[Matt1963]

In the second picture this assumes that you don't have to over dig even farther. Soil conditions effect the over dig.

 

[Matt1963]

OVERDIG! That makes sense. So the main reason is efficiency and as well as the over dig area if it remains mostly undisturbed in the bigger/deeper porch allowing the soil to not have to settle then it's good and they go 42" below after the basement is backfilled for porch footings. But like @Matt1963 is saying that is assuming they do not even have to dig even further which then in turn that entire porch area would be done with the basement like the smaller porch and filled with dirt and then do the slab as usual I'm assuming since it's possible it could be in the overdig area.

Both plans do have basements and to the right are the garages.

Because you have a 42inch min cut (Frost) then you most likely will be ok in picture two. (It's 12 inches were I am)

To be in a safe working hole, labor and industries in my state makes me be 2 ft clear at the bottom of a hole, then I can go straight up 4ft and then 1 to 1 slope from there to the upper grade.

So on an 8ft cut (basement) you would then bench cut about 4 feet up and excavate the porch from there. Footing and walls would tie in and just step to grade. One pour.

Other factors can apply.

Are you trying to figure out budgets based upon designs?

When estimating these kinds of excavations and foundations one must do an educated guess and then explain to the owner how such estimates were done.

Most builders are much more comfortable after the hole is backfilled.

 

[Matt1963]

Makes sense so the the bench cut area is pretty much left alone and the 2ft clear area would just be back filled and as you stated there is a 42" cut after the fact so porch can be trenched out after and either poured or cmu block. The reason behind this was to see how excavation efficiency plays a role and this can be done with budget based on design, yes.

The goal is to excavate once, pour once and backfill once. That's typically the most efficient way.

Site conditions don't always allow for that.

 

[Matt1963]

I spent 3 hours explaining that bridge to a new engineer one day. He made 4 phone calls before he passed it and let us pour concrete. We lost a day over that.

Did you leave out "new YOUNG engineer"