My roommate has lived in our apartment for 4+ years and I just moved in a month ago. We live in a nice area on the top floor of a four-unit building and are one of two apartments (the other two units are offices). The building also has double doors on the ground floor that are locked outside of business hours. The layout of our apartment is unusual – the door to the right at the top of the landing leads to the kitchen, bathrooms, living room, and my room, and the door to the left at the top of the landing leads to her room. It's actually a railroad apartment since our rooms are connected through a door but we keep the door locked and she goes across the landing and through the door on the right to use the shared spaces.
My roommate would like to leave our apartment door unlocked whenever we are home. Her argument is that if this were a normal apartment layout, she would be able to freely access the bathroom at night without bringing her keys. She's also afraid she'll leave her keys on my side of the apartment and be locked out in the morning and she says that she's lived in the apartment 4+ years and has never had any issues leaving it unlocked.
I am very much the lock-the-door-at-all-times type. I can deal with leaving the door unlocked during the day when I know she's home but I feel uncomfortable with leaving it unlocked at night, especially because if someone were to gain access, they would be able to access my room but not hers.
I've suggested leaving it locked at night and not at other times but having her keep a pair of spare keys on her side of the apartment so she can get back in in the morning if she does get locked out at night but she doesn't want to deal with having to bring her keys back and forth at night. How can I convince her of this without causing tension?
I'm glad that leaving your apartment unlocked for the past four years hasn't led to any trouble for your roommate, but it is absolutely reasonable that you demand this practice comes to an end. Good sleep is hard to get, especially so if you're worrying about the bogeyman. I had to read the description of your apartment's layout a few times to get a mental picture and I can see how it's a nuisance for your roommate to carry keys during late night runs to the bathroom, but your sense of security trumps this annoyance.
I think a good compromise, if your landlord approves, is to invest in a door lock with an electronic keypad. For around $200 it will give you peace of mind and eliminate your roommate's need for carrying keys and worrying about locking herself out.