have an example of over engineering residential?

I can do most of the structure but I’m too busy with 8 million other things so it saves me a bunch of time and saves my client some $ to have someone else do structural. Can’t say I’ve seen much that was over engineered. 

I have done my own structural calcs on probably 75% of my projects in the past, but I'm now using structural engineers more often, as my projects are getting larger and more complicated, and I like the idea of someone else taking the risk. But I still run my own calcs so I have a reasonably good idea of what sizes should be, and on small, simple projects it can be hard to find an engineer to do it for a reasonable fee. 

I do have a BS in structural engineering (a few classes shy of what's needed for an accredited BSCE) but I never practiced. 

I find that engineers can be a bit conservative, but since I know where their numbers come from, I usually understand why. There are exceptions, especially when it comes to newer ideas. I have had several of them require much denser foam insulation under footings than my math (and many projects I engineered myself) say is necessary; recently one required a concrete slab in a new crawlspace with little hydrostatic pressure. But generally they are just going far enough to protect their liability, which is ok with me. 

I can generally do gravity loads but when it comes to shear I'm out of my element. Basic gravity load calcs get me close on beam sizes for design considerations but I'm in a high seismic zone so I always rely on an engineer as the lateral stuff is more complicated. 

hell no

Not if I want them done right

I have a good sense of the rule of thumb for residential projects or structures in general. I can talk to an engineer and get what I want. But no way I'd do it myself. We have a lot of things assigned to a structural engineer alone and only to a structural engineer with a current license. California uber Alles;))
Saves a lot of money and liability.

G4tor, for simpler beam calculations and stuff like that. Yes. Note: If you can have an engineering consultant, use their services. For sanity check on structural design, sure. Like others already wrote here, I agree with them. Some of the structural engineering, stuff, I would not do. 

In high seismic areas and related stuff, wherever I can use prescriptive path, I'll use it but can still size footings and all. Fairly basic stuff if I know the soil bearing capacity. The details like rebar and such may require specialize requirements that the engineer would detail out and specify but I can already get the footing in the ball park since 20%-30% rebar won't necessarily require a change size of the footing especially if I am a tad overbuilt in my foundation. If the engineer specifies a smaller footing or foundation, we'll just have a conversation over it. Sometimes I might be far more overbuilt in my approach of the foundation than the engineer so I would more or less want to know where the engineer is coming from. If you have a good S.E. then everything should be alright on the structural. 

Jonathan is right on about making sure the lateral forces stuff is properly engineered by an engineer. The gravity load stuff are fairly basic. In design, I can already approach design with already geometrically stable form that is geometrically stable on the lateral. You can learn a few things from the Incans. Low rise homes are generally fairly stable but you can already start with "best practices" principles by avoiding the things you should not do like "soft story" designs. You don't need to be an engineer for that. 

The next step is assuring a strong stable connection between the first floor above any foundation wall of crawlspace foundation system. You want to minimize where possible "hinge points". Basic fundamental engineering. Otherwise, you can have this problem (for example): https://www.google.com/search?...

During early preliminary level design, sure do some of that yourself if you can. As you process through the design process, you have your engineer work out the structural engineering specifics. In early design phases, you can use "rule of thumb" principles and sizing with a little above minimum. 

If the calculation says use an 8x10 beam, maybe size up to 8x12 or 10x12 or 10x14 beam. Then size up the support column from 6x8 or 8x8 to say... 10x10.  Of course, there are more complex situations that may require further thinking like maybe you'll specify a wall that functions as a shear wall. Sometimes you need to specify diagonal bracing to establish the needed stability in the wall. 

As you get into that mess, you'll want to have a competent structural engineer on board and a good comprehensive soil report from geotech if possible. When your project is on a hillside or other sites with special risks (such as sink hole risks being pervasive in and around the project site), it's necessary and a must. It can be really costly if you don't.