How do I handle a transition from concrete to dirt underneath a flagstone patio?
(1) Is this even advisable, or should I just extend the concrete out to the entire area I want for the patio before laying stone (adding considerable cost)?


Rolls-Royce job (Money no object): Extend the concrete slab out to the area you wish to pave or remove the existing slab and start allover again.


  (2) Assuming I don't extend the concrete, what should I do when the stone transitions from the concrete subsurface to the dirt? I'm nervous that cracks will occur over time, and I'd like to do whatever I can to avoid that.


Good enough job (and what most paving pro's would do, at least the ones I know back in the UK): Excavate 6 to 8 inches (150 to 200mm) from the area you want to extend into. Back fill that area with sub-base (crushed rock) material. Back fill in 2 to 3 inch (50 to 75mm) layers, each layer should be "properly" compacted. This will give you a sub-base that is "nearly" as good/stable as the existing concrete slab.


Something to consider, regardless of chosen method, how is the paving area going to drain? Do you need to put in some sort of drainage system , thus preventing the surrounding softscape from flooding due to the new hardscape?