Hard telling what advice to give you because we don't know where you are on the world wide web!
But, what I can tell you is before you spend any money on architects and contractors, you would be well served to check at your local town/city hall, to see if this is an allowed use where you are located.
If this is not an allowed use where you are, your architect probably is not going to tell you. He/she is going to draw up the plans and you are going to pay for useless plans.
I know, I have seen this happen.
I'm the guy that makes the extra apartments go away!