First thing people need to know is your geographical location. That determines what you can grow.

IMO even more important is the actul size of the plot and what size ADU are you planning and do you have permission to do so /IMO that set of stairs is a death trap so maybe look at a multi level deck systemWhat space is in that shed addition ?

Yes, for design the location is not so important. But for plant choice it is.

Identify how you want to use the space including the ADU, outdoor dining or lounging, kids play areas, room for dog to play or do business, etc. FIgure out the size and location for the ADU. Then work on a design around that. Draw it out on paper and/or work with a landscape designer to plan for you. Then you can plan out hardscapes, trees, etc. You'll want to make sure you know where you want the ADU so you don't end up placing hardscape or expensive landscaping somewhere you end up having to rip it back out again for the ADU.

I can't believe there is no ground floor access to the back yard! That would put a damper on my eating outside, I'd be sure to trip carrying a load of salad down those stairs! The deck obviously near the house, I totally agree rip out the current one if you can afford it, and make something tiered. Invest in that, it will make the most difference!! At the lower level, maybe include space for a small outdoor kitchen (grill, big cooler, storage, etc.) so you don't have to go up and down the stairs as much. Also, keep the fire pit away from the tree and near a water outlet!! I recommend a small small fire space in an urban setting. They sell cool "smokeless" raised stainless fire pits and they are quite small too, great option.

https://www.solostove.com/ranger-plus-stand/

If you're just going to add onto the back of the house, then disregard the part about redoing the stairs. Just figure that out first, don't lock yourself in if you plan on adding but I too wondered about codes. Check on that too. Maybe just start with the fire pit, a grill, some lawn furniture, upgrade the turf, and see how you end up using the space before a major hardscape investment. If you long for something pretty to look at, get a few big pots of annuals for this year. Save the pots and fill with new stuff next year and add on from that . . .

Thanks so much for the comments! The add on is the laundry room and another full bathroom. The stair are actually up to code, there are reinforcements underneath and the pole was just replaced. Can’t afford the take the stairs down and do a deck right now, but would love to at some point. There is ground level access to the backyard, you come out the door of the add on, the picture doesn’t show it well.

We didn’t plan to build the ADU until next year, but maybe we should consider doing it first.

As already said, start with the ADU -- planning for it, at least. Site, size, utilities (water, electric, ??) -- and is it permitted by your city? Will you be sharing the deck, fire pit, and other outdoor spaces with the ADU occupant or is some privacy needed? Where will the ADU occupants park, how will they access their dwelling?