The US state of Idaho Tax Commission has announced that same-sex couples in the state who were legally married out of state must recalculate their federal tax returns as if they were “single”, or “head of household”, before being able to file in the state.

Liz Rodosovich said on Thursday that the Commission had altered its 2013 income tax forms following the decision of the US Supreme Court to strike down the Defense of Marriage Act.

The Internal Revenue Service ruled that legally married same-sex couples in states which recognise same-sex marriages, would be treated as married for federal purposes.

Idaho’s rules now require extra work by same-sex couples, as those who file a joint tax return as a same-sex couple must file their Idaho state return as single or head of household.

Before filing the return they must also recalculate their federal tax return, as if they had used the same status, so either single or head of household.