What is the typical kVA in a one bedroom flat?
2 KVA per home (2000 watts) is what the power plant uses when they're bragging about how many homes their plant powers. They're talking average use. 

What really happens is you have high extremes of power use.  The family chef has the oven, 3 burners, a George Foreman and the toaster all working at once (and a crockpot for sauces), the water heater is constantly recovering from dishwashing, and all that heat in the house has forced the AC to 100% duty cycle.  Once dinner is called, all those loads will poof disappear.

You have to size the system for peak use. 

OK, so you think "fudge factor", but this is like a fudge-factor of fifty.  No, you gotta sharp-pencil engineer this out.  Or just put in really good service and not worry about it. 

Maybe you're thinking about it upside down

You're asking "how little service can I provision".  How about asking "how much can I get for sane cost". 

Adding electric capacity is dirt cheap.  Spend an extra $500/unit on the parts to do it right, and it'll last 40 years, and that's chump change for something that'll last 40 years.  It's a loser's game to pinch pennies on service, only be forced to upgrade later. Especially if, once permitted and occupied, the city requires a licensed electrician to do the work.  As a homeowner subdividing yourself, you can DIY that stuff for pennies on the dollar. 

Ample service also saves money on appliances.  Electric on-demand water heaters are half the price of gas, and don't require a vent. My landlord put in a $1200 gas one because he had no choice.  Ditto dryers - tenant-grade electric washer-dryers are commodity. 

Better service also means better tenants. Tenants who are constantly fighting tripped circuit breakers are going to believe your electric is hokey and you're a slumlord, and are not going to stay. At extremes, "Can a circuit be added for my electric car" is a wonderful question because it means a wealthy tenant who'll be staying awhile.  You want to giggle with glee going "Oh yeah! We can do that!"

If the conversation with the electric company goes something like "Well we could bring out 400A service, but it'll cost ya (more than you wanna pay rght now)", then make everything else ready for it, so they can just cut it in without additional work. 

If the game is make max money as a landlord, squinting at price tags at Home Depot is barking up the wrong tree.

Commons service

Keep in mind Code will certainly require you to have a separate meter for commons-area loads.  This could be impressive, like an electric dryer or shared on-demand water heat; or it could be trivial, like five LED lights.  

I have to say, in the latter case, I would consider going off-grid solar for a few LED premises lights.  It's far cheaper than the cost of a monthly meter fee.