Spy plane technology has been used to map the borough and find more than 300 suspected cases of beds in sheds.
Harrow Borough Council used the aircraft to detect illegal developments in gardens, which are then rented out.
Before the operation, planning officers estimated there were 75 cases of unauthorised 'beds in sheds' in Harrow.
Now there is a list of 319 potential cases after the evening flight, during which a thermal imaging camera was used to take aerial shots of the borough.
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Yellow areas of colour in the images show buildings with uninsulated roofs which could possibly be illegal structures.
Council leader Cllr Susan Hall said: “The pressures of migration in London, fuelled by recent waves of new arrivals from Eastern Europe, are creating new stresses on local authorities that we just haven’t had to contend with before.
“What was once thought even just 18 months ago to be a problem for the centre of big cities is now rippling out to the suburbs.
“The beds in sheds phenomenon means there is a hidden community springing up in the back gardens of our cities – in our borough alone the thermal pictures we receive suggest there are four times as many as we first suspected.
“These are people who use council services – like bin collection – but for whom we receive no extra funding.
"In many cases the people in beds in sheds are at the bottom of a pyramid of exploitation.
“But we get complaints from their neighbours that we cannot ignore, and it is not fair on them that these houses should turn into cash cows for opportunist landlords.”
Slough in Berkshire, has used the same technology in 2013 to map out possible beds in sheds.
Harrow Council says planning officers will now visit the properties identified and will be comparing the images with other records to establish if there are any more.