Is there a way to tell if my attic can be used as livable space?
We're buying our first home, and at 1394sq ft it has a pretty massive attic area with a pull down ladder in the garage. I had considered building stairs in the garage, making a room over it into a man cave area, and making the rest of the attic more useable space than just rafters and insulation.
Is there a requirement for adding joists up there to support flooring, walls, and added weight of people? Or can I just put down a subfloor on the current joists and treat it like a normal floor?

Doubtful that you can use the existing "joists" as your floor joists. They are originally designed to hold a ceiling load and nothing else. Things to consider in converting your attic to living space.

1. Height - the obvious thing to look at first. At the very minimum you need 7' clear headroom with 7-1/2' in half the attic space for it to be considered living space. In taking a quick measurement for height you'll need to measure from what will be new 2x10 joists as well as 2x12 rafters. If you don't have enough height then either give up on the attic project or realize that it will be a larger project requiring the addition of dormers or raising the roof.

2. Framing (trusses) - If you attic framing is all engineered trusses (made from 2x4 or 2x6)then you have a lot of work to do to reframe. Not only will you need to add at least 2x10 alongside each bottom cord of each truss but you will also need to sister 2x12 along the top cord. Plus, since trusses are usually set 24" OC, you'll need to add extra framing between each existing truss. You will also need collar ties in place before you can cut out the center portions of each truss. A big word of caution here: THIS IS NOT A DIY JOB. While it can be done it's a structural modification that needs to be done by a pro.

3. Framing (stick built)- Depending on what you have there will determine how much framing you need to add. As a minimum, you will need 2x10 16" OC for your floor load and 2x12 16" OC for roof load and space for insulation. If your place already has 2x10 and 2x12 framing in the attic area, then you are well on your way. If not, then you have a lot of work to do.

4. Heating and cooling - You will need a separate unit or you'll need to modify your existing unit as a zone system. So there needs to be enough space for the unit, access for maintenance and ductwork. And of course, the several thousand dollars needed for H/A.

5. Access - You'll need stairs that fit into rise and run requirements as well as headroom.

All the above are things that can be determined before you start and before you commit to any money. If you are not versed in these areas than call in a contractor who does attic conversions. He could likely give you a go or no go answer after just a few minutes.

Thanks Jaybee for a great reply.
I've yet to even go up and look in the attic, but I have a picture from the home inspection of some uninsulated pipes running above the garage and the distance between them did look pretty wide for putting anything on them to support the weight of a person.
I had figured since I was going to be up there running CAT6 and probably cutting space for those pipes in the joists and installing a guard so I could lay down plywood for additional storage space I would see what could be do about making it useable space.

It's hard to tell from just the picture, but it doesn't look like normal 2x4' or 2x6's were used. They look bigger, but I'll take better measurements tomorrow.
I'm not so much worried about heating and cooling, I live in South Carolina, it doesn't get all that cold all that often here. Ventilation would be more what I would look for.
Access would be stairs in the garage going up essentially where the pull down is now.

I'm not committing to anything until I get a well thought out plan in my head on how it will proceed and what will be done, if it's even possible. I just wanted to find out what I should look for to know if it would be possible.

Well, like I mentioned, the first thing to check is just to see how much room you have there. Shallow pitched roofs (below 6/12) are poor candidates for attic conversions.

You can't discount the H/A needs. Attics are easy to heat as hot air from below rises. On the flip side, they are difficult to keep cool. That's the reason you need at least 12" rafters - to give you enough room for R-30 insulation. I forgot to mention in the other post: Take a look around to see if you have any existing ductwork already in the attic. Not a deal breaker but does ad an extra complication.

Whatever you do, be very cautious about adding a floor - even just for storage. In most cases the framing is designed to hold the ceiling load and not much else.

Well, this project is a total bust. The attic space is completely filled with the framing aside from a small space at the access point above the garage about the same size as a full mattress.
I'm pretty disappointed that we won't have the storage we thought we would and I know I'm gonna have to recruit a smaller friend to help me run my data cables because there's no way I'm fitting my 6' self through all that lumber up there.

Oh well, thanks again Jaybee. I'm sure I'll have more questions for you as we go along.