Loft is open to living room/dining room downstairs, and there are big windows all across that side of the house, so you can enjoy the lovely view from the living room or the loft.
Do I need to worry that kitty might 1) try and jump up onto the loft's railing, miss, lose her balance and fall or 2) think she can go through the railings and jump down from loft to main floor?
Our current house has a landing that protrudes into the foyer; she eschews the last few steps and usually jumps up or down through the rails on the landing, but that's a ~3-foot drop, not a 12-foot one.
My parents' house has half a second floor ...hard to describe, but basically an open 2nd story hallway that goes through the middle of the house, w/ rooms off the hallway. Their adventurer kitty once tried to walk on top of the banister rail, but slipped and fell 15' to the tile floor. We were all quite concerned, but she was 100% fine except for perhaps being a bit embarrassed. She did not try walking up there again.
Both Adventurer Kitty and 'Prophecy' Kitty have jumped several times from the 10'-12' landing at my family's summer house. For Adventurer Kitty it is just a little leap, she's quite agile. We do have to keep an eye on Prophecy Kitty to discourage him from jumping, as he doesn't land on his feet. He's certainly a bit exceptional though, rather ...challenged in the art of being a cat. He has been known to fail in making the 2' jump onto the couch.
I personally wouldn't worry too much about your kitty. Most cats (though not all) have a pretty good eye for distance, as well as, of course, that knack for landing on their feet.
We have a loft style house with 3 cats. They all like to sit on the railing over the main floor, have yet to have one fall.
Though a friend of ours has a loft and cats and one night we were eating dinner at the table when a black streak landed with a WHUMP next to me... yep, cat jumped of the railing onto a hardwood floor. Cat was fine, a little dazed, but she is kind of an odd cat to begin with.
Cats are pretty good at self-preservation or they become "Darwinized."
I would not say my cat is the brightest crayon in the box, but she's not poo brown either.
So I shall try not to worry about her taking a header off the loft ... though she has tried leaps from the couch to the top of the dog crate and missed (a distance of about 4 feet). But that was a while ago.
As my ancient retired barn cat heads toward the end of her life, we've been telling stories, and we were just talking about this one last night. When I took her in, I was living in a teeny tiny converted farm building apartment that was one room with a loft over half of it, that you reached by climbing a metal ladder. I totally expected her to stay on the main floor, since there wasn't any good way into the loft other than the ladder. Wrong. She figured out how to go from the floor to the counter to the top of a wardrobe that was against the wall, and from there up into the loft. I didn't know she'd been doing it until I found a cat-sized circle of hair on one of my wool sweaters. Once she figured out how to get into the loft, it became her favorite place for as long as we lived in that house.
So, look hard at ways your kitties might discover to get up and down from the loft... they can be very creative when motivated!