a. I have a house on the beach.1. I have a house ON the beach. and I have a house AT the beach.
b. I have a house at the beach.
There does not seem to be any difference in meaning between the two sentences. Context would indicate if any meaning is intended other than that which is apparent.
a. My house is always full of friends.2. My house is always full OF friends. and My house is always full WITH friends.
b. My house is always full with friends.
Those two sentences seem to mean somewhat different things, but without context I cannot be sure of that. Anything I would say would be speculation only.
a. If I have work at the hospital, I get up early.3. I'm a nurse. If I have work AT the hospital, I get up early. and If I have work IN the hospital, I get up early.
b. If I have work in the hospital, I get up early.
It is hard for me to see how there could be any real difference in meaning between the two sentences; however, I would expect the first one to occur much more often. Surely if you have work at the hospital it is going to be done in the hospital. Context would reveal any differences in meaning (if any) between the two constructions.