The main new feature that The Sims: House Party introduces to the base game is the ability for sims to use the phone to organise house parties; social gatherings where many sims can gather at a sim's household and interact together at once. House parties can often be good opportunities for sims to raise their mood, meet new people and raise their relationship scores with other sims, however they also usually require some effort from the player, can cost money to prepare for and if the guests are not enjoying themselves will end quickly.
In addition to house parties House Party also adds five new radio stations for sims to listen to: beach, country, techno, disco and rap. The expansion pack also adds a new clothing type: costumes. Sims can change into a variety of costumes from Hawaiian wear to togas.
The furnishings and build mode items that House Party adds can be organised into three different themes: Futuristic, Traditional Hawaiian and Old Western. Some of the notable items that the game adds include bars, buffet tables, punch bowls, camp fires, mechanical bulls, costume trunks, turntables, dance floors, dance cages and rave lighting.
Male/Female Entertainer- Can be hired by interacting with the Fancy Feet Cake Treat. After they perform a short dance which raises the fun meter of all observing sims they will hang around the house. They may try to flirt with sims in the room and should that sims lover or spouse see this it will lead to a negative impact on the relationship.
Gabby the Mime- If a party is going poorly Gabby will appear to entertain guests. The unfortunate downside is that the mime is a kleptomaniac and will steal objects from the sim's home.
Caterer- Sims can hire the caterer for the day for the cost of 350 simoleons. The caterer will refill buffet tables and punch bowls when possible and when dismissed will also clean up the buffet tables and bowls. He may occasionally try to interact with the guests, however this usually goes poorly due to the caterer being a bad conversationalist.
Drew Carey- Should a party be going very successfully Drew Carey will arrive in a black limousine and greet the guests. Sims often gather around him and become ecstatically obsessed with him.
Party Crashers- Parties may randomly be crashed by Paul Dropinsky and/or Paulette Dropinsky. The party crashers generally wander around the house, use whichever objects they feel like, and can actually be a positive influence on parties, however on occasion they will also break objects.
Campfire Ghosts- When sims are telling ghost stories around the campfire a ghost may appear and haunt the lot for the remainder of the night. The more creative the sim telling the story, the more likely a ghost will appear.