I recently had a house built and paid $500 extra for 300 amp service for a future pool. Today, I bought a hot tub which uses 220 volts and must be hook up to a 50 amp breaker dedicated. I went to my electrical circuit breaker boxes out in the garage and see no available spaces to install a 50 amp breaker. I do have a second box which is also full. Both boxes have 150 amp main breaker switches. Does this make the entire house 300 amp service?? Also what would I have to do to install breakers for the pool and/or hot tub if there are no spaces available for breakers??

Most panels allow for half size or tandem breakers, read the label and see if they are listed.