How much head height do you actually have? There are certain minimums that are required by code. How tall is the dormer space? You do realize that even if you have 7' of head height, it won't be that tall after doing the required finishing steps? How do you know that your attic was built strong enough for live loading? That is not at all standard in construction. It's an upgrade. Is it stick built or trusses? That also makes a huge difference in whether or not you can alter the roof supports without the house falling down. In addition to your attic joists, your first floor framing needs to be strong enough to handle the weight of a second story live load. The foundation also needs to have been engineered with a second story in mind from the beginning. You are going to need a structural engineer's inspection and report to even know if what you want is feasable to do without basically demolishing the house and starting over. The garage is at least a simple enough of a structure that if it needs ro be taken down and rebuilt you won't be talking about anything too complex.

Option B will certainly be cheaper because most attics are only built to accept storage items, not the weight of a living area. You'd have to reinforce the attic joists in order to meet structural requirements, and that would intrude into your headroom. Also, even if you did plan from the beginning and installed larger joists, unless you have around 9' of ceiling height, you won't have room for a living space and the required insulation. You'd have a lot more construction complexity with this choice. Option B will also need all of the above mentioned alterations, plus additional insulation in the garage ceiling, but it would undoubtedly be cheaper than option A. However any of these projects will require a lot of work from licenses trades to accomplish. Creating bathrooms close to an existing plumbing stack does help with some of the plumbing costs, but we're talking the difference between 15K and 12K for your plumbing bid. In the overall numbers of the project, 3K isn't a lot of difference on a 100K project in order to have a bathroom where it works best for layout functionality. Option C would be to take the 100K that the project would cost and add that to the amount you could realistically sell your home for and go shopping for an existing home with the additional space you need.

make sure you nail or screw them down. my stepdad put flooring in his attic and did not nail any down. it was only where stuff was to be sitting, not for walking on. well, after moving stuff around and throwing out a bunch, the floor is now walking area. last time i was up there one of the boards went out from under my foot and i ALMOST went thru the ceiling. the boards are now nailed down. in many areas we doubled up 1/4" and it is fine. any more than a 16" spread needs to be 3/4".

Well, the first question has to be...wait for it...where will your basement be? Will it follow the perimeter of your first floor? Will it be something more, or something less? It's simply impossible to take an off-the-shelf floor plan, without basement, and say we want to put a basement under the house and where should the stairs be located? You might as well ask where should the stairs be to the second floor? All levels of a house have to be developed together and in synch, just as the outside and the inside have to be developed together and in synch. So to add basement and stairs, there will have to be some considerable revision to the first floor. Do you see? What will be in the basement? How do those spaces relate to the first floor spaces and circulation? Do you want the stairs to the basement to be central and visible? Or out of sight and in some less frequently used part of the house? So many options, depending on your choices.