Building Garage with an Apartment Studio

I live in Northern part of melbourne. More precisely Meadow Heights. I was thinking about building double garage and a small apartment at the top. I was quoted $100k.

If I decided to sell the house, would this sort of construction add the same $100k value to the overall property price?

My main motivation is that I could rent it out for now, even as a shared accomodation and in the future (if i don't sell) get my money in capital gain.

Have a read of this thread which addresses a similar Q:

When you are spending $50K - no issue.

When you are spending $100K then you need to ask a whole lot more Qs:
1. Am I overcapitalising? i.e. Will my house be worth more then $100K more - I need to see some comparables in my suburb to determine this.
2. How much rent will the studio bring in?
3. Is there a way to do it more cheaply for the same valuse add + rent
4. Am I producing something which will appeal to a broad cross-section of the market or am I limiting the property's appeal?