I want to build a waist high garden bin. Is 12" deep enough for vegs

it will be ok for lettuce and greens, radishes. maybe turnips and carrots. Not sure about squash, definitely not deep enough for tomatoes. I put toms in a 5 gallon bucket & bury the plant up to it's leaves and stake them.

Check online to see how to build a trug it has a deep V I think it is for growing veggies. 12" of soil is deep enough for veggies

If you want to plant tomatoes or potatoes you would need something deeper. You could plant them in a deeper container that is set onto your bin, sort of two planters in one. Many other herbs and vegetables would do fine as long as you didn't let them dry out. A 12" deep planter generally needs more water

Veggies like carrots and lettuce should be good for