Illegal building is when a building permit is not issued for construction works that should have a permit.
Carrying out works without a permit can compromise the safety of occupants and cause problems with insurance, re-selling and other legal matters.
A Building Permit cannot be issued retrospectively for the works already carried out, therefore you cannot obtain an Occupancy Permit or Final Certificate to say the works are compliant.
It often costs more to resolve illegal building issues than to get a Building Permit before starting work. Administration fees, structural investigations, engineer reports and building reports may be required to investigate a complaint, making it an expensive process.