I’ve rented the upper floor of a house to a couple and now there seems to be another couple living with them. What can I do?

Unfortunately there may not be too much you can do about the extra people living in the unit unless you can show that the unit has become overcrowded, you’ll have to check with your local municipality to determine whether there is a bylaw that addresses this issue. If the property is in the city of Toronto the bylaw permits 100 square feet per person and that would include the entire unit with the exception of bathrooms and hallways.