If considering claiming a percentage of mortgage interest based on floor area as a tax deduction for a home office / study, one should first do the sums to calculate whether it is financially worthwhile or not, as claiming a proprtion of the mortgage interest paif for the family home automatically means that one will be liable for capital gains tax on that proprtion of the floor area claimed, when the family home is finally sold. If it is calculated to be financially advantageous to claim it as a tax deduction with the ATO, then it is advisable to get a registered valuer to value the family home before the claim is first submitted to the ATO, to provide a "capital base" for calculation of the capital gains tax due when the family home is sold. If a proprtion of the mortgage interest is not claimed, then the family home is of course completely exempt from CGT when sold.

Also, one needs to be aware of the "letter of the law", in that if a percentage of the mortgage interest is claimed, the home officce/study for which it is claimed should be used exclusively for business purposes and not used for any other 'non-busineess' purposes (e.g. as a guest bedroom), although the chances of being audited & detected for doing so are of course very low. if randomly audited by the ATO, however, it would be a good idea to remove the fold out bed (& any boarders in residence!) from the office / study before the ATO man visits! Same sort of rules as for investment properties, which if negatively geared should not be used as a family holiday home...