Illegal Flat

I am not sure if this is the correct forum but I need some adivce about what I believe to be an illegal flat next door to me.

I live next door to a two storey house that has been owned by a family for about seven years. At the beginning of this year, the family disappeared and a different family started living in the top story and another group of people were living on the bottom storey. I don’t believe the house was sold because the house was not handled by an agent and after the original family moved out, I did see them from time to time visiting the people living at the bottom of the house. Recently the people at the bottom moved out and new people moved in. I don’t believe the people in the top storey or bottom storey are related because they are of a different ethnicity. I know for a fact there is no kitchen in the bottom because I have been through this house previously. I can also see into the bottom storey and have seen a rice cooker and other cooking things set out on a table. Although there are stairs between the stories in the house, the people at the bottom use the sliding doors as their main entrance and walk up and down the side of the house to the street. As it is very close to our property we can hear them clattering up and down the stairs at all hours of the day and night accessing their “flat”. My concern is possible overcrowding and fire hazard due to people cooking without a proper kitchen. Who do we need to report this to in order to have it looked into and made into a safe residence?