If your home is over 30 years old, settlement against the house probably isn't an issue anymore. I'd dig down to whatever your local frost depth is, pour a footer, and build up from there. Can't see a need to pin the footer to the house unless there's a local condition that dictates it.

Thanks oaktown. These are all great considerations. We sat down with the landscape architect originally to discuss all of these things (well most of them at least, but some things you bring up are new). He was suppose to take them all into account in planning this but to me, the plan just doesn't seem that thought out. This is the plan without the trees and plants shown on. He adds that after we confirm the hardscape (which I guess is the cement areas, decks, retaining walls etc.) Right now the land is clear so there is nothing to work around (attaching a picture taking from the back left). The basement is a walkout so anyone can access the house from the lower patio through there. The deck is on the first level of the house an the stairs would lead to the elevated area of the yard (so basically it is only 3 steps to reach land) which is level with the first floor. To get to the under-deck area from there you would take the steps through the retaining wall (which he forgot to add....I will sketch and attach below). That area is at the level of the basement. I actually wanted to adda slide from the deck to the patio area for fun but we weren't planning a large staircase. The retaining walls die into the land and beyond them, the yard will be sloped for lawn mower access/rolling/winter sledding. We just moved from an apartment in a very urban area so we have never had a yard and aren't really sure how we would use it! We have young kids so we want it to grow with them. The driveway goes straight to the road, it't not too long. I will attach the main floor plan as well. The sliding door in the kitchen and the mudroom door both connect to the deck. Sorry if this is still unclear. In the picture below, the wooden board across the back of the house shows where the deck will go. The land still has to be graded so that it will be flat below that, then there will be 2, 4ft retaining walls.

at 16x20 you will be able to do it with a single deck board (we have to use 2 for ours). We have ours at 16'x28' (usable 14 x 28) but this is off our main level so we will have a grill space, seating space & maybe dining space (although I hear the bees are bad so we may not do that). I found this article helpful when planning ours https://decksbye3.com/how-to-figure-out-the-right-size-deck-for-lounging-dining-or-grilling/ Regarding stair landings and safety...kids are young for a blink of an eye. We have straight stairs inside our house as well and it wasn't a consideration of whether our kid would fall down them when we bought the house. Sure, it might help in the "just in case" scenario that there's a shorter run of stairs, but I wouldn't force it into the design. As I said, I considered adding a landing as well - not really due to falling but more for aesthetics. I think landings look nicer. If it had taken up less of our yard to do it, I would have. But ultimately I wasn't going to pay more for a landing when it doesn't do much other than make things marginally safer. Plus the straight stair will be a LOT more fun to sled down in winter. :D

Generally, gravel and poured concrete will be the least expensive patio surfacing, concrete pavers next and natural stone at the top of the price range, both for the cost of materials as well as the labor needed to install. Decks and patios are not necessarily interchangeable :-) Decks tend to be constructed above grade....often well above grade!! And patios or paved terraces at grade (or sometimes below). There will be substantial extra costs involved if replacing an above grade deck with a paved patio, not the least of which will be filling, grading/leveling and easing the transition from indoors to out.