Welcome to Cloudy Nights.
Your original post stated you are looking to build a "roll-off" observatory. Does that mean you want to be able to roll the entire building away from your pier/ mount/ scope? I don't know of anyone who makes a kit for something like that. However, the roll-off observatories I've seen are typically small, so it wouldn't be hard to build one from scratch, or from a pre-fabricated garden shed. You'd just have to figure out what kind of tracks you want your building to roll on, and how to tie it down so it stays put when you're not using it. Sometimes I wish I had built something like that. You can do a search in these forums, or on the internet, for "outhouse observatory".
If you want a "roll-off roof" observatory, where the roof rolls and the rest of the building stays put, the others have provided you with several ideas. Several people I know have hired Backyard Observatories and I can say they do good work and are quite fast. I've heard they are also willing to work with you to customize one of their stock plans so you get exactly what you want. The people I know had contractors pour their slabs, then BYO arrived at the site with all the materials and tools required. Two or three days later BYO left, and my friends had functioning observatories, didn't even have to buy batteries.