I should preface this post by saying that kitchen lay-out is my stronger suit but I had an idea for your bathroom. This plan assumes that you can move the waste line and plumbing lines for the toilet. The areas that would need privacy in a house of 2 boys and 2 girls are the shower/toilet area, IMO, so it made more sense to put these in a room together, not the toilet and vanity. This way one of your daughters could be at the sink while a son is in the shower. My brother and I had a bathroom like this growing up and it worked for us. Oh, and putting the toilet in the same room as the shower means less chance of siblings playing pranks on the one in the shower. ;-) If you install transom windows above the shower, door and vanity area, you'll get natural light into the shower/toilet area. You could also check into a frosted glass door for even more light into the shower area without giving up any privacy. I added a window seat below the window to give you more storage and to give that area a purpose. Besides, window seats are cute. ;-) If clearances will allow it, you might be able to lengthen the vanity up to 1', which would mean your vanity would be long enough for 2 sinks, making it even easier for your bathroom to serve 4 kids.

Garage/home walls have to be fire rated. It would function better if the garage was set apart from the main house with either a courtyard or breezeway between the two so that you could get more natural light into the house and still be able to utilize the visual connection between the two. A garage that needs to house two cars stacked on a lift will need to be taller than you are planning to account for not smashing the lighting on the ceiling. You might want to house something beyond lower height sports cars. Our 4 post lift in our garage won't allow a tall vehicle like a SUV on it and then raise to full height, and we vaulted our already 11' tall ceiling to 15' in the lift bay. Even with Corvette stacked on top of Corvette, it's really tight with the lighting when the lift is up enough to get that second vehicle under it. Only Formula cars without the wings would really work to have the double stack be comfortable at that height, and they have issues with the ramps, needing longer ones to get the ground clearance for the chassis. We also climate controlled our garage area, as that keeps the vehicles in much better shape, and it's a much more pleasant working environment. While you plan on having guests here, this isn't a very guest friendly house. There isn't enough room allotted for them, either in the public or private spaces. I might suggest the separate garage have a second floor guest suite(s) and make the main house more about your groundfloor space on one side, and the public space on the other, linking to the garage space visually through the breezeway. Double height space is very difficult to heat and cool, so I would suggest having hydronic radiant floor heating for winter comfort so that the people in the rooms are closer to the heating source.(We did that in our garage, so if you happen to need to be on a creeper under something, you don't have a giant cold mass of concrete sucking the warmth out of your body.) For summer, if you rarely get temperatures above 85, then I probably wouldn't do AC at all since you are in a low humidity climate. I would focus on doing some type of cupola with operable vent windows that will allow the hot air that rises to be vented externally. That's an old fashioned technique, but very effective in climates that don't heat and humidity to need full time AC. Overall, you really need a good architect to take these ideas and make them more functional and designed specifically for the site, to take advantage of it's positives. You will need that anyway for permitting, and in CA there are plenty of creative people who would love to be a part of an interesting build. Take a look at some back issues of Architectural Digest and see if you come across any designs that you like, Then contact a couple of those folks. This needs a large amount of technical expertise to wade through the red tape of building and permitting. So, you need someone local as a builder, well as an architect whose work you admire to work as a team to accomplish this build. Hire good people, and then listen to them. A good design is an interactive experience that makes the work better. Good design is "invisible" You don't even notice that things are where they need to be and work the way they "should". Bad design is obvious, especially if you live with it. It makes life harder. Don't make your life harder!

I understand the need for saving money on what you build. However keep in mind that a rectangular foundation and walls will be cheaper to build than a bump-out for stairs. That bump out will end up costing you more than adding as part of the normal rectangle. More expensive foundation wise, the tie in to the roof, more material to build u-turn steps etc. The plan posted by bungeii is a really good workable plan. As solie suggested you could remove the master closet use space from bedroom 2 for the master closet. Bedroom 3 will be pushed into bedroom 2. That would take off around 112 sq ft and make it close to 1300 sq ft. You would also be able to get a larger closet for the office/bedroom and it would be a good size to squeeze guests in too in addition to office space. Another thing to cut down on costs is to keep bathrooms, kitchen, and laundry close to each other as the plumbing is shorter to route. For the bath size - as you age it will be better to have space to be able to move around easily or if you'd need help with anything.

Jessica P, we are building your third house plan's bigger sister: https://www.architecturaldesigns.com/house-plans/country-home-plan-with-marvelous-porches-4122wm We felt that the extra 300 sq ft to make the room sizes slighty larger was worth the minimal cost increase, which is why we choose the larger plan. It has better size bedrooms, and a larger kitchen and living area. With this open floor plan, remember you need room for pathways between rooms which takes up usable space, and makes the dining room/living room become smaller. One major criticism of this house is the covered porches, however this was a selling point to me. We spend so much time outside, that we felt it was okay to lose some natural light for awesome outdoor spaces. If you have any questions about this plan, let me know!!