A deck pier block is in many ways just a simplified version of a “precast foundation”, a foundation type recognized by building codes. They’re subject to all of the same requirements as a typical footing, regardless of not being cast-in-place. They must have a sufficient bearing area (the area of the block that sits on the earth) and be a minimum of 12-inches below grade, or below the local frost depth. They cannot, however, be simply placed at grade level.
The connection of the deck support post to the blocks is not consistently regulated throughout the country. Some regions with high winds and concerns of uplift forces may not approve them, as there is typically no physical connection between the post and block. For lateral forces, like a post getting hit with a lawn mower, we don’t want the post kicked out. Therefore, building codes require lateral restraint at the base of a post. While pier blocks commonly have protruding concrete lugs that surround the post, there is only opinion as to whether they are of sufficient strength.
The small bearing area of pier blocks is limited in supporting much load. Therefore, beam and joist spans must be limited and more blocks must be used over typical construction. Overall, pier blocks are best suited for low- or ground-level decks, where smaller framing materials are common and additional posts and blocks are not an eyesore. Uplift and lateral forces on the posts are also of less concern in ground-level decks.