As far as I know, which isn’t much, there needs to be a clear separation of essential and non essential circuits, how does one do it in an existing db or are you supposed to install another db and extend the essential circuits wires and install breakers in that db ?
Oh come on, don’t sell yourself short! 
Put all the non-essential stuff to the left and all the essential stuff to the right (or the other way around if more convenient). Make wiring neat so it is clear that the two are not interconnected. Label properly. That should be enough, but let me add my own disclaimer here, I defer to the real sparkies on this topic.
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Depending on what is inside the existing DB, at times it is cheaper labour-wise, to install a surface mount DB for the Critical loads.
Also at times, the existing DBs are rather crowded, so if one needs more space, surface mount makes a lot of sense.
Like I added a 24way surface mount checked and checked again … it is full already. Should have gone 36way … should/could, pfft me.
Disclaimer… I’m not a real sparky.
I’ve split my own board twice (two different houses). Sparky approved afterwards.
Familiarise yourself with what you need to do the split and what you’d need to add in the inverter.
As @TheTerribleTriplet says, usually the existing board is too crowded and sometimes the house was wired during severe copper shortages which means the wires are simply too short to move much. So the secondary surface mount board is a good idea, or even as a replacement over the old board (provided there’s enough wire).
The saving is mostly in the time it takes to find the correct neutrals for each circuit.
Don’t cheap out on the earth leakage.
Don’t forget surge arrestors.
Double/triple check those neutrals to avoid random tripping.
Plan better than the original sparky - leave spare room in your new board.
Consider additional changeover switches for some non-essentials… That way you can have solar still run the pool or heat the geyser during load shedding.
Have a sparky check it afterwards - your life and your inverters warranty may depend on it.