A landlord advocate says Toronto homeowners are facing a growing problem: Fake tenants who convert rented homes into illegal rooming houses.

Paralegal April Stewart, a self-described "landlord advocate," represented an Ontario couple who rented their home to a man they believed to be a good tenant. The couple said the tenant then turned their home into an illegal rooming house, subletting rooms to as many people as possible.

"I can tell you in my practice in Ontario as a paralegal only representing landlords for 15 years, I have never seen a situation like this one before," Stewart told CTV's Canada AM on Thursday.

Stewart didn't get into detail because the case is still before the courts, but said that it is alleged that the same man operated similar scams at other properties. Stewart said it is believed that as many as 20 people lived in one of the properties at one point.

"Most of them were newcomers ... Very little command of the English language and therefore quite vulnerable," she said.

Stewart said the best way to avoid being caught in a similar situation is to spend the time interviewing potential tenants before making a choice.

"Out of the cross-section of the cases that we see, there's usually a problem in the very beginning screening process, where enough time is not being spent to really vet who this person says they are and what their intentions are for the property," she said.

"To be a landlord and a tenant in Ontario, it's like a marriage, and getting out of it is just as difficult."

Stewart said that the landlord process is regulated in Ontario, and property owners have the right to collect information and verify the particulars provided by a potential tenant.

"For example, if someone says, 'My name is John Smith. This is where I live. This is my date of birth,' and they commit that to a rental application, the first thing that the person who's doing the screening should do is ask for a piece of photo ID."

She also recommended running a credit check, not just to find out the financial details, but to determine if what the person has told the owner is true.

For tenants, Stewart recommended asking "hard questions," including who lives nearby, how many people live in the property and whether they will fit in well in the new setting.

"If they sign a contract and they decide after a couple of weeks that this isn't going to work out, they may be in for some litigation," she said.

Stewart recommended landlords or tenants in a tough situation contact the provincial landlord and tenant association. There are also self-help centres and online legal resources in most provinces.