Boston Council has issued a reminder to landlords that the deadline is looming to apply for licences for House in Multiple Occupation (HMO).
Under new Government rules landlords and letting agents have to apply for an HMO licence for most properties in which five or more people from two or more unrelated families live.
Those failing to have done so by October 1 will be committing a criminal offence, a council spokesman said.
Landlords and agents who do not licence a licensable HMO will be prosecuted or face a civil penalty of up to £30,000 for each offence as well as a potential rent repayment order for up to one year's rent.
They could also be added to a national database of rogue landlords and property agents and receive an order banning them from letting out a house in England or engaging in property management work.
As part of the new measures, the Government is introducing national minimum room sizes for rooms used for sleeping and mandatory HMO licence conditions relating to refuse storage and disposal.
Mandatory HMO licensing is being extended because of concerns about the number of HMOs operated by landlords who rent out sub-standard, overcrowded and dangerous accommodation, exploit vulnerable tenants and in some cases defraud the public purse, the spokesman said.
The council has housing standards officers proactively inspecting private rented properties in the borough to improve property conditions, which will help identify any unlicensed HMOs.
Applications can be submitted online at www.boston.gov.uk/hmolicensing