I think it is a matter of personal preference. I have my only sink and a dw in my island and I specifically requested that our island be designed for one level. I wanted the benefit of more uninterrupted flat surface. The island is relatively large and I still wanted to maximize the amount of work area on one flat surface. I also have an open floor plan where my island faces into my great room which is one reason many people prefer the visual buffer of a two level island. I am not a particularly neat person but after two years of living with it I am still happy with my decision. I am however, more inclined to keep my kitchen tidier now. I'm not sure that is necessarily a bad thing. If it makes a difference, one basin of my sink is pretty deep and dirty dishes can be stacked up in between dw loads and they are pretty inconspicuous.

There is no pat answer for your problem but a good place to begain is by asking if down stairs was warming well in the past. If so make sure (A) All elements/burners are operating (B) Both return and supply ducts are not leaking (C) Cooling coils are not dirty restricting air flow. After that,it's probably time to call in the pros unless you are exceptionally mechanicaly talented. If you are indeed talented and have tools we can walk you through the testing process.

The tile is the stuff that needs the THICKER subfloor. Is the tile in good shape? Did you see cracking of grout lines or popping tiles? These are all signs a subfloor is moving too much for the tile to remain properly bonded to the substrate. Wood is more forgiving - especially if you are running the wood PERPENDICULAR to the joists. So long as the wood is running perp. to the joists you *should be fine. A thicker subfloor is nice, but may not be necessary. You have the joy of having plywood (not OSB) as you substrate. It is probably doing all the work needed. What is the flooring on the rest of the level? Is it carpet? When it comes to OSB, you can have great OSB and you can have sh!tty OSB. For that reason I like the safe option of using plywood (although some call this overkill, I like plywood's ability to grab nails/cleats/staples etc). It is also very easy to GLUE to. Some OSB products are TOUGH to get glue to grab on to. And a wide plank oak (anything 4" or wider) will require glue assist (glued down to subfloor) as well as mechanical fixtures (nails/staples/cleats). Once you are down to the 1/2" plywood, are there any floor height-changes that you need to be aware of? How about door clearance (internal and external doors)? So long as you have all the clearance you need, the plywood is a nice option throughout the home. The cost of plywood for the whole home may be part of the whole decision. A nice plywood runs about $1/sf. Installation is another $1 - $2/sf for labour. With glue-assisted installs (or any permanent floor in general) it is nice to have a layer of subfloor. That way if future removal of the glued down product is needed, it is easier to detach the top layer of plywood from the bottom layer of plywood. It means the demolition does NOT get down to the joists (that's tough to work around....12" wide HOLES between the joists with nothing but 'air' between you and the ground below).

The fifth pic of Beth H. : -- the full width double-decker porch in white would make really good first impression if/since this is the front side of the house. Consider a full width screened porch for the back side of the house -- a shed roof with hip corners. https://www.pinterest.com/pin/528398968759773936/ If you changed the upstairs door to a window, you could do the same on the front of the house as well. If , however, you are on a tight budget, a portico strong enough to serve as a balcony/emergency exit for the upstairs door or window (if you change it out) that begins at the left side of the upstairs door and ends at the right side of the downstairs door could suffice https://www.pinterest.com/pin/528398968759773936/