Pick out half a dozen or so designs that you like. Give them to her and ask which are the best 3 she likes. In the conversation (over a few weeks - no rush) discuss colour options, cost of plumbing, light and power, plus a concrete driveway. Throw in a water tank for rain harvesting to make it enviro friendly.
Get her to commit on these items and the value. Then wait a while and see if she starts to waiver on the shed idea. If she doesn't get cold feet, then she is up to something. Like maybe she wants the storage space, or is thinking about running a business from home, or a granny flat for the outlaws, etc...
20 years, I bet you have built up some equity on your home. It's not the shed, its how easily are you persuaded to increase your debt and start something new.
Wait a while and it will all flush out....eventually she will let you in on her grand plan.
Hmmm....if you build a shed does that mean the cars go in the shed and the garage on the house gets converted into a.......new kitchen, dining room, TV room, something or other that one of her friends has done to their home? Seen a few too many house and kitchen magazines lying around recently?
I've got had a 5 bay shed. I am now reduced to 2 bays for me and 3 for the wife. She bought a new horse trailer, and I have to build an extension to the shed to fit it in. 5 becomes 6 and I still only get to use 2 !
Be very very careful......
And congratulations on 20 years.