Root Cellaring by Mike & Nancy Bubel. Best comprehensive book ever about root cellars. Includes plans for building your own root cellar, places in your house that could also work for vegetables, recipes, length of time most vegetables keep, optimum humidity and temperatures for vegetables and more. I own this book and found it to be great. Not expensive either. Check it out.

http://www.amazon.ca/Root-Cellaring-Natural-Storage-Vegetables/dp/0882667033/ref=sr_1_1?ie=UTF8&qid=1385599901&sr=8-1&keywords=root+cellaring

I agree - the Bubel book is the best. Also check out this article in Mother Earth News: http://www.motherearthnews.com/diy/basement-root-cellar-zm0z04zsie.aspx#axzz2pLx0r0QJ

In Ontario, typically they are poured concrete. Get at least 4' or 1.2m deep to get below the frost line, a good damproofing membrane on the exterior, and even some rigid insulation would be good along the top 2' perimeter. A concrete cap would also be good and the cellar can double as a hardened shelter against tornado's, etc. This is what we have and it works great.

Do you need details on walls, footings, rebar, etc? They are fairly common in Ontario and Canadian Climates but I can scrounge up some common details if you need them.

The sizing of your house may determine the practical sizing of an assigned space in your basement.

I've done 3 root cellars, the last 2 virtually the same.
One can choose to abut one bare basement wall ,or build into a corner abutting 2 walls. The latter subjects the r.cellar to colder temps unless all walls are insulated. I would only use highdensity foil-clad foam insulation,(with wood stud frame on the outside of the inner cellar foam layer) as that is least mold-supporting and is effectively a vapor barrier.The ceiling should also be the same material.
In Southern Ontario, i chose the corner which was close to a window just 6" from ground level, as it was large enough to transfer boxes of veggies thru to the r.cellar. To the window's left , on the inside basement, was the cellar entrance door, and to its right was a sturdy bench for sorting veg. before putting in r.cellar.
In this case the dimensions of the r cellar was about 8x8 inside, with the 36" door in the middle of the wall, opening towards the rest of the basement and so that you enter the middle of a U-shaped shelving configuration, with a 2'deep shelf depth at the lowest level, about 2' from the floor...for the veggies appropriate for that level of temp/humidity average, and good full-box-handling height.Then 16" higher, a 16"deep shelf all round the U, too.Then above that a 12" shelf, all round....mainly for squash if not too cold/damp....otherwise they were under the beds.
On the floor,under the 2'shelving, i had 6 rolling platforms about 2x2 and 2x3 with HD casters, for containers, wood crates of leeks,celeriac, Rutabagas, cabbage, beets, and for carrots in sand.
There were no supporting posts on the floor nor in the shelves' corners, but all free-supported wall-to-wall....for easy shifting.
The shelving was 2x4 frame with 2x4 shelftops with gaps for thorough circulation.
If the house has good perimeter drainage and is radon free, i would cut a square-foot hole in the concrete exposing the subsoil for likely continuous ambient moisture.
Otherwise, keep a watering can sprinkler handy and douse the floor as needed.
Always have 2 vents installed 3-4" piping, with the plastic slidegates inline to regulate optimum temp/humidity.The inlet pipe should go direct to the floor and not drift down along the wall. The other vent at the ceiling level should allow the slightly warmer air to exit. Both should be shielded on the outside from prevailing winds and account for unforeseen snow levels.
This base size can be extended length -wise, but then the shelving would need to be structurally altered/sized.
I used cedar.
Keep a humidity gauge inside and a min-max thermometer so you can track/record your particular atmospheric conditions.
Don't forget a central light fixture with a directional feature, and maybe an infrared socket in case the temp needs to be quickly adjusted.
Use a preframed Insulated steel entrance door with magnetic/rubber seal
That's off the top of my head's memory for now.
Inside house is my preference just for convenience in a prolonged storm/outage, etc.rather than an earthbermed detached unit unless right close to garden and washing/prep facilities. Nice to have both, for volume and disguise/cache.
Another one was under the ramp off a bank-barn...huge!

In Ontario, typically they are poured concrete. Get at least 4' or 1.2m deep to get below the frost line, a good damproofing membrane on the exterior, and even some rigid insulation would be good along the top 2' perimeter. A concrete cap would also be good and the cellar can double as a hardened shelter against tornado's, etc. This is what we have and it works great.

Do you need details on walls, footings, rebar, etc? They are fairly common in Ontario and Canadian Climates but I can scrounge up some common details if you need them.

Do you have a photo of this root celler / possible tornado shelter ?