Landlords of houses in multiple occupation (HMO) in South Kesteven must apply for a licence by October 1 to comply with a change in the law.
From that date all HMOs occupied by five or more people and are home to two or more households will legally require a licence.
Landlords who have not submitted a valid application to the district council by the deadline will be committing an offence and risk an unlimited fine on conviction in a magistrates' court.
Currently, only properties of three or more storeys need a council licence.
The Council has written to landlords with more information about the changes.
Cabinet Member for Housing, Cllr Nick Neilson, said: "We welcome these measures, which will help us tackle rogue landlords. They are designed to help prevent overcrowding and ensure properties are fit-for-purpose.
"It is also important that landlords make regular inspections in case tenants have moved in new occupants without their consent or knowledge as the landlord will be responsible if circumstances change and a licence is required."
Landlords can apply for a licence online at http://www.southkesteven.gov.uk/index.aspx?articleid=8229
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