Hi all, we are in the planning stages of an 1100 sq ft addition to our 1967 four season cottage in South East Ontario Canada (zone 5b) and are considering various types of foundations for the addition. We will of course be engaging engineers and designers for the project but want to have more knowledge before we start conversations with them.
Background: Existing house/cottage has a partial basement along with partial crawlspace, all concrete block, to which we added closed cell foam when we purchased the house in 2007. As well as foam on the walls, the crawlspace floor has 6 mil poly on dirt floor, edges tape sealed  and batt insulation in the floor joists above.  The door to the crawl space is left open to the partial basement, and there is a small baseboard heater in the crawlspace which is operated on low in the winter, controlled by a thermostat on basement wall. There is also a continuously running small fan just inside the crawl space door.  All of this (along with lots of lighting, lol) has created a comfortable dry crawlspace, with no issues.  The original crawlspace block walls are only 3 deep, 1 1/2 of which are above ground.  The addition will be attached to the crawl space end of the original building. With no need or desire for a basement, and not wanting to have to install deep frost walls for a crawlspace, we were thinking that a slab on grade fpsf, topped with floor joists to align with the original building  and provide some area for mechanicals, would be a viable option (the engineers can figure out how it will be joined to the original block wall).  However,  this seems like a duplication of floor material to my sense of frugality. 
Question: I came across a 2004 paper (US) that discusses fpsf’s and it shows an option for  a fpsf unvented, heated, crawlspace ( see attachment, page 25, fig 13). Since we have had  a good crawlspace experience with the existing house we’re wondering if this might be a method to discuss with the engineers. To my mind, it would allow for less deep crawlspace walls, good foundation and perimeter insulation, and more room for mechanicals, as well as aligning with the existing foundation. However, this paper  is older, and I haven’t been able to find any more recent discussions on this type of fpsf. 
Any and all thoughts on this would be appreciated, including code concerns. 
Note that we are seasoned homeowners-renovators, and much of the labour  on this  retirement  project (lots of time, but low budget 🙂 )we’ll provide ourselves,  with the added expertise and advice from our general contractor brother, as well as other trusted subs. Thanks!