Each building type has a specific size and can only be placed in a location that can accommodate that size.
The available building types are:
Large building types:
Medium building types:
Small building types:
The three tiers of buildings
Each of the building types listed above can be upgraded twice.
To upgrade a building you will need to have all of your building spaces filled and pay the fee which may include materials and gold.
You will also need to obtain advanced building blueprints which will be available via quests, world drops and faction rewards.
Upon reaching level 2, some of your buildings will provide bonuses including
15% Follower Recovery Time
5% Passive Melee Buff
10% Travel Speed
15% Garrison Supplies
5% Passive Caster Buff
10% Ore Gathering
5% Herb Gathering bonus.
When your buildings reach level 3 some will yield even more benefits in the form of raid buffs. These buffs are designed to last 4 hours and persist through death.
Building interactions with other players
Your friends and guild members will be able to trade materials and visit your Garrison even if you are offline. You will need to add these people to your friends list in order for them to be allowed in.
Follower Bonuses and additional resources
How many followers can a garrison building provide? This will depend on building type/size but assigning more than one follower to a gathering type building will allow the building to produce more resources per hour. Resource building types can also yield additional resources if your followers have proficiency in that particular tradeskill.
Some buildings will allow you to choose a spec option when they have reached level 3. The choices you make will be reflected with minor changes in appearance and furniture layout. Screenshots of these changes are available on the individual building pages linked above, take a look.