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Place pre-fabricated garden cabin in slope backyard

I intend to place a 9 sqm wooden garden cabin in my backyard, which is on a slope. I'd say between 30 and 40 degrees. I was planning to build a retaining wall (no more than 1.5 m high) to flatten the ground as much as needed to make the cabin's foundation. I want to know if I need building consent for that. I will do it by myself.