We have rented out property for several years. As already mentioned, all income should be declared. Your mother can offset many things against the rent, i.e. mortgage interest (note, you can only offset the interest portion of your payment!), outgoings such as insurance, repairs, etc. If it's furnished she can offset 10% of the rent (after water rates) against wear and tear, or the actual cost of replacements. Being honest with the taxman is not as frightening as some people think. Look on the Inland Revenue website, there's loads of info there.