Hounslow Council in West London has fined landlords a collective £180,000 in the past six months, with an average fine of £14,000 each.
Most of the violations were for unlicensed Houses in Multiple Occupation (HMO).
Inspections check whether landlords have the correct tenancy agreements, licenses, and health and safety standards.
Councillor Tom Bruce, the council deputy leader and cabinet member for assets, said: “We stand firmly with tenants and communities, supporting people’s right to live in secure, well-maintained homes.
“Most landlords provide safe and decent housing, but we will not tolerate those who exploit renters or neglect their responsibilities.”
He added: “Where landlords fail in their duty of care, we will act decisively, issuing the strongest penalties available whenever possible.”
