If you want to dig terrain in an existing building I'd suggest raising terrain though existing supports first, that way any mistakes are less likely to fully remove support.

Originally posted by Khalysto:

Edit: After re-reading your post, it seems you want to install a basement under an existing home.. I do not advise this as the terrain can be a little unpredictable when you're digging it out. It's easy to compromise the foundation of the existing structure. You *can* make it work if you're careful, but I'd recommend building a new structure with the basement as a part of its initial design.

Can confirm, do not do this.
Unless you want to have the easiest way to disassemble your house.....

If you find an Elder Boss spawn location that is high enough in elevation you can dig under it and end up with a very spacious basement leading up to the top that you can use as a base.

I do spiral basement. that way you can still get support in the center.

Originally posted by pyremind:

I thought I would be so clever and put my portal and chests and money piles down in a basement... took me a while to realize I am horribly undermining my house structure.. and it doesn't seem like adding wood beams can help since I'm still extending the house off of the bedrock. How do you do basements?

Heya, basements are EZ PZ and you can do it with wood.

Step 1: Dig a hole, dig a diggie diggie hole. The dimensions of this hole MUST BE smaller than the footprint of your home. If your home is 5x5, the hole should be 4x4. Keep the building limitations of wood in mind as you consider the size of your home and basement. As has been mentioned, you can build larger homes / basements with better (stone / iron reinforced) materials, but you can do it with wood if you stay small to start.

Step 2: Should be a part of step one, but be sure you install ladders on the wall so you can get out of your diggie diggie hole. Consider putting this ladder where you want the actual entry point to your basement.

Step 3: "Pave over" the hole you made with wood flooring. I typically like using the 1m poles to start so the foundation is raised off of the ground, then I connect the 4, 1m poles, one for each corner (but sometimes I'll add 1m poles along the sides for longer buildings such as 5x8 or 5x10). Start at one corner and start placing your wooden flooring. Repeat this until your floor is installed.

VIOLA! you now have a basement. I'm happy to run through and demo the process if you'd like. Lemme know!

Edit: After re-reading your post, it seems you want to install a basement under an existing home.. I do not advise this as the terrain can be a little unpredictable when you're digging it out. It's easy to compromise the foundation of the existing structure. You *can* make it work if you're careful, but I'd recommend building a new structure with the basement as a part of its initial design.

design before you build :)
one way is to raise your terrain up in a (ring, square, etc) and use that as your grounding for the walls, roof/floor across it and go down into it, that is your basement.
another way is just to dig it out like you did, but you need to have planned for this so you can ensure your support is good for it.
basically, you can't do a basement under a home that uses the middle part of the ground in some ways as support. You have to do ALL your support off outer walls if you go that route.
Not using any middle support limits your total height a little bit. Less than you might think, but its still a few tiles lost when you push the loading out instead of straight down.

if you keep the walls close enough, your upward limitations are still on "ground", anchored into the "walls" of the basement. generally, the best solution is to use iron-reinforced wooden beams and go as high as you, realistically, want to.

With advanced materials beyond mere wood, you will eventually be able to construct structures with better support, like longer log beams, iron cages and beams, or stone and marble columns.