First, contact a structural engineer and ask for his evaluation about converting your attic to living space. You want to make sure your 1920 home can handle the additional load.

We are very nearly at the end of completely gutting and remodeling a 1/2 story in our 1913 bungalow. It was finished living space before we started, but because we were contemplating expanding an existing shed dormer (making it much larger) and opening up the ceiling to make it a "cathedral" ceiling, we did consult a structural engineer. This was very necessary. We had to add a structural beam to the center of the roof in the main living space and we had to leave some cross members between some of the rafters. But we managed to get the two big things we wanted - the much larger dormer and opening up the ceiling - and it has made the space so open, airy and sunny!

Try to plan what other work on the house you can tackle while you are working on the attic. Does the roof need to be replaced? Maybe you should do that first? Will the work in the attic open up the ceiling joists from the floor below? Is there electrical to downstairs light fixtures, etc. that could be replaced while you are working upstairs? Or do you need to add insulation to that ceiling before you floor the attic? Is your home balloon framed? If so, can you run wiring or plumbing up the walls from the basement? (Oh my, how these projects do tend to grow!)

We did a lot of the work ourselves, but you do have to know your limits. For example, we knew we needed a qualified electrician because we needed to replace all of the electrical. We tried to hang drywall ourselves and it was a job that was too big for us and we eventually contracted that out. The drywalling pros really "wowed" us, after the experience of trying it ourselves!

We did all of the demo ourselves. We did the floorplan. My DH has a degree in "industrial arts", even though his profession is something far removed from that, so he understands construction, design and is handy. We did use a GC for some of the bigger parts of the project, mainly because he could manage the subs (and back then, subs were hard to get). We did the interior wall insulation ourselves. We have done all of the finish trim/woodwork with the help of our BIL. What you can do yourselves will definitely save you some money.

Also, be careful about what permits your city may require. Often homeowners try to avoid those, and we certainly dreaded paying the fees... but the city inspector actually helped us understand that the insulation we were going to use between the roof rafters didn't meet our code requirements and we could have ended up with a mold problem. Also, if you plan to sell the house and the work required a permit but you did not get one, you could end up with some problems. So, it is best to look into the permitting requirements.

Keep in mind that DIY is in "your spare time"! It took us several years (interrupted by other home renovation projects that took precedence and took our money - like the roof and the kitchen) before the upstairs project arrived where it is now... but we are finally almost at the end! If you can manage it, it can give you lots more living space.