Organic matter will break down as you add it, so it's not going to increase the elevation all that much. But it does not all go away, either.

Also, the process of improving soil by adding organic matter will usually lead to a more fluffy texture, so the soil will increase in volume over time.

I'm not sure exactly what you mean about it being 'ground level', but if you can handle an inch or two of 'fluff', you should be OK. I don't think you'd have to remove a foot.

Tilling that soil, and adding air, will raise the level of soil aas much as adding organic matter will. Add enough OM to that soil to make a difference more then likely will not raise it more than an inch or two so removing 12 inches would be a lot of unnecessary work. As the entrained air in the soil is removed the level of the soil will drop back to its previous level.
Many people do remove the sod that is there, unnecessary and removes the best soil you have, and that creates a depression, and that is a lot of work and then you need to add back more organic matter to replace that which you removed with the sod.
When I have enough time I cover the new planting bed with newspaper, cardboard can be used also, and cover that with a mulch (to help hold the paper in place and hide it) and wait for what was growing there to die, usually about 6 months. The beds I make that way are about level with the surrounding soil after the Soil Food Web digests the paper and mulch and whatever else was growing there.

How much money do you want to throw at this?

You would need to acquire about 300 cubic yards of organic matter to get a 12" layer to till into your soil. That's lots of dumptruck loads of it. Removing soil to avoid raising the soil level would require an equal amount of truckloads.

Raised planting beds are the easiest and most effective way to deal with difficult situations like this. You get a higher yield for less money and less work.

Make shallow planting boxes and dig about 6 inches of organic material (compost, bark fines, whatever you can get) into the top 6-12 inches of dirt.

Optimum bed width is 4-5 feet, so you can reach the middle from the pathway. Leave enough space between beds to maneuver a wheelbarrow. On the bright side, your paths are already graveled and ready to use.

This will confine the organic matter where you need it the most, where you will be planting.

Do a couple of beds a year, adding more organic material from your compost heap as you get it.

Plant deep-rooted plants, such as okra and squash, and when they die back, cut them off at ground level, leaving the root mass in the ground. It puts organic matter several feet into the ground as the roots decay.

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Use straw bales - straw bale gardening - and keep digging the decomposed bales into the dirt under the bed areas, along with any compost you have made.

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Or do a combination ... start with the straw bales and at the end of the season, use the bales in a raised bed as mulch.

If someone were to go out to a landscape supply house, or other seller of organic matter, it can become quite costly to make a good, healthy soil. However, for many people there is ample quantities of organic matter available every year where deciduous trees grow and drop their leaves, except most people think that is a waste product that must be gotten rid of as quickly as possible, so the next spring they can go to their local garden center and spend money on things like compost and peat moss.
Make, don't buy, compost with the waste you produce at home and utilize the free organic matter you can find.

The good news is that decomposed granite is loaded with nutrients.

So . . . can't he just add some organic matter and start planting? Decomposed granite is added to some of that expensive bagged stuff that people buy, so why wouldn't it be a good thing to have/plant in?

My garden is in the ground, but I hoe up raised beds that I don't walk on, and they end up higher than the pathways. They stay a couple inches higher than the pathways, because I layer manure and hay on them every year, and also because I walk on the pathways which lowers them by compacting them. All of it works fine. (There are no built raised beds -- just some of the dirt that I mounded up a bit into beds about 3 - 4 feet wide). Would this work for you? Or just skip the mounding and plant.

"decomposed granite" is basically gravel ... very little organic matter, very poor water retention, and many plants do not thrive in it.

Unless you are growing those plants that specialized to thrive in gravel beds, you need to make the soil more hospitable to them.

Where I worked we used man-made "decomposed graninte" i.e. crushed granite.

We used class 5, 2 and sand.

What size is yours?

We also used to unroll sod rolls on a granite drive-way to preserve them. They would root themselves in a matter of days.

If you are going to do an area that large, the entire area, get some one in their with a plow and plough it up, then put a few truck loads of dirt on it and plough it again.
I guarantee you, while it may not be the most pleasant soil to work with it will grow some items well.

You could also get some one in their with a skid-steer pile it up and sell to landscapers.
It is a thousand percent better for paving base than pit gravel.

I thought decomposed granite was a small step before clay. I was looking at a place some weeks ago here in mass, the topsoil is black and like kindergarten paste. The neighbor said he was told by the septic engineer that the area is decomposed granite.

Decomposed granite, like compost, comes in various stages of decomposition. It is everything from the fist-sized chunks that fall off the granite cliffs to the fine sand that fills the arroyos. You have to define it by particle size.

Absolutely. My family lives in Albuquerque and it's decomposed granite with a bit of limestone thrown in. The particles range from kitty-litter size down to silt. It locks up like concrete when it's compacted.

The OP might do well to consider moving the material around to make mounds, berms and trenches. Doing so he/she could create places to bring in soil for beds for particular types of gardening.

Although I haven't worked much with decomposed granite myself, I think that it will support many types of plants as it is. But those plants that need a different soil can be nurtured with imported clay soils.

In Alaska last year, I helped make some garden beds built up on a rock and gravel fill. The location was a beach that had been built up with a boulder retaining wall and backfilled. The 'soil' was very 'young', full of stuff like decomposed granite.

After making the raised beds (we used a chainsaw mill to make the boards from yellow cedar), we filled them with whatever soil we could scratch up. We screened the soil to remove the stones and used the stones between the beds for walking paths. Hard work.

Some vegetables did ok in that raw soil, and cold wet climate; lettuce and radish for example. We used kelp, sawdust, and humus from the forest as mulch and soil amendments. Very different than the clay back here in Boise.

I wonder what climate the OP lives in.

The best definition of what decomposed granite is is that it is granite that has weathered to the point that is easily fractures. Decomposed granite may well have a lot of nutrients locked in that rock that will not become available to any plant unless the Soil Food Web releases it and they cannot work at that unles there is a habitat, adequate levels of organic matter, they can live in.
Perhaps the link below may be of some help.

Here is a link that might be useful: The Soil Food Web Primer

Alaska raised bed garden, heavy on the DG, and mostly silt and humus. Onions, potatoes, radish, broccoli. The raspberries in the back are off to a slow start.

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