@alowhum There will be a wealth of information out there on the net and probably also at your local library. look for resources about remote cabin power systems and also even the larger 5th wheeler type caravan systems will be helpful reading. You will also need to check your local building codes to see if there are any rules to be adhered to.
When we built our house (about 15 years ago, long before i discovered MySensors :) we ran a low voltage backbone cable throughout the house. From this backbone we ran smaller cable drops to every room in the house. Many of these are still not in use but as others have said the cost of running wires is relatively cheap when you are building. Of course we still have mains power as well and use the low voltage system to compliment this.
In those initial years the 12v system was used primarily for lighting systems such as night lights etc. As time progressed we installed 12v outlets in the bedrooms and these now are used to power bed/reading lamps. The small 12v downlights have also proved to be very easy to convert over as well. These were initially halogen bulbs but now there is a good range 12v LED replacements for just about any type of socket. Once I discovered MySensors and home automation then the 12v system became even more useful now powering several nodes and even the gateway.
Right from the start we ran our system from a battery with solar charging and in 15 years it has never let us down. Voltage drop will always be a thing to watch, but with LED's low power draw and as you have surmised the ability of arduinos to run well below 12v it will usually be of little practical concern. Unless of course you are thinking of running more than lights and nodes.
