What can I deduct from an owner occupied duplex?

[Doris Daily]

Hi everyone, I just purchased a duplex recently and my mother is renting out the other half. I'm wondering about tax deductions and how I would go about writing off utilities, internet, property taxes and so on and so forth? Do I split this half an half? My mother does have the keys to my side of the duplex and uses the washer/dryer and amenities quite a bit. Since her side is smaller, how should I write off these expenses accurately? Can I just write it off 50/50 legally since she has access to my side of the house? I have heard it is 50/50 but have also seen that people are supposed to make the measurement of deductions based on square footage.