Can a restaurant serve alcohol off-site?
In most states, yes. A licensed restaurant can get a one-day or caterer's permit to serve at a specific event, with written permission from the property owner and a lead time of ten days to three weeks in the states that publish a checklist. The permit does not come with insurance. Some liquor liability policies are written for the premises on the declarations, so off-site service needs an endorsement or a caterer's policy.
Most groups assume they cannot
A client asks for the bar at their holiday party, and the events director says no because the license is for the restaurant's address. In most states that is not where the answer ends. The license is the starting point for a permit that lets the same restaurant serve at a named event somewhere else.
The permit
The licensed restaurant applies, not the client and not the venue. The application names the event, the date and the address. The states that publish a checklist ask for written permission from the property owner and allow ten days to three weeks for approval, so the clock starts when the booking is confirmed, not the week of the party.
The policy
The permit makes the service legal. It does nothing for the insurance. A liquor liability policy written for the restaurant's own premises may not extend to a client's office or a rented hall. Off-site service is added with an endorsement on the existing policy, or written on a caterer's policy for the events.
If the client buys the alcohol and the restaurant's staff pours it, it is still the restaurant's service. A guest who drinks too much and drives home is the reason liquor liability exists, and that risk is the same at the client's party as it is in the dining room.
Before the event
Two things, ideally before the December bookings close. Start the permit for each event where the group wants to serve, with the property owner's letter in hand. Then ask the agent one question: does our liquor policy follow us off-site, and if not, what does it take to add it? Venues often ask for a certificate naming them as well, which the agent can usually issue the same day.
The sublimits and exclusions worth knowing on any liquor policy are in The liquor liability gap. If the event also means employees driving food over in their own cars: Does a restaurant need hired and non-owned auto?