A landlord was fined £3,000 for renting out a decrepit outbuilding as a family home.
Kunaratnam Kuganesan told Acton Magistrates' he did not know they were living in the converted garage and they should have been living in the main house, in Hurley Road, Greenford.
But the court did not accept this last Thursday and also ordered him to pay £1,133 costs.
The 48-year-old was collecting rent, had been issued a prohibition order, forbidding anyone living there, in 2007 and warned by the council in February 2008.
Kuganesan, of Garrick Road, Greenford, was prosecuted after council officers found the family was still living in April.
Councillor Vlod Barchuk, cabinet member for safer communities, said: "I'm delighted with the outcome of this case which highlights the growing problem of landlords exploiting tenants by renting out substandard garden outbuildings.
"This is an issue the council is determined to stamp out."