A mother and five children say their house is so badly affected by damp, mushrooms have started sprouting.
Donna Jamieson, 31, says a vacant house next door in Coleridge Street, Blackburn, is causing the problem. She has called her home a "mushroom farm".
But the owner of the house has refused to allow anyone access to the property, meaning Ms Jamieson's landlord cannot take action.
She says the damp is affecting her children's health.
Blackburn with Darwen Borough Council's environmental health inspectors have said the house is unfit for habitation.
They have made her family a priority for rehousing, a spokeswoman said.