Every time you teleport to your house, you will get a small bar where you can choose various tools for customization. You must press the fifth button (look at the screenshot above) to enable editing.
After pressing the first button, the property layout screen pops up. You can notice 7 plots where you can build something. The central plot is for the house (you can choose type of the house there, although they are race and level restricted).
Surrounding plots are used to build various objects. Most of them require FabKits, obtained through path missions, reputation and regular vendors and random world/dungeon drops. You can build objects that start challenges or mini-dungeons (you can do them with your friends even); you can build portals to different zones (those FabKits can be bought at zone reputation vendors), yards where you can farm crafting resources, the crafting station, etc… You can upgrade some of them (FabKits are required for that too, for example, Festival tier 1 to Festival tier 2 requires Festival tier 2 FabKit).
From time to time you will have to repair those buildings/objects, before you could start using them again. Also, and this is important: if you build some other object over another, you won’t get the FabKit from the demolished building; if you want to build that object again, you will have to obtain its FabKit. Decor items can be planted outside, anywhere you want.