I rang IRD. I have a flat in my house. I was told to complete the form for a rental property. And to measure the area of the flat and the area of the house and calculate the percentage area of the flat. Then that percentage can be deducted from power, rates, insurance, wifi etc. You can also claim expenses for cleaning etc. There is also the cost of any food etc supplied. You would also be able to claim depreciation on the chattels ie beds, TV, bedding etc. A previous poster is correct, You can claim for the period the area is available for rent not nights booked.
I don't think you can claim for capital costs for setting it up eg new bed, bedroom furniture, bedding unless you have set up a business. But I am not sure about this. I also suppose this raises the whole question of "is it a business" and is GST involved?