Here is a proposed crawlspace foundation for a modest vacation home. The out to out dims do not match modularity for either LOGIX or AMVIC 6" core blocks, but they make all the stick framing modules work above. Formed and poured walls would be cheaper, but we would need to foam-line the walls, and with ICF, we can self-build, instead of hiring it out. Anyone see a problem? The one-block drop area is so we can get some maneuvering room for installation of mechanicals.

I would say it should work OK, but I suggest you rethink the drop down area. I see you're going to have 4' walls all around. I presume you are planning on building the floor deck on top of the ICF so this will give you 4' clearance between ground and floor. I would think that should be plenty for whatever mechanicals are going down there. You're planning on having the crawl space access in the floor, right? Rather than doing the drop down area build an oversize access hole. You could make the cover in two pieces so you'd only need to lift a small door for regular access down there. You can cover the access door with your flooring to match the floor all around it and it will hardly be noticeable.

If you really do need, or want, the 5'4" clearance check the cost of just doing the whole wall 4 blocks deep without the drop down area. You'll save the extra footing pour, and you already have about 1/3 of the extra wall planned anyway.

Unless you're digging all this by hand, the small size of the drop down area will complicate the excavation process.

Thks for that, dmaceld. We have just dropped the idea of the drop, and will squeeze the mechanicals into the overhead of about 44 inches. A lot less expense this way.

I would check your direct vent hot water heater for height first. We did the same thing on a project and no one made a hot water heater (direct vent) short enough. We had to install a on demand hot water heater.

Mono pour the footings and walls and the rat slab in one shot.

For what you are doing I would go with 6".