the vacant seat.
This means that the Town/Community Council can choose themselves who they wish to appoint to fill the seat, rather than holding an election.
County Borough Council
When a councillor's seat becomes vacant a 'Notice of Vacancy' will be displayed to advertise the fact that there is a vacancy.
The notice explains that an election can be called by a request made in writing to the Returning Officer by 2 electors from the electoral area. If an election is called it must be held within 35 days of the notice.
The exception to this is that if a vacancy occurs within 6 months of the date of the next scheduled elections, the vacancy is carried over to the next elections. The seat will remain vacant during that time and no by-election will be held.