I suspect the septic field is going to be the biggest problem; the field will take X square feet (X is dependent on perkability), plus more feet distance away from any dwellings, which may not be many but there aren't many feet in any direction on that lot. BTW, if you are willing to get serious about the environment, AND educate the local officials, it **MAY** be possible to reduce the required size of the septic field by using electric toilets and a greywater system. The practibility of a garage is dependent on your wallet and the soil type. The more you are willing to spend really big bucks, the less important it will be as to how many boulders will have to be removed to make space for a garage. OTOH, no one will know just how expensive earthmoving is/will be until the job has been completed. This is the one and only item in housebuilding that has an estimated cost entirely dependent upon guessing about unforeseeable events (boulders, springs, marine clay, etc.). [The next worst guesstimate will be for a well, but at least wells have a few guidelines based upon neighboring wells.]

Personally, I think you can never go too high in garage ceilings, so go high - you can always store stuff where you don't need the height. I can't really offer too much on the other things - is it worth the extra expense to YOU to go l-shaped? What's going to suit your requirements best? I can't imagine asphalt as the way to go for a garage floor, as if you're doing retaining walls and filling, the floor will require some structural integrity, plus you will want something dimensionally stable - something that won't move with time, and asphalt will, you will in fact notice that there will be depressions from the wheels etc, and heavy stuff will sink in...asphalt is in fact a liquid at most temperatures, it's just a very thick one.... I wouldn't worry too much about finding plans online or wherever for an L-shaped garage, a situation like yours is unique and anything will need to be custom designed to suit. If you're using a home improvement contractor, if they're going to be designing it, (you'll need to submit plans to your local council) see what ideas and suggestions they may have, they may suggest things you mightn't have considered, in other words, don't "pre-design" it too far in advance - if you're planning to do the work yourselves, then you need to research as much as possible. I guess the most important thing is to work out how much you want to spend, as it will make a lot of the decisions for you...

I would suggest a small scaled sofa (scaled for the space) on the long wall, and float the tv. https://www.wayfair.com/Williston-Forge--Solid-Wood-TV-Stand-for-TVs-up-to-55-X111677938-L6-K~W000566827.html?refid=GX444236625790-W000566827_824255789&device=m&ptid=902647982793&network=g&targetid=aud-354999624471:pla-902647982793&channel=GooglePLA&ireid=75415605&fdid=1817&PiID%5B%5D=824255789&gclid=EAIaIQobChMI_4mDnoq76gIVBMDICh31uQ7SEAQYBSABEgKEuPD_BwE https://www.wayfair.com/furniture/pdp/ebern-designs-jankowski-tv-stand-for-tvs-up-to-75-tdi10000.html?piid= https://www.potterybarn.com/m/products/williams-media-console/?gclid=EAIaIQobChMI34TcpY276gIVi5yzCh3mBg9eEAQYASABEgJQ6PD_BwE&catalogId=84&cm_ven=PLA&cm_ite=4204561&sku=4204561&cm_pla=Furniture+%3E+TV+Stands+%26+Media&cm_cat=Google https://www.overstock.com/Home-Garden/Furniture-of-America-Royce-Industrial-60-inch-TV-Stand/9914811/product.html?TID=SUI:FP_DETAILS

@mama goose_gw zn6OH Thank you for your thoughtful feedback. I was definitely considering the dishwasher swap, with dishes in drawers instead of upper cabinets. There's two reasons I'm hesitant. 1) I have never been a big fan of dishes in drawers. Getting dishes is a frequent activity, so having to bend down each time seems like it could get painful on the back. I could be totally wrong on this. 2) Our typical routine for weeknight dinner involves brining clean plates to the stove for loading and then taking to the island to eat. Setting the table for more "formal" dinners usually involves using the "good" dishes, which are stored elsewhere anyways. (The plan doesn't show it, but there are uppers on either side of stove and dishes are planned to be on the left.) Trash drawers are going to go under both main and prep sink. There is definitely room to make the island wider than the standard 4 feet and hence move it a little closer to stove. My concern there is that it would require a seam in the counter material (quartzite). Maybe a skilled installer could make that almost disappear. I'm just wondering how much that would bother me. I should have provided more context on the bank of floor-to-ceiling cabinets. The design is below. The middle section is indented with a counter ledge and has the microwave and coffee/breakfast stations behind the appliance garages. This is why the fridge can move to one of the two ends, but not the middle. If I'm adding the island prep sink, then I'm actually happy with the fridge at the end because it means I can just pivot to pull things from fridge and put on island. I had originally wanted to keep the island completely appliance free, but now realize the prep sink there is a must given the rest of the configuration.