It's a great idea - and you don't have to reinvent the wheel. It's already done widely, notably in off-grid solar homes, robustly wired RVs, tiny homes, people who want a comfortable home when mains power fails, etc. You can adapt existing concepts, but you have to know when to let go of them. When someone naysays, they are not letting go of the right concepts. It happens.
Justification
"Merely to save money" won't work. Your house still needs mains wiring in the legally required locations (mainly receptacles) because Code requires it, and you can't sell your house without it, which means you can't get or keep a mortgage on it. This means any DC system must be an "overlay" at least as far as receptacles. I see nothing in NEC that requires 120V lighting, though.
"Because I'm off-grid" is a superb reason. "Because it's cool" is fine - it's your house.
Because I want more is a fantastic reason. It's the 21st century for Pete's sake. Houses shouldn't lose power. All you need to fix that is an overlay system using off-grid solar tech, and prices for those are in free-fall, and you can get in as little as a few hundred dollars if you scale carefully. Panels, charge controller, batteries, you can have a very nice system. With a little more work the system can power refrigerator and a well-chosen furnace (or simply a non-electric furnace) and you stand a chance of being able to ride out storms. You won't be able to wash and dry clothes, but could watch Netflix on your TV. (notice most routers, Roku and many TVs take 12V input, and the central-office has a submarine-sized battery bank to keep telecom up at all times, and genny backup.)
Distance and Voltage
When dealing in low-voltage, distance is a very serious factor. This was Edison/GE lost the War of the Currents. So cottage vs. McRanchion matters. Corner vs middle of building, matters. You need to think about this. It's not a deal-breaker, just needs either smarts... or lots of feeder. I recommend "smarts".
Voltage is a huge factor, and your choices are 12, 24 or 36 volts. 12V has far-and-away the best selection of gear, with USB fobs for instance sold at literally any gas station. But it is the most difficult to haul distances, requiring thicker wire. 24V gear exists, but more hunting is required to find it. In return, you get 4x the transmission efficiency, which is a Big Deal. 36V doubles transmission efficiency again, but gear is scarce, so probably not worth it. Actually a lot of 12V gear uses switching supplies (e.g. LED drivers) which buck, and so inherently have the ability to work on 12V and 24V (and in some cases, 36). The question is whether they enable it and had the device listed for it. This bucking also makes them resilient to voltage drop within reason.
Anything over 36V is out of the question. It no longer qualifies as "low voltage" and DC can get rather bitey and arcey at those voltages. You don't want to mess with that.
You might as well put a desktop shortcut for a decent voltage drop calculator, because you'll be "living there". It's hard to find a good one that doesn't lie to you... most of the good ones are optimized for NEC mains installation, and have bugs when dealing with 12V. This is the best I have found.
Here's a concept to let go: 3% voltage drop. That's a mains electrical "meme" that guides a lot of really bad design, much to the delight of cable manufacturers. It is not in Code anywhere. What's in Code is the 310.16 and 240.4D current limits, and one thing I like about Southwire's voltage calc is it takes those into account while letting you choose any % drop. You are going to have voltage drop. Plan for it. But it's not the end of the world: likely your supply is 13.8V if a battery is supported, and even at 15% you're still at 11.7V. And bucking loads just won't care.
The grounding dilemma
Remember electricity wants to get back to source, not ground. In AC mains, we wire as an isolated system -- ground is excluded from the normal current loop, it only carries ground faults. There is also one neutral-ground bond in one place only which functions as a "fault catcher" and to keep it from floating/rattling at high voltage, which isolated systems are prone to do. That does not make neutral equate to ground. EE's hate this because they are used to GND/that little symbol being the symbol for the normal current return (what we call neutral).
What about a low-voltage DC system? Good question. The whole point of isolating mains is that it's hazardous - that doesn't apply to low voltage DC. Even if you built the DC system isolated, you'd still want to bond negative to the grounding system to keep it from floating/rattling. I can't honestly see a reason not to merge the DC system's grounding system and current return and building frame, provided it's rated for the current. There's no question on a RV, boat or train; they are vehicles and this is SOP. However in buildings, your steelwork is not as massive, nor bonded as well (vehicles are welded), and it may be a moot point anyway: you must use proper wiring methods and they don't make 1-wire Romex. You will surely be stuck dragging 2 if not 3 wires along regardless, so using the building for return may be moot.
The ultimate decider here is your AHJ, so have that conversation with him/her first. If s/he nixes it, run the negative wires. (AHJ = Authority Having Jurisdiction, i.e. your electrical inspector).
Topology
Think about every place you want DC power to go, and how much in watts (or amps if you have decided your voltage). You may be able to exclude large sections of your house entirely. Think about where to put distribution. Generally you want short runs to each load, you don't want to carry 12V long distances on thin wires. Every segment needs to go into the voltage drop calculator to help you make good wire-sizing decisions. I imagine most loads will be <4A going <20', so 14AWG wire will yield a <0.42V drop.
Every junction box needs to be accessible. But a distribution point only needs to follow the service panel rules if it has circuit breakers or fuses that you would reset.
Each circuit must be fused based on the thinnest wire in it: 15A for 14AWG and 20A for 12AWG. You are allowed to use larger wire if you want to.
Anytime a switch is involved, think about relays to reduce the carrying distance of power. Do the math both ways, but a relay can help, since the power doesn't need to travel the relay loop.
You have to break out the calculator, make the map and do the math.
Example:
This is just a quickie to convey the idea, much more could be done with it. This is with a long, narrow house with all the action on the ends, so 2 separate LV systems. Trying to run a fat trunkline down that long middle didn't make any sense, nor did running 15 "smaller" wires. There is some fat wire between the distribution points and battery. A choice was made to move the cable modem and router to the utility room to be near the battery, instead of wasting time trying to haul power to its difficult location in the living room. Easier to move phone and Internet lines. Since branches were kept short, #12 and #14 wire could be used, except for the bedroom-living room run for its lighting. There's only 40' of fat feeder and 20' of smaller feeder.
Load factors
One thing said a lot is "look how much power lights need." 6500 lumens was proposed for an "average room". I was like "you're really gonna want a dimmer on that." Fortunately DC light dimming is simple and reliable, unlike AC.
And why should all the lights in the house be on at the same time? Maybe the right answer is let them dim... intentionally engineer in some voltage drop, to warn the occupants not to leave lights on unnecessarily. That makes even more sense in an off-grid/battery scenario. where such carelessness would burn up battery. This is the kind of design decision you get to make.
Beyond that it really comes down to a calculus of "how much are you willing to spend on wire for proportional voltage drop". No problem getting fat wire - 4/0 mobile home feeder at $3.50/ft is more than you'll ever need.
Voltage conversion
Generally you want to do as little voltage multiplying/dividing as you absolutely have to. Inside a normal house, thicker wire will be cheaper than a choppy, lossy voltage pump.
That's especially true for 120/240 inverters - they are very lossy, especially when they are oversized, and they lose even with no load. A lot of off-grid/solar projects fail because the user fails to account for this enormous vampire load.
I make an exception for supply - solar, wind, generator, microhydro, "use your car battery to recharge your sagging batteries 3 days into the ice storm" etc. You probably can't do much about your power source being quite far from your battery. This is a great time to pump voltage up as high as you can, pretty much to wire insulation limits. This can be as simple as putting the solar panels in series instead of parallel, or winding the windmill generator differently, or using an inverter to pump your car's alternator up to 120/240V for haulage.
