Converting a 3 car garage in Black Rock0

Hey everyone.   

I'm looking to get more value out of my rental by converting the 3 car garage into a 2 bedroom apt . So i have some questions.

My garage is almost 900 sq ft with an  electrical meter already attached to it and some plumbing running to it  due to a sink and toilet setup. The garage is detached and sits behind my house facing a different street than the front.  I wad told by the town that they will allow me to convert 800 of the 900 sq ft for a dwelling .

1 Is there anyway possible to get permission to use more sq footage ?

2. Do I need a architect for the technical drawings ? If so , any recommendations?

3. What can I expect to pay for such a conversion ? It's going to be an " up to date " rental . 

4. What kind of permits have to be pulled if any ?

5 Are there any common mistakes to watch out for ?

6. During the process, any ways I can save on the costs of my project?

all help is greatly appreciated in advance !

Also if Amy contractor recommendations, let me know