Let's start with your ideas concerning voltage.
12 V suck. Really.
Let's do some simple math. Assume you want to illuminate the whole area with at least 150 lx. A rough calculation gives the need for 6000 lm for basic brightness. In fact you will want to spend additional power on a lot of spots and you'll loose much light depending on the remittance of furniture, floor and carpets. You don't write anything about the number of rooms. Every wall makes it more difficult. My basic guess is, you will need 200 W at least to supply everything for full illumination.
Cabling and supply will have to carry 15 A or more to the lights. While in a 230V installation 15 A are a usual load for 1.5 mm² cables, in a 12V cabling it is a problem.
In a 230 V installation a voltage drop of let's say 5 V is acceptable in terms of efficience and functionality. You will loose some percent of your power in the wiring and all appliances will work as expected. 5 V drop in a 12 V environment will make your lights stop working and efficiency abysmal.
If you really want to go for a SELV-supply installation, go for 24V. Cabling will be feasible and there's a vast number of lights and control gear out there for that voltage level. In fact 24V is defined as the "professional" voltage level in voltage controlled LED lighting and 12V is the "amateur" voltage level.
Now for installation in general
You will have to control your lights somehow. To adress this you will have to either use the features your already existing installation has or to add some functionality on top of that. If you refuse to use the switches already present, you will need either extensive cabling or wireless controls. I'm not a big supporter of all this wireless stuff but it may work if you carefully select your components.
A central PSU fostering hundreds of watts may be much more expensive than carefully selected individual control gear. On top if you exceed certain amperage on the output of your PSU you will have to provide circuit breakers for 12 V DC for the SELV circuit depending on regional codes. And circuit breakers for DC are ridiculously expensive.
If you want to dim and control your lights individually, keep in mind, that you still will have to supply individual control gears for single lights or local groups of lights regardles of the bus you're going to use. If you compare prices for integrated dimmable control gears with 230V input and dimmers for 12V, you will notice that it won't save you any money to put the voltage level conversion together into one device, as the number of electronic devices will stay the same.
If you want to add your own wiring to your home, take care of some details
- if you install additional wirings in existing piping take care not to exceed the maximum number of wires. You will have to consult an electrician.
- When using buses like DMX which aren't suited for building installation take care your ECGs use isolators to prevent "sum up of leakage currents".
- When using buses like DALI be sure not to mix it with SELV-circuits inside a common cable. DALI has only basic insulation, it is allowed to route it together with 230V but not with SELV.
Update on 12V problems
I just want to go into some more detail, why 12V systems are problematic and inferiour to higher voltage SELV solutions.
LED lights operating on 12V will contain some circuitry to ensure a fairly constant current over single LEDs or groups of them. There are two groups of lights available for 12V. Strip lights and retrofits for halogen systems. When searching the market you will find few other types.
In strip lights a small group of LEDs will operate independently on 12V to enable cutting the strip at convenient lengths. So there must be a current regulator for only few low power LEDs. As one can imagine that can only be a linear regulator to keep a reasonable pricing. A linear regulator is not necessary a bad choice for LED circuits. For 12V it is only possible to put 3 or 4 LEDs in series. For most white LEDs 4 LEDs are too much in terms of forward voltage. The linear regulator won't have enough headroom to regulate. So the 3 LED will typically reach a forward voltage of 9V something. The regulator has to waste the remaining 3 V plus minus the tolerance of the supply. That's around 25%.
Due to run length of the strip and the supply cable the tolerances on the voltage can build up significantly.
The retrofits in turn have to deal with 12AC. They sometimes have some sort of switching regulator, but that is rare. They waste some energy in a rectifier, need a reasonable capacitor and have a linear regulator again. The capacitor typically is an electrolyte one, which doesn't like heat. Unfortunately a LED retrofit for 12V AC systems is prone to heat up strongly, as they don't have good thermal connection to housing and only feeble means of convection cooling. Lifetime is limited by the capacitor not the LED.
If you compare that to higher voltages, most of these problems are reduced drastically.
- Absolute voltage drop over cabling and strip length will be smaller for the same output power.
- Voltage drop relative to supply voltage will reduce by second order.
- Considering an equal power drop for a linear regulator (let's stay with 3V headroom for this estimate), the regulator's loss will only be 12.5% of total power consumption.
- As a smaller tolerance of supply voltage can be expected, the system can be designed to be even more energy efficient. This is done by selecting appropriate forward voltage binnings of the LEDs.
- For higher voltages there are only dedicated DC supplies, so there is no further need to deal with rectifying and the need for filtering flicker.