I also renovated houses and own an interior design and renovation business. 

Better materials such as stronger  lumber were used when the earlier vintage properties were built and many  contained design characteristics such as stone or brick work and intricate millwork that are too costly to replicate today meaning many buildings are significantly below todays replacement cost. 

Ironically, platforms designed for real estate (Zillow, Apartments.com, Rentler, etc.) produce very few tenants for me, compared to FB Marketplace (and I also find a lot of their functionality to be user un-friendly, from the landlord side). ...you'd think that platforms custom-made for real estate would make the process a snap for landlords, but it takes me a lot longer to post and manage rental listings on Zillow or Rentler than it did on Craigslist.

They have some things designed for higher end investors.

Following this, I consider it another pro of our situation that I have the ability to do significant property renovations myself, and between my wife and I, we make a pretty good design team.

BiggerPockets is full of resources designed to help you along your journey, whether you are just getting started or a seasoned investor.

., how much design work does the civil engineer need to do. 

Congrats - love the design

I have a general contractors and interior design license.

The time between when the corrections are presented and the turn time for the designer is really a drag (keep on them).5.