I bought a fourplex in Sacramento area, each unit is a 2bd/1ba 958 sq.feet. Only living room has a wall AC and a wall heater. I decided to remodel each unit and put mini-split systems in each room. I bought 12 heads/compressors, paid $600 per 12,000 BTU units. The contractor that was going to install it, said I don't need a permit, because he was going to plug them into the wall outlets in each room. Well... he turned out to be a dumb idiot, because it turned out that each room had all wall plugs wired together to a 15 amp breaker. That means if you turn on an AC in a room and then turn on something else in that room, you might burn down the house.
I had to get the electrical permit - $800 and do a proper electrical wiring on the outside of the house from the main panel, another $6200. Then get another permit for air conditioning installation for $700 and the installation cost of $550 per room + materials. Overall it became a $24K adventure...
It looks and works great, the tenants love it and I am sure I'll get the money back at sale if the appreciation keeps at the current level. If you ask me if I would buy another property without AC, I would say no way!