Building Regulations (WA) define the kinds of building work for which a building permit is not required. These include small garden sheds, temporary site sheds, certain fences, certain masts and antennas, small retaining walls, small pergolas and the like... it states that a building permit is not required for the construction of a freestanding Class 10a building (means does not contain any habitable rooms such as a living room, bedroom, etc. or is not frequently used) that:
If youre planning on a 15m2 cabin, then you would almost certainly need a building approval. If you kept it to less than 10m2, then would need to consider what you plan to use the cabin for etc. as it still may require approval.
To be absolutely sure, best to confirm with your local council.
Hope that helps.