When I had my house built the plumbing & electrical "engineering" was done by the installing tradesmen. More than know the code, they knew how the local inspector was interpreting it that year. I was available during construction and simply walked around with the electrician to establish light and outlet locations. I did have some special lighting control ideas, for those I gave him sketches and we mutually chose suitable parts. At my city permits office I have the impression that they really don't want to deal with electrical amateurs anymore. Take a look at your county's building department website.
I gave my general contractor scale sketches of the floor plan & elevations I wanted and he had them fleshed out and drafted for permit submission. The city asked for one overhanging room to have PEng approved support. As the work progressed I shopped for finishes & fixtures and gave the contractor notes.
I did not design the house in nut and bolt detail for him and I did not tell him how to build it. I did not want to build it myself either, co-workers had told me how many years that takes. From supervising contractors in my own job I also knew the difference between discussing things and telling him how to do his job.
The contractor completed the house in 9 months but I was still involved more than half of the evenings and most weekends in researching and answering his questions on style choices and in shopping for stuff. I looked in at least 7 carpet stores to get what I wanted, ditto for tile, plumbing, lights, appliances, kitchen cabinets, paint, blinds, etc. That work exists in self built homes too. At the end of the year I gave back my last 4 days' vacation to my boss because of the office time I had spent on the phone.
Good luck on the new home, Bill
