In June, Gurdeep Kaur from Hayes was convicted by Uxbridge Magistrates of one offence under the Town and Country Planning Act.
Last week, Isleworth Crown Court imposed an order that Kaur should repay £406,000 in profits from renting out rooms in the house and an adjacent outbuilding.
The case resulted from joint action by the London Borough of Hillingdon’s planning enforcement and trading standards teams.
The council worked with the Home Office’s immigration team, which discovered that Kaur had illegally let out rooms in the main house and an outbuilding to 15 tenants.
This was despite the previous issuing of an enforcement notice ordering her to turn the property back into a single house.
At the confiscation hearing, Kaur was ordered to pay £406,712 and fined £20,000 for the planning offence.
The council was also awarded £4,019.46 in prosecution costs.
Kaur has three months to pay the money or face a prison sentence of five years and still have to pay the money back.
Keith Burrows, the council's cabinet member for planning, transportation and recycling, said: "This is the highest amount the council has ever secured using proceeds of crime laws. Gurdeep Kaur blatantly flouted the law and for that she must pay a hefty sum.
"Let this be a warning that we will take robust enforcement action to stamp out illegal activity," he said.
In September, Westminster City Council welcomed a crown court's decision to order a man who illegally converted his home into eight flats to pay total fines of more than £500,000.
In June, a man in the West Midlands was ordered to pay out almost £79,000 under the POCA after he ran an illegal caravan storage business on his land without planning consent.