Best Of

Whatever else you do, unless for some reason you need two independent circuits out there, just run the bigger conduit and wire for 50 or 100 amps. The extra cost in comparison with the digging and electrician's time is trivial.

I would run maybe a 1" for the 20A you want now, run a second spare conduit for the 100A in the future.

If you run the 100A now, eliminate the 20A and just set a sub panel

We are working on an ADU in our detached shop, we have capacity for a second 100A for that building.

I'd contact an electrician for wire and conduit sizing for that distance.

Around here you need a permit, licensed electrician and inspection before the power company will do the service upgrade.

No need to run both, just run the gauge of wire for the highest project load and transition at a box (exterior or interior)

When I did the construction of an addition for my existing house with new service, I ran a one and a half inch line from house to garage in a 2 inch out to the connection point for the mains.

If I did it again, I would run a 3 inch out to the connection point for the mains. It was a pain pulling the aluminum supply lines through the 2 inch PVC with two long 90 in line.
Do yourself a favor and leave a couple pull strings inside the run to the garage

The inspector required metallic detectable tape to be installed above the lines.

for the cost difference in the larger PVC, why not leave the option for a 50 or 100A upgrade?

1-1/2” PVC would be my suggestion

I believe 18” burial, 12” under driveways is how the code reads

I put yellow caution tape in the trench a few inches above the conduit