From PsyHelp

You can expand your house beyond the basic Common Room and Storage with a number of different add-ons, each with different functions and properties.

Add-ons and their Properties

• Airship Mooring
  • Used to build an Airship for PvP
  • Access to the Aeronautical Society
• Apiary
  • Feed bees every two hours to harvest certain items
  • A level 5 Apiary will award you with a badge
• Attic
  • Adds 20 spaces to your Storage
  • Has a locked chest with various goodies (and not-very-good-ies)
• Basement
  • Holds 100 items regardless of item size
  • Wipes all comments on items
  • Expand 100 units by trading in Deeds to 1000 Units
• Colossus
  • Reduces Storage fees by 10%
  • Grants you a badge
• Dungeon
  • Showcase defeated Monsters
  • Has mysterious door that can be unlocked in December for items
• Eiffel Tower
  • Gives out wine and fireworks on Bastille day
• Fireplace
  • Keep a fire burning for various gifts
  • Gives a fireplace mantle to display items on users profile
  • Grants two badges: one for keeping the fire burning for 500 hours and one for 1000 hours
• Forge
  • Smelt and de-smelt certain metals and alloys.
• Greenhouse
  • Grow stuff
• Indoor Swimming Pool
  • Host Swimming Race events
• Lake
  • Add Rubber Ducks, Boats, and a Lake Monster
  • Nivaine's Ring
  • Bathtime Badge
• Library
  • Reduces maximum house size by 100 in return for unlimited storage for books
  • Access to the Batcave
• Map Room
  • Keeps track of the areas your pet has explored as a Gatherer
• Menagerie
  • Showcase Prey (similar to Dungeon)
• Moat
  • Provides safety
  • Fish for things
• Refreshing Spring
  • Half-hourly action
  • Profile Title Choices
• Shrine
  • Light Candles to cause effects and cast spells
• Smokehouse
  • Cure Steak into Jerky and Spicy Jerky
• Tower
  • Have your Flying Monkey fetch (mostly) alchemical things
  • Transmute things
• Windmill
  • Saw Logs into Wood, and mill Wheat into Flour efficiently
• Wishing Well
  • Gain access to the Mysterious Shop and other things.