If you have an office based at home, you are entitled to claim a portion of the space it occupies within the house. IRD defines office space as an amount of time in a specific area used for generating income-earning activities.
Some examples of what you can claim are: rent, insurance, interest (not principal) on your mortgage, rates, water, power and gas. Home phone and broadband is apportioned on usage. Your bathroom or kitchen is not considered an office space, even though you may use these areas during your workday.