Just keep the legs and remove the overhead rails and put a block/lock at the top of the door so nobody opens it. If you decide to have a garage back down the road you attach the overhead track and the cables.

I agree. Remove the rails, leave the vertical rails, put a couple of screws in the garage door to secure it, then build a wall in front of the inside of the garage door, and will look like a regular room. Four walls for your office, while still giving the look of having a garage on the outside.

It's "almost" that simple except vertical rails always continue a 90* turn before horizontal rails connect. You will need to cut rails which destroy's possability of reuse.

All good advice, especially the walling off the door. The only other catch I can think of is if your floor is sloped will water head to the hall or office or if you are putting a subfloor in so your desk isn't crooked. And I want to say this is blasphemy - your man card is hereby revoked.

thanks to all! Special shout out to GN Builders who included pictures! Very helpful and clear.