The Assembly term has been extended by a year after the Northern Ireland (Miscellaneous Provisions) Act 2014 received Royal Assent.
The bill, which also bans double-jobbing between Stormont and Westminster or the Dail, will extend this Assembly term to 2016.
It will then bring Stormont into line with the Edinburgh and Cardiff assemblies by giving MLAs terms of five, rather than four, years.
Previous Secretary of State Owen Paterson had expressed concerns about the constitutional implications of extending the Assembly term to five years after MLAs were elected for a four-year mandate.
The act, which does not provide for a Stormont Opposition, also allows the Assembly to reduce its size from six to five members per constituency – which would cut Stormont from 108 to 90 MLAs – if there is cross-community support.
Secretary of State Theresa Villiers said it was “designed to improve how the democratic institutions in Northern Ireland function”.