Build a standalone root cellar or dirt floor basement?

I'm going to be adding on to my house in the next few years, about 16x24 space.

I also want to build a root cellar.

I've read the Root Cellaring book several times and can't decide what would be better - to build the addition with a root cellar as the basement (Concrete sides, dirt floor)? Or to just build the addition on a regular slab like the rest of my house and build a root cellar elsewhere.

I do have some bermed-ish areas close to my house where I could dig in to do a root cellar
Cost is definitely a factor here
I don't think water table will be an issue although no guarantees until I dig, of course
Not sure if the moisture from the root cellar would be an issue? I would insulate the crap out of the ceiling of it so that the cold temps wouldn't affect it.
I do plan on building an RMH in the addition eventually, I'm not sure if having a basement would pose problems with that or not.

Would the basement be a walk-out?  If so, I'd probably put it in the basement.  Lugging dirty carrots through the house, down the stairs and into a cellar isn't fun.  But lugging them through a walk-out basement door and then 10' through a more utilitarian basement space to a root cellar is fine.  And more convenient for food retrieval.

If it's in the basement, you'll probably struggle to keep it cool and damp enough.  Dirt floor will really help with the dampness.  Insulate the snot out of the sides that meet the basement and the ceiling and vapor barrier them as well.  That should keep the moisture out of the house.  Consider not insulating the exterior foundation around the root cellar (if allowed by code?) to allow more cold to get into the root cellar.  I think there's an ideal amount of exposed foundation above grade for each cellar based on winter temps.  Mine has about a foot of exposed foundation wall between the cellar and the -10F outside air in winter.  My block wall foundation likely isn't insulated on the outside like they do now. That combination is about perfect for my climate.  In a warmer climate, more exposure could help you keep the cellar cold.  

As Mike said, if you can make a door to access it, I would go with your dirt floor basement idea.  It seems an unnecessary time and money expenditure to build a separate root cellar if you are adding on anyway.  I would add some holes in the exterior wall for PVC pipes so you can control ventilation and temperature more easily.

Root cellar with outside and inside access. Bring in your veggies through the outside entrance, and go downstairs on cold winter days without having to put your coat and boots on and shovel out your root cellar. Insulate well on interior door/walls and ceiling.  North side of the house if your can. Sand on top of earth might be something to consider for your floor. Easy to add moisture without making mud.

Building a RMH on top of your root cellar - remember to account for the concentrated weight of all that mass and build extra support under that part of the floor.

Lugging carrots through the house, down the stairs and into a cellar definitely isn't fun. I'm in the process of doing it right now... same thing for potatoes, beets, turnips, leeks, cabbages, chicories ... I so wish there was an outside access to the cellar! Inside access is a good idea too, but if I could only have one access I'd go for the outside access.

One thing to consider: will rodents get into the cellar? They always manage to get into ours. We store everything in rodent-proof boxes, so they can't eat anything, but they make a mess with their droppings. Our cellar has concrete floor which makes it easier to clean. I don't know how I could clean the droppings if the floor was dirt. Even without the rodent problems I think I'd want to clean the cellar thoroughly once a year, before bringing in new veggies in the fall, and a tile/ concrete floor makes it easier, or so I believe. Lack of humidity is no problem in our house (quite the contrary).