Tap con pt plates flat to the slab,find leval,install small posts under the joists with cleats conecting the joint. J

we had sort of the same type issue. We have a wooden deck on one side of the house, then this horrible dirt strip that wrapped around a patio and sidewalk to the front of the house. So we had to tie into an old concrete patio poured first, a newer brushed sidewalk poured some time later, and a wooden deck. We solved the thing by having new concrete poured over the entire dirt area. Then, a concrete restoration company came and stained all ages of concrete one color, taped it into random stones (you could do bricks or anything), stained it in about 4 colors, pulled up the tape and sealed the entire thing. It looks amazing, and was under $6000 in Southern California. This was about 700 sq. ft. You have a much smaller area, I think. I like the seating wall idea also!

My dad will be 94 in May & he installed a flagstone patio on top of the concrete patio about 8 yrs ago. He swept the concrete & washed it & when it was dry he mixed up wheelbarrel full of 1 part mortar cement, 1 part Portland cement( Can use something like clean cut off bleach jug for measuring mix with 2 parts clean sand. Important thing is mixing well while it is dry, we mix it carefully with a hoe & face mask or hankie tied over your nose & mouth(make it in triangle- area in center goes over your nose & ties behind your head) will keep it out of your lungs, keep kids away. Add water carefully as you can get too much,continue mixing with the hoe in long pulls toward you turning the mix over until there is no powder left & moist enough to lay stones in but no water sitting in it(& not soupy-won't dry properly)There are no amounts of water as it depends on whether sand is moist, humidity in air,etc. If extremely hot use bit more water so it doesn't set up too fast. The key to getting a nice job is to have all your stones laid out for the 1st batch. If your patio were 10x20 lay out stones starting against house & out 1 direction say 10 ft. way He did about 2-3 ft wide all the way to edge as he didn't plan on cutting anything. Flagstone is different thicknesses so you would fit them way you want so colors & shapes look good & move them back toward center of patio so you could get your mortar down to desired thickness. This was a patio with no cracks or unevenness. Main point is you don't have time to figure out what is going to go where if you slap the mortar down & try to fit stones together, dried out mortar will crack. So you need enough of your pattern figured out to use up 1 wheelbarrel of cement as you have to use it up before you quit. You quit at edge of the stones so when you start in again the new fresh mortar goes up against the already set stones.(Good idea to wet old towels to keep it from drying too fast) You can't tell where my dad stopped for the day by doing that way. You do have to do it way I said. Anyone that throws the stuff together in wheel barrel & adds water without having dry stuff thoroughly mixed is wasting their time & money, it will crack & crumble. I've mixed cement for my dad several times, replaced block wall(quake) after I knocked all the old cement off of them & out of them(some job) when I thought I had the mortar mixed he would say-Do it some more!! Good Luck!

The reason footings work and don't shift is because they are part of a larger system that keeps them in place. Once left on their own, they will move around with frost heaves. I'm in North Carolina, US and our climate is not nearly as cold as yours. My neighbor took out an inground swimming pool and left some of the wall material in the ground to take the less expensive route. It is now a major problem for them as things keep shifting and their back yard has sink holes some big some small but it's problematic. My parable...I was renovating an old house and was getting exterior painting estimates. The painter asked me, "Do you want it done or do you want it done right?" Do it right, take out what you need to take out to resolve the problem.