After renting apartments for 11 years, about 5 years ago my family moved into a home that we bought. It was a one family apartment, but we split it into two apartments, one to rent out and one to live in. When we purchased our home, it was still in the building stages, so we were able to design it in a way that it would need as little construction possible to turn it into a two family house. Officially when we were given it by the builders it could only be one house, and only after divide it into two, but we could do many of the preparations beforehand.
The things we did to make it a rental unit already by the builders while under construction were as follows. We turned our kitchen into a bedroom (so we'd have a three bedroom apartment) and the dining room/living room into a kitchen/living room/dining room, and we made a place for the washing machine and dryer. The upstairs that was supposed to be 4 bedrooms was changed the most- we made 3 of the bedrooms smaller so that we could drastically increase the side of the other one and get rid of dividing walls to turn it into a living room/dining room/kitchen to the extent that it is bigger than the living room/dining room/kitchen in the apartment I live in. We got rid of the laundry room to make the main room bigger. And we put a water and drainage line into the area where the kitchen would be. Another thing we made sure to do was to separate the electricity and water lines so that we could put a water and electric meter separate in the apartment, as well as allow the tenants to heat up their own hot water and not share a tank with us.
Once we moved into our apartment, we hired a contractor to finish up the work. He put in a wall at the top of the stairs to separate the apartments, broke a hole in the wall for the door that led to the outdoor stairs, and added a hot water tank for the apartment. Then my son and I built and installed an Ikea kitchen, then had the gas company add a gas line into the apartment for the kitchen.
It was a large investment at first, but it has paid off. I'm able to keep the rent in the apartment relatively cheap, around the same or even less than the apartments of similar sizes where I live. In the 5.5 years that I've been a landlord, I've kept the price the same, even as prices here go up.