Home is only five years old but the builder only put in 100 amp service with twenty slots. He used every available slot/circuit breaker. I want to install a spa/hot tub that is 240 volts. I like to find out if I can just add another box or do I need to go up to 200 amp service. I like to go with the most inexpensive option. Thank You.

See if your panel will take tandem or half size breakers. If your not sure let me know the make and model number.