My friends living in Eugene, OR have one planted maybe 8 feet away from the northern wall of their house. It's been there for at least couple of decades and it took some pruning to prevent the tree from rubbing on the wall but the tree grows so slowly it has not been a problem. Also, their tree receives only an hour or two of direct sun so you could grow yours under maple as long as you stay on top of watering and feeding it as it will have to compete for these with an established tree nearby. And maples tend to be greedy.

I think you will be quite safe planting it 12' away from the house. Most established pseudocamellias I see around here are rather narrow in profile, spreading nowhere close to 30'. And as mentioned, they do grow slowly enough that should a conflict arise, there will be adequate time to foresee and make any minor pruning adjustments. But I doubt they will be necessary :-)

I would find this location superior to that of under the maple canopy. Maples are indeed greedy and it is inordinately difficult to establish other plants within their root zone and canopy. And the stewartia is best displayed as a specimen anyway.

Pointed top of stewartia won't look any good poking up into the canopy of a larger tree. And is likely to be deformed by its proximity.

Other than recognizing that Japanese stewartia is a specimen tree of some size and placing it accordingly, a key point in the PNW climate is to never let it dry out during hot weather.