No need to know anything.. the contractor will not stupidly building another story just like that..
(based on if owner say it can or cannot support).
Whether your existing stucture can take extra load or not doesn't matter.. They know how
to evaluate. They know how to reenforce the current structure and how many extra columns
are need to be added and how to make the proper foundation..

The main point is, yes, the technology to transform your house from 1 to 2 storey or 2 to 3 storey is there.
As long as you got the money.. it doesn't matter how is your existing house structure.

The worst case is, demolish existing house and rebuild from scratch 

i am also interested to know.  how much would it cost to demolish & rebuild a 1s to 2s?  even to demolish a 2s to 2.5s?

When I said "they", I meant the contractor + engineer + architect + ... It's my fault for just mentioning the
contractor alone..

In order to do this, as usual, you need approval. To get approval, you must have a plan and the plan must be approved by architect.. For the architect the approve the plan, the architect must know exactly the load of the structure, etc. If architect approved it, chances are the building should be ok (but accident do occurs like MRR2 crack, collapsed Terengganu's stadium, collapsed highland tower, etc). Normally, the contractor just follows the plan given to them.

I've seen a few link houses that has been converted with an extra storey.. I'm just not sure whether the town council approves them or not.