Fori, my parents just bought an Eichler on the Peninsula as their retirement home project. It's mahogalicious! In fact, they too have the kitchen opening directly to the right as you enter the front door. Paneling as far as the eye can see. My mom is despairing because the paneling darkens the place but she can't/won't paint it out of historical respect. I agree on the virtues of a smaller more private entranceway as a place to take off one's hat, check lipstick and correct slip peekage before tromping straight into the living room. Ideal if you have room for a coatrack, shelf for purses, and a mirror. We have a tiny little 5x5 vestibule with a second door and love it. Besides being great for stopping thermal loss, it's the perfect cat airlock. People do forget that in sunny climes, you really don't need-capital-N a mudroom for dirty boots & dripping coats. But you could use my plan above and add a nice big coat closet or mudroom with shoe bench to the right of the door, either only on one side, or across the whole shebang, giving you another doorway between hall and office/laundry room. This would also be swell if you go shoeless in the house. My Japanese friends have special shoe racks with guest slippers in that very spot. With that said, I'm off to shovel the 10" of new snow we just got last night here in Maine. My outlaws are arriving today for a long visit, 'nuff said.

You definitely need an engineer to design the modification for you. The way they did it in the link, by basically bolting ledger boards on 3 sides, would only be appropriate if the 2 side walls are load bearing. If they just attached the ledgers to joists, they could get some serious sag down the road. It can be as simple as the link, or some of the drywall might need to be removed to provide more support for the new floor. Costs vary so much regionally, that it's hard for anyone to give you an estimate. And even a local contractor wouldn't be able to estimate price unless he/she knew what structural changes needed to be made. So I'd probably start by talking with a contractor or 2, as they could give you some sort of wild ballpark. If it's in the range of your budget, then hire a structural engineer to design it, and then bid the fully designed project to several contractors. As for the window, if it's that low to the (new) floor, it should be replaced by one with tempered glass. That's code.

OK it makes more sense now seeing the other rooms. I like the idea of moving the bookcases to the living room as you mentioned. Have you considered a tall slender storage cabinet for the solid wall there in the play area? Some of the toys could be behind the doors. And you will still have room for a chaise or whatever you choose. I have several of those types of cabinets they are not very expensive.

The house style is Gothic Revival, a Romantic style of the mid 19th century, with gable cross bracing instead of the more common decorative verge boards. Cross bracing later became a popular feature of the Stick and Queen Anne Victorian styles. The internet plan sites tend to classify this style by whatever names are popular. From "Cottage Resiednces" by Andrew Jackson Downing 1842