Legal Question in Landlord & Tenant Law in Illinois

Check your lease first and see if there's any mention about it. Hiring an electrician to prove your theory out first would probably be a good ideabecause although the most open thing would be to involve your landlord, your neighbor and an electrician, the accusation itself could cause you problems. If it turns out you are correct, you may have rights under the Chicago Residential Landlord & Tenant Ordinance to offset a portion of the bill. My sense is that you live in a building where a single apartment was broken up into two (maybe front/back or upstairs/down?) and the landlord didn't separate electrical service for the two. You may also want to contact the Building Department or Fire Department because depending on various factors this could be a building code / fire code violation; understand that this can get your landlord into trouble too and this will have consequences. That's why talking to your landlord first is a potentially good and bad thing. But electricity can be deadly on its own.