Building a home office in your backyard is really hard. It's a good idea (though expensive) to go with a pre-built if you don't wanna figure all the details out. If you DIY the design and build, you have to figure out:

- What type of floor? (2x4? 2x6? will it hold me and my chair without collapsing?)

- What roof type? How big should the overhang be? What's easier to build yourself vs looks nicer but a real pain to build?

- How many windows should be in my office?

- How do I build something within my budget?

- What size of shed should I go with for my purpose? (workshop? office?)

I'm working on https://myhomecat.com - which lets you do all of this. If you enter your address and the kind of shed you want (i.e. 10x12 office), you should be able to get something fully code-compliant that figures out all of your details. It's still early so it doesn't cover every edge case, but the idea is there and it works really well from the feedback I've gotten.