I usually work on infrastructure as code, and I usually swap out the third layer of the design for the first layer of the implementation. You need a fully-specc

I usually work on infrastructure as code, and I usually swap out the third layer of the design for the first layer of the implementation. You need a fully-specced functional design and a high-level technical design, but I've found that in many cases the technical implementation details can change as the project is developed, and that means either the low-level technical design and the implementation go out of sync, or there a heavy bureaucratic process in place to prevent that -- but the natural solution is to have the low-level design and the high-level implementation to live in the same place. That still requires the rest of your process, and perhaps even places more emphasis on those documents. The infrastructure implementation needs to follow from the high-level technical design which needs to follow from the functional specification. Because as you say, the implementation needs to be reviewed against a specification. What are you reviewing if the spec is incomplete?