Adding second dwelling to PPOR. What to consider?

Hi we are in Perth southern suburbs. We bought our PPOR in 2004 and have always lived in it. Have now got 48% equity and ready to start investing. The block size is 950sqm but house position too awkward to subdivide the land.

So, we decided to build a 60sqm 2 bedroom unit at the side . We'll give it a private courtyard and alfresco area giving it a land area of 120sqm. The floor area of the main house is 180sqm not including car port and patio.

I understand we will be subject to Capital Gains Tax when we sell the property (probably 5 years).

What I am trying to work out is the cost base. It is "reset" when we start earning income from the property (read Bantacs pamphlets but couldn't work this part out). What does this mean?

We can claim holding costs. Is that from date of purchase or date of renting a portion of the property?

How to determine the portion. I think it is floor area not land area but is that "habitable" floor area not alfresco, carport, outbuildings?

Is it better to have a higher proportion that is NOT income producing?

If we want to do renos or extensions to the main house too should we wait until the unit is rented out so the costs are a portion of the whole income producing property?

Lots of questions and I really appreciate anybody taking time to answer whatever they can.

Caz

PS 2 income earners, PPOR is in sole name of highest tax payer so CGT will be at highest rate. We have a family trust but it doesn't own any of the property, if that is of any use.