hard disagree. The residential building code isn’t terribly hard to adhere to – especially in new construction – and nearly every bit of it is written with the health and safety of building occupants in mind. I’d much rather deal with a bit of bureaucratic oversight to be sure my house and/or my neighbor’s house doesn’t collapse in a stiff breeze, or blow up from a gas leak, or kill all its occupants in a fire, or turn into a heap of rot after the first heavy rain, etc., etc. You might have the skills and ethics required to do the job right without somebody looking over your shoulder, but not everybody does, and I’d venture at least half the big home building firms would cut every corner they could in the absence of code enforcement.

There’s no reason you couldn’t still do that, as long as you pull the necessary permits and your work can pass inspection. Most jurisdictions make specific exemptions in the contractor licensing rules for homeowners working on their own properties.

Just got my late, not-so-great furnace and AC replaced with a new cold-climate heatpump setup, and in the process moved the indoor equipment from a too-tight niche in the main floor of the house into the basement where it really should have been to start with. Now I need to frame up a wall where the furnace access panel used to be, properly tie in the return ductwork, and (eventually, need to relocate some other utilities first) add a linen cabinet in the vacated space. Next big stage in the huge-slow-moving basement Reno I’m in the middle of is to get the 60-year-old galvanized steel water supply line replaced, and then I can start inside plumbing work.

That’s true if the right side is a flue, but with neither fire brick nor flue liner in evidence, I suspect it’s purely decorative – I’m interpreting OP’s text description to mean that we’re looking at one of two lined flues, and one of two unexpected void spaces he found when removing the cap. I don’t think the void is actually connected to anything, and instead is just decorative, but if I’m wrong and you’re correct, then yeah – there are more serious issues to deal with here

I think you’re okay here – the code requirement about solid or fully-grouted blocks applies to the masonry supporting the flue liner. What you’re looking at in the photo is a decorative brick wrap around the structural portion of the chimney. My main concern would be to ensure that this area is properly capped and sealed so that critters and rainwater can’t get into the cavity and find their way further into your home.

Chomated copper arsenate treated wood has been functionally banned in the US since 2003, and replaced with Alkaline copper quaternary or copper azole preservatives, which are generally safe, though I wouldn’t go eating off a plate made from them or anything. The bigger issure with wood foundations is that even when treated they’re necessarily going to have a limited lifespan – wood still degrades from groundwater contact over time without insect or fungal attack, and water will wash the preservative agent out of the wood over time, eventually letting fungus and termites in.

The server I set up for my friends is named “Portable Creatures: Exhalation of the Wilderness” for a reason.

With old masonry wall construction, you need to be very careful about how you retrofit insulation to the walls. Insulating the interior makes it very likely that you’ll have moisture-related degradation of the stone and grout making up the wall, as you’re cutting off drying potential from the interior side of the wall. If it’s possible, insulating the exterior is your best option, otherwise you’ll need to be very conscious about selecting vapor-open insulation and finishes on the interior side, which will limit your options considerably. This article from Building Science Corporation goes over some of the details, and offers some options for interior-side retrofits that might not cause the wall to fail down the road.