Since it's difficult to estimate the costs relating to letting when the property is also used by you, a so-called standard deduction for costs is granted instead. The standard deduction is 15 percent of rental income, which means that only 85 percent of the rental income exceeding NOK 10,000 is counted as tax liable income. 

It is only when letting so-called rental cabins, where the entire rental income is taxed, that you can deduct costs related to the letting.