BRANZ answers some questions over the sizing of domestic garages and the ability to use NZS 3604:2011 Timber-framed buildings to design them.
Free-standing non-habitable garages up to 30 m2 in area are covered as Importance Level 1 buildings. To be designed using NZS 3604:2011, garages that are larger than 30 m2 must be dealt with as Importance Level 2 structures (see Table 1.1 Classification of buildings).
Garages with a suspended timber floor must have the floor structure specifically designed, as the floor loads are outside the limitations of NZS 3604:2011.
Summarised from BRANZ Guideline, February 2015.