OK...Here goes. I added a basement under my garage which doubled my storage and workshop. In fact I moved my entire workshop down below. Here's how I did it. Since my garage was a 2 car model I decided to run an 8 inch block wall down the center. I started by making a cut in the floor just on the other side of the center line and removerd all the concrete from one side only. Next I began excavating the dirt down the center line. I did this all by hand by the way. When I got to within 6 feet of the east wall, the garage runs north and south with the door facing south. I dug a 4 foot trench to the east foundation. There I found out there was no foundation, the entryway and family room shared the same slab, so with the help of coworkers we excavated out the dirt and put in an 8 inch conctete block wall running the entire length of the east wall. We did this by laying up a 32 inch wall with a saw tooth and then excavated to the left and laid in the block we moved left to right until the entire wall was done. The tricky part was digging around the footings of the center wall, the house, and the south footing. A friend showed me how to dig out only enough to set in a footing with 2 8 inch block. We stacked these up saw tooth style and poured them full of concrete. Next we dug out next to the block and proceded to add block and footings as we went interlocking the saw tooth block. We made sure we only took out enough dirt to allow for 2 inches of rigid ins. behind the block. The footings along the door and west side as well as the interior wall poised a completely different problem and that was solved by the same process the difference was triming the foundation even with the block and laying up a new wall that when it got even with the foundation and exsiting block we drilled holes and inserted 5/8 rebar which we filled with concrete. This was done in a saw tooth fashion. Adding only a two block type wall each time. First we would do the right hand side and then the left hand side etc. As the basement took form we continued tying the new block to the old block which was 5 block deep into the ground. When we got to the top or the garage floor we laid in a half block so we could pour a 10 inch rebar floor. Since I had a wall running down the length of the barage we poured the floor in 2 pours. The wall had an eight foot opening with double 4 foot steel doors comerical grade. This allowed access between the two rooms that were created. The second pour wasn't as hard as the first because I put in a subterranean car lift that allowed me to store a third auto in the basement. I even excavated 10 feet under my driveway for additional storage and a storm shelter. The entire project took over 18 months. But the good thing about the entire project it was inside work and we only took out that dirt that allowed us to lay in block to support the existing foundation. My friend brought over his Heavy Const. book and showed me how the big boys do it and we followed their examples. The book is called Principles and Practices of Heavy Construction by Ronald Smith. There is a chapter addressing just such problems. Its called underpinning. But instead we decided to lay up a completely new wall and tie the existing wall to it. Once the concrete floor was laid the entire structure was tied together. The work was done during the evening hours with a trailer in the next bay that was filled with dirt. A friend needed the dirt so getting rid of it was easy. This is NOT a project for the faint hearted. It takes time and patience because you don't get a second chance if it all falls down. But with planning and slow steady work you can do it. My project costs about $13,000 but I had a lot of help and after it was done a few contractors said they wouldn't even consider the project. I did have to move my water line and a sewer line but as luck would have it they fit neatly behind the new block wall. The only outward appearence of the entire project was the PODS sitting in the driveway storing my STUFF while we worked. We stored the block, sand and mixer in the garage. I only purchased the amounts that would fit into the garage. I looked for sales too. This allowed us, me, to work 24/7 rain or shine. I now have a dust collecting system for the shop. No more dust in the house and my wife can get her SUV in the garage!!!!! The footing was 12 inch vee block laid sideways and filled with concrete and rebar with the ends sticking out to accept the next section of footing. This also allowed the new basement floor to be tied to the wall via the rebar. There are no cracks. YET.

Hopefully this works. This is what we are thinking of doing. 14 x 14 mudroom with raised 6x8 laundry (to take advantage of plumbing on wall) and 28x40 garage. Is it worth the extra trouble of figuring rooflines and frost etc. Thanks for any thoughts you might have. Here is a link that might be useful: renovation plan

Helpful responses here. OP, I just wanted to tell you that DH and I built our house, after it was framed, mostly on weekends, and DH installed all the board and batten siding on top of a laborious Fine Homebuilding suggested rain screen. I started out wanting super-contemporary minimal trim, then quit paying attention as I began painting. As a result, our windows have a similar eclectic trim profile which drives me crazy if I pay attention. So I don't. But I feel your pain. Good luck with your very cute house!

If you are planning for several kids, do not reduce the number of bedrooms. They can share one bathroom. Also plan for a playroom. Don’t plan for a guest suite unless you have family that visits often. And (maybe just me) a large master bed and bath seems unimportant vs your other priorities.