If total cost is $110, then it would be about $20 to finish otherwise framed space. It is similar to finishing an unfinished basement if this area was planned as a "finishable" space. I am assuming that roofline is there and stairs are already there, and subfloor is already there (and designed for load). The real cost variable is HVAC - if you finish it later, then your HVAC won't be sized right and you may need a unit just for that space which can add a big chunk. Rough costs per sqft electrical $4 Paint $2 Drywall $2 Insulation $2 (mild climate) plumbing $0 - assuming no bathroom HVAC $4 Carpet $2 Trim $2 (includes door) Builder $2 (low to make numbers work...) HVAC is quite variable - it can be less if you are just adding a duct or two and not changing the size of the original unit. The comfort of a bonus room is climate and insulation and HVAC design dependent. Done terribly much of the time. I have a bedroom in a "bonus" area and it is just fine without spray foam. It also only has 1 duct and no return but it isn't 900 sqft either. The walkin closet in that area is absolutely frigid with no duct.

Do you plan to use the 3rd bay in the garage for every day parking or is it more for storage or for a car with more occasional use? If the latter you could consider doing a tandem garage on one side. If you want a front facing garage I would try to set it back some so it isn't the thing you see sticking out first. Although frankly when you are on wider lots or acreage you often see the side facing garage just as much if not even more than the front facing ones so I don't think in those cases that a side facing does nearly the same for a house as it does on a narrower lot, where it doesn't become as visible from the street. Our neighborhood have many side facing garages including ours, and I see those garages and sides of houses a whole lot more than I see the front of the houses as I'm driving by or even as I'm out running I see the sides more than the front. I actually wish I had thought about that and changed it to a front facing that was set back a little instead as it would have worked better with house access from the drive way etc. Only the houses that are placed along a curved part of the road do I see less of the garage. One has it on the side, but in the way back and still attached and that one you do see a little less of it than the majority that have them to the front side.

I hear you Pam. I'm in an existing house also, single pane wood windows, decent insulation. with my 15 SEER heat pump my electric bill hasn't exceeded $60 since install in June of 2012 install. My sister's utility bill is always $150 plus 13 seer with elec strip, double paned windows better insulation packags. My house is tight, hers is fairly leaky. air sealing & heat pump/higher seer its a good fit, for me. the $800 "savings" that my sister chose has cost her every month...and until unit is replaced..will continue to cost. savings in her situation wasn't savings at all. and when she replaces...her DH will insist on cheapest...again. it is all in understanding what the investment returns. savings & comfort was my goal, not cheaper up front. the difference in August sept in RH & coolness of our houses are day & night. with a variable speed air handler my Rh stays 55%. getting it lower..and air COLD (wonderful in hot summer) requires the whole house dehumidifier. I designed my system, sized ducts & ductwork inside living space to remove RH, the aprilaire was a great deal that I couldn't pass up...but I have to confess I only used it in august/sept. otherwise 55% RH is ok with me. and the $40 winter bill...wonderful!! best of luck.

Not all new construction homes in Arizona are built with side entry doors anymore. I don't quite understand it and seems like a potential fire trap. What if the garage caught fire with someone in it and the door entering the house was locked? Sure, you might be able to remotely open the overhead if you could find the remote in time or manually pull it open, but how long could that take? Not everyone is capable of lifting up those heavy doors. I see many homes in the community where I live that don't have side doors. Buyers usually don't notice until they move in and go to take the trash out (usually through the garage).